what is a active margin

Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision.

Plate tectonics

"Plate tectonics From Wikipedia, the free encyclopedia (Redirected from Plate boundaries)navigation search The tectonic plates of the world were mapped in the second half of the 20th century. Diagram of the internal layering of the Earth showing the lithosphere above the asthenosphere (not to scale)Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός ""pertaining to building"") [1] is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth 's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century.

The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s. The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform.

Earthquakes, volcanic activity, mountain -building, and oceanic trench formation occur along these plate boundaries (or faults ). The relative movement of the plates typically ranges from zero to 100 mm annually. [2]Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.

Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.

In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle.

Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe. [3]Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle.

Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography (the ridge is a topographic high) and density changes in the crust (density increases as newly formed crust cools and moves away from the ridge).

At subduction zones the relatively cold, dense crust is ""pulled"" or sinks down into the mantle over the downward convecting limb of a mantle cell.

Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.

Contents [ hide ]1 Key principles2 Types of plate boundaries3 Driving forces of plate motion3.1 Driving forces related to mantle dynamics3.2 Driving forces related to gravity3.3 Driving forces related to Earth rotation3.4 Relative significance of each driving force mechanism4 Development of the theory4.1 Summary4.2 Continental drift4.3 Floating continents, paleomagnetism, and seismicity zones4.4 Mid-oceanic ridge spreading and convection4.5 Magnetic striping4.6 Definition and refining of the theory4.7 Plate Tectonics Revolution5 Implications for biogeography6 Plate reconstruction6.1 Defining plate boundaries6.2 Past plate motions6.3 Formation and break-up of continents7 Current plates8 Other celestial bodies (planets, moons)8.1 Venus8.2 Mars8.3 Icy satellites8.4 Exoplanets9 See also10 References10.1 Notes10.2 Cited books10.3 Cited articles11 External links11.1 Videos Key principles The outer layers of the Earth are divided into the lithosphere and asthenosphere.

The division is based on differences in mechanical properties and in the method for the transfer of heat. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient.

This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like ( visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/year ( Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year ( Nazca Plate; about as fast as hair grows).

[4] The driving mechanism behind this movement is described below. Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust ( sial from silicon and aluminium ).

Average oceanic lithosphere is typically 100 km (62 mi) thick; [5] its thickness is a function of its age: as time passes, it conductively cools and subjacent cooling mantle is added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed.

For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively.

[6] Continental lithosphere is typically about 200 km thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents.

The location where two plates meet is called a plate boundary. Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and widely known today.

These boundaries are discussed in further detail below. Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation [7] and to mantle plumes.

As explained above, tectonic plates may include continental crust or oceanic crust, and most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans.

The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents.

Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements ("" mafic "") than continental crust ("" felsic "").

[8] As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate ), while continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).

Types of plate boundaries Main article: List of tectonic plate interactions Three types of plate boundaries exist, [9] with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: [10] [11]Transform boundary Divergent boundary Convergent boundary Transform boundaries (Conservative) occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed.

The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer).

Transform faults occur across a spreading center. Strong earthquakes can occur along a fault. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion. Divergent boundaries (Constructive) occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin.

As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of mid-ocean ridges (e.g., the Mid-Atlantic Ridge and East Pacific Rise ), and continent-to-continent rifting (such as Africa's East African Rift and Valley and the Red Sea), are examples of divergent boundaries.

Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision.

At zones of ocean-to-continent subduction (e.g. the Andes mountain range in South America, and the Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate is heated it releases volatiles, mostly water from hydrous minerals, into the surrounding mantle. The addition of water lowers the melting point of the mantle material above the subducting slab, causing it to melt.

The magma that results typically leads to volcanism. [12] At zones of ocean-to-ocean subduction (e.g. Aleutian islands, Mariana Islands, and the Japanese island arc ), older, cooler, denser crust slips beneath less dense crust. This motion causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands.

Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called ""foreland basins"". Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic continental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.

Plate boundary zones occur where the effects of the interactions are unclear, and the boundaries, usually occurring along a broad belt, are not well defined and may show various types of movements in different episodes. Driving forces of plate motion Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL.

The vectors show direction and magnitude of motion. It has generally been accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere.

Dissipation of heat from the mantle is acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming.

The current view, though still a matter of some debate, asserts that as a consequence, a powerful source of plate motion is generated due to the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement.

The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.

[13] Although subduction is thought to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among scientists.

One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movement, as some patterns may be explained by more than one mechanism.

[14] In short, the driving forces advocated at the moment can be divided into three categories based on the relationship to the movement: mantle dynamics related, gravity related (mostly secondary forces), and earth rotation related.

Driving forces related to mantle dynamics Main article: Mantle convection For much of the last quarter century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through the asthenosphere.

This theory was launched by Arthur Holmes and some forerunners in the 1930s [15] and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged a static Earth without moving continents up until the major breakthroughs of the early sixties. Two- and three-dimensional imaging of Earth's interior ( seismic tomography) shows a varying lateral density distribution throughout the mantle.

Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density is mantle convection from buoyancy forces. [16]How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics.

Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There are essentially two main types of forces that are thought to influence plate motion: friction and gravity. Basal drag (friction): Plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere.

Slab suction (gravity): Plate motion driven by local convection currents that exert a downward pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Lately, the convection theory has been much debated, as modern techniques based on 3D seismic tomography still fail to recognize these predicted large scale convection cells; therefore, alternative views have been proposed: In the theory of plume tectonics developed during the 1990s, a modified concept of mantle convection currents is used.

It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas, which find their roots in the early 1930s with the so-called ""fixistic"" ideas of the European and Russian Earth Science Schools, find resonance in the modern theories which envisage hot spots or mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators).

Modern theories that continue building on the older mantle doming concepts and see plate movements as a secondary phenomena are beyond the scope of this article and are discussed elsewhere (for example on the Plume tectonics article). Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below the Earth's crust, which then provide basal friction to the lithosphere.

This theory, called ""surge tectonics"", became quite popular in geophysics and geodynamics during the 1980s and 1990s.

[17] Recent research, based on three-dimensional computer modeling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere. [18]Driving forces related to gravity Forces related to gravity are usually invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above.

Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges.

[19] As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load.

The result is a slight lateral incline with increased distance from the ridge axis. This force is regarded as a secondary force and is often referred to as "" ridge push "".

This is a misnomer as nothing is ""pushing"" horizontally and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature.

Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate which produces a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges, and mantle plumes and hot spots, which are postulated to impinge on the underside of tectonic plates.

Slab-pull: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this current understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches.

[20] Recent models indicate that trench suction plays an important role as well. However, the fact that the North American Plate is nowhere being subducted, although it is in motion, presents a problem.

The same holds for the African, Eurasian, and Antarctic plates. Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them. This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism.

This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin. [21]Driving forces related to Earth rotation This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.

(November 2015) ( Learn how and when to remove this template message)Alfred Wegener, being a meteorologist, had proposed tidal forces and centrifugal forces as the main driving mechanisms behind continental drift; however, these forces were considered far too small to cause continental motion as the concept was of continents plowing through oceanic crust.

[22] Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929. However, in the plate tectonics context (accepted since the seafloor spreading proposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see below) during the early 1960s), the oceanic crust is suggested to be in motion with the continents which caused the proposals related to Earth rotation to be reconsidered.

In more recent literature, these driving forces are: Tidal drag due to the gravitational force the Moon (and the Sun) exerts on the crust of the Earth [23]Global deformation of the geoid due to small displacements of the rotational pole with respect to the Earth's crust;Other smaller deformation effects of the crust due to wobbles and spin movements of the Earth rotation on a smaller time scale.

Forces that are small and generally negligible are: The Coriolis force [24] [25]The centrifugal force, which is treated as a slight modification of gravity [24] [25]: 249For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and the geographical latitudinal and longitudinal grid of the Earth itself. Ironically, these systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to the Earth equator and axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, ""fixist theories"").

Later studies (discussed below on this page), therefore, invoked many of the relationships recognized during this pre-plate tectonics period to support their theories (see the anticipations and reviews in the work of van Dijk and collaborators).

[26]Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possible principle driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond the discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model.

In 1973, George W. Moore [27] of the USGS and R. C. Bostrom [28] presented evidence for a general westward drift of the Earth's lithosphere with respect to the mantle. He concluded that tidal forces (the tidal lag or ""friction"") caused by the Earth's rotation and the forces acting upon it by the Moon are a driving force for plate tectonics. As the Earth spins eastward beneath the moon, the moon's gravity ever so slightly pulls the Earth's surface layer back westward, just as proposed by Alfred Wegener (see above).

In a more recent 2006 study, [29] scientists reviewed and advocated these earlier proposed ideas. It has also been suggested recently in Lovett (2006) that this observation may also explain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on the planet.

In a recent paper, [30] it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific Ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces).

In the same paper the authors admit, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.

They demonstrated though that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. The debate is still open. Relative significance of each driving force mechanism The vector of a plate's motion is a function of all the forces acting on the plate; however, therein lies the problem regarding the degree to which each process contributes to the overall motion of each tectonic plate. The diversity of geodynamic settings and the properties of each plate result from the impact of the various processes actively driving each individual plate. One method of dealing with this problem is to consider the relative rate at which each plate is moving as well as the evidence related to the significance of each process to the overall driving force on the plate.

One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates.

It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.

[20] This view however has been contradicted by a recent study which found that the actual motions of the Pacific Plate and other plates associated with the East Pacific Rise do not correlate mainly with either slab pull or slab push, but rather with a mantle convection upwelling whose horizontal spreading along the bases of the various plates drives them along via viscosity-related traction forces.

[31] The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics.

Development of the theory Further information: Timeline of the development of tectonophysics Summary Detailed map showing the tectonic plates with their movement vectors. In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 book The Origin of Continents and Oceans [32] and the scientific debate started that would end up fifty years later in the theory of plate tectonics.

[33] Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was called Pangea later on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to ""icebergs"" of low density granite floating on a sea of denser basalt.

[34] Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast, and from the matching of the rock formations along these edges.

Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris and Gangamopteris, and the therapsid or mammal-like reptile Lystrosaurus, all widely distributed over South America, Africa, Antarctica, India, and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in the southern hemisphere. The South African Alex du Toit put together a mass of such information in his 1937 publication Our Wandering Continents, and went further than Wegener in recognising the strong links between the Gondwana fragments.

But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Distinguished scientists, such as Harold Jeffreys and Charles Schuchert, were outspoken critics of continental drift. Despite much opposition, the view of continental drift gained support and a lively debate started between ""drifters"" or ""mobilists"" (proponents of the theory) and ""fixists"" (opponents).

During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove.

One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since the mid–nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time.

Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called ""polar wander"" (see apparent polar wander ), i.e., it was assumed that the north pole location had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own ""polar wander path"".

During the late 1950s it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956, [35] and by Warren Carey in a symposium held in March 1956.

[36]The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on the bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology [37] which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals, published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley.

[38]Simultaneous advances in early seismic imaging techniques in and around Wadati–Benioff zones along the trenches bounding many continental margins, together with many other geophysical (e.g. gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins. All this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 that continental drift was feasible and the theory of plate tectonics, which was defined in a series of papers between 1965 and 1967, was born, with all its extraordinary explanatory and predictive power. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology.

Continental drift Further information: Continental drift In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what is called the geosynclinal theory.

Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time. Alfred Wegener in Greenland in the winter of 1912–13. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves —have similar shapes and seem to have once fitted together.

[39]Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid Earth made these various proposals difficult to accept. [40]The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth.

[41] This had previously been estimated by its cooling rate under the assumption that the Earth's surface radiated like a black body. [42] Those calculations had implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years.

Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.

By 1915, after having published a first article in 1912, [43] Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans. [32] In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this ( Abraham Ortelius, Antonio Snider-Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit ).

Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick.

Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology. However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift.

Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930. [44]Floating continents, paleomagnetism, and seismicity zones Global earthquake epicenters, 1963–1998. Most earthquakes occur in narrow belts that correspond to the locations of lithospheric plate boundaries.

Map of earthquakes in 2016As it was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt, the prevailing concept during the first half of the twentieth century was that there were two types of crust, named ""sial"" (continental type crust) and ""sima"" (oceanic type crust).

Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that a layer of basalt (sial) underlies the continental rocks. However, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath.

The concept that mountains had ""roots"" was confirmed by George B. Airy a hundred years later, during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by the mid-1950s, the question remained unresolved as to whether mountain roots were clenched in surrounding basalt or were floating on it like an iceberg.

During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in specific areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth.

These zones later became known as Wadati–Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States.

The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) [45] to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.

Meanwhile, debates developed around the phenomena of polar wander. Since the early debates of continental drift, scientists had discussed and used evidence that polar drift had occurred because continents seemed to have moved through different climatic zones during the past.

Furthermore, paleomagnetic data had shown that the magnetic pole had also shifted during time. Reasoning in an opposite way, the continents might have shifted and rotated, while the pole remained relatively fixed.

The first time the evidence of magnetic polar wander was used to support the movements of continents was in a paper by Keith Runcorn in 1956, [35] and successive papers by him and his students Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.

This was immediately followed by a symposium in Tasmania in March 1956. [46] In this symposium, the evidence was used in the theory of an expansion of the global crust. In this hypothesis the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth.

Certainly there is no evidence that the moon has expanded in the past 3 billion years; other work would soon show that the evidence was equally in support of continental drift on a globe with a stable radius. During the thirties up to the late fifties, works by Vening-Meinesz, Holmes, Umbgrove, and numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory.

In particular, the English geologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convection currents within the mantle might be the driving force. [47] Often, these contributions are forgotten because: At the time, continental drift was not accepted.

Some of these ideas were discussed in the context of abandoned fixistic ideas of a deforming globe without continental drift or an expanding Earth. They were published during an episode of extreme political and economic instability that hampered scientific communication. Many were published by European scientists and at first not mentioned or given little credit in the papers on sea floor spreading published by the American researchers in the 1960s.

Mid-oceanic ridge spreading and convection Further information on Mid-ocean ridge: Seafloor spreading In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution 's research vessel Atlantis and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents.

They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions. [48]The new data that had been collected on the ocean basins also showed particular characteristics regarding the bathymetry. One of the major outcomes of these datasets was that all along the globe, a system of mid-oceanic ridges was detected.

An important conclusion was that along this system, new ocean floor was being created, which led to the concept of the "" Great Global Rift "". This was described in the crucial paper of Bruce Heezen (1960), [49] which would trigger a real revolution in thinking. A profound consequence of seafloor spreading is that new crust was, and still is, being continually created along the oceanic ridges.

Therefore, Heezen advocated the so-called "" expanding Earth "" hypothesis of S. Warren Carey (see above). So, still the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?

In reality, this question had been solved already by numerous scientists during the forties and the fifties, like Arthur Holmes, Vening-Meinesz, Coates and many others: The crust in excess disappeared along what were called the oceanic trenches, where so-called ""subduction"" occurred.

Therefore, when various scientists during the early sixties started to reason on the data at their disposal regarding the ocean floor, the pieces of the theory quickly fell into place. The question particularly intrigued Harry Hammond Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U. S. Coast and Geodetic Survey who first coined the term seafloor spreading.

Dietz and Hess (the former published the same idea one year earlier in Nature, [50] but priority belongs to Hess who had already distributed an unpublished manuscript of his 1962 article by 1960) [51] were among the small handful who really understood the broad implications of sea floor spreading and how it would eventually agree with the, at that time, unconventional and unaccepted ideas of continental drift and the elegant and mobilistic models proposed by previous workers like Holmes.

In the same year, Robert R. Coats of the U. S. Geological Survey described the main features of island arc subduction in the Aleutian Islands.

His paper, though little noted (and even ridiculed) at the time, has since been called ""seminal"" and ""prescient"". In reality, it actually shows that the work by the European scientists on island arcs and mountain belts performed and published during the 1930s up until the 1950s was applied and appreciated also in the United States.

If the Earth's crust was expanding along the oceanic ridges, Hess and Dietz reasoned like Holmes and others before them, it must be shrinking elsewhere. Hess followed Heezen, suggesting that new oceanic crust continuously spreads away from the ridges in a conveyor belt–like motion.

And, using the mobilistic concepts developed before, he correctly concluded that many millions of years later, the oceanic crust eventually descends along the continental margins where oceanic trenches – very deep, narrow canyons – are formed, e.g.

along the rim of the Pacific Ocean basin. The important step Hess made was that convection currents would be the driving force in this process, arriving at the same conclusions as Holmes had decades before with the only difference that the thinning of the ocean crust was performed using Heezen's mechanism of spreading along the ridges.

Hess therefore concluded that the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is ""consumed"" in the trenches (like Holmes and others, he thought this was done by thickening of the continental lithosphere, not, as now understood, by underthrusting at a larger scale of the oceanic crust itself into the mantle), new magma rises and erupts along the spreading ridges to form new crust.

In effect, the ocean basins are perpetually being ""recycled,"" with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, the new mobilistic concepts neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Magnetic striping Seafloor magnetic striping. A demonstration of magnetic striping. (The darker the color is, the closer it is to normal polarity)Further information: Vine–Matthews–Morley hypothesis Beginning in the 1950s, scientists like Victor Vacquier, using magnetic instruments ( magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor.

This finding, though unexpected, was not entirely surprising because it was known that basalt—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral ( magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time.

As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns.

When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra -like pattern: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping, and was published by Ron G. Mason and co-workers in 1961, who did not find, though, an explanation for these data in terms of sea floor spreading, like Vine, Matthews and Morley a few years later.

[52]The discovery of magnetic striping called for an explanation. In the early 1960s scientists such as Heezen, Hess and Dietz had begun to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest (see the previous paragraph).

New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust.

This process, at first denominated the ""conveyer belt hypothesis"" and later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges. Only four years after the maps with the ""zebra pattern"" of magnetic stripes were published, the link between sea floor spreading and these patterns was correctly placed, independently by Lawrence Morley, and by Fred Vine and Drummond Matthews, in 1963, [53] now called the Vine-Matthews-Morley hypothesis.

This hypothesis linked these patterns to geomagnetic reversals and was supported by several lines of evidence: [54]the stripes are symmetrical around the crests of the mid-ocean ridges; at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;the youngest rocks at the ridge crest always have present-day (normal) polarity;stripes of rock parallel to the ridge crest alternate in magnetic polarity (normal-reversed-normal, etc.

), suggesting that they were formed during different epochs documenting the (already known from independent studies) normal and reversal episodes of the Earth's magnetic field.

By explaining both the zebra-like magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis (SFS) quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural ""tape recording"" of the history of the geomagnetic field reversals (GMFR) of the Earth's magnetic field.

Today, extensive studies are dedicated to the calibration of the normal-reversal patterns in the oceanic crust on one hand and known timescales derived from the dating of basalt layers in sedimentary sequences ( magnetostratigraphy) on the other, to arrive at estimates of past spreading rates and plate reconstructions.

Definition and refining of the theory After all these considerations, Plate Tectonics (or, as it was initially called ""New Global Tectonics"") became quickly accepted in the scientific world, and numerous papers followed that defined the concepts: In 1965, Tuzo Wilson who had been a promotor of the sea floor spreading hypothesis and continental drift from the very beginning [55] added the concept of transform faults to the model, completing the classes of fault types necessary to make the mobility of the plates on the globe work out.

[56]A symposium on continental drift was held at the Royal Society of London in 1965 which must be regarded as the official start of the acceptance of plate tectonics by the scientific community, and which abstracts are issued as Blacket, Bullard & Runcorn (1965). In this symposium, Edward Bullard and co-workers showed with a computer calculation how the continents along both sides of the Atlantic would best fit to close the ocean, which became known as the famous ""Bullard's Fit"".

In 1966 Wilson published the paper that referred to previous plate tectonic reconstructions, introducing the concept of what is now known as the "" Wilson Cycle "". [57]In 1967, at the American Geophysical Union 's meeting, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other.

[58]Two months later, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions, which marked the final acceptance by the scientific community of plate tectonics. [59]In the same year, Mc Kenzie and Parker independently presented a model similar to Morgan's using translations and rotations on a sphere to define the plate motions. [60]Plate Tectonics Revolution Main article: Plate Tectonics Revolution The Plate Tectonics Revolution was the scientific and cultural change which developed from the acceptance of the plate tectonics theory.

The event was a paradigm shift and scientific revolution. [61]Implications for biogeography Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present-day life found on different continents but having similar ancestors. [62] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora. Plate reconstruction Main article: Plate reconstruction Reconstruction is used to establish past (and future) plate configurations, helping determine the shape and make-up of ancient supercontinents and providing a basis for paleogeography.

Defining plate boundaries Current plate boundaries are defined by their seismicity. [63] Past plate boundaries within existing plates are identified from a variety of evidence, such as the presence of ophiolites that are indicative of vanished oceans. [64]Past plate motions Tectonic motion is believed to have begun around 3 to 3.5 billion years ago. [65] [66] [ why? ]Various types of quantitative and semi-quantitative information are available to constrain past plate motions. The geometric fit between continents, such as between west Africa and South America is still an important part of plate reconstruction.

Magnetic stripe patterns provide a reliable guide to relative plate motions going back into the Jurassic period. [67] The tracks of hotspots give absolute reconstructions, but these are only available back to the Cretaceous. [68] Older reconstructions rely mainly on paleomagnetic pole data, although these only constrain the latitude and rotation, but not the longitude.

Combining poles of different ages in a particular plate to produce apparent polar wander paths provides a method for comparing the motions of different plates through time.

[69] Additional evidence comes from the distribution of certain sedimentary rock types, [70] faunal provinces shown by particular fossil groups, and the position of orogenic belts. [68]Formation and break-up of continents The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents.

The supercontinent Columbia or Nuna formed during a period of 2,000 to 1,800 million years ago and broke up about 1,500 to 1,300 million years ago.

[71] The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents). The Himalayas, the world's tallest mountain range, are assumed to have been formed by the collision of two major plates.

Before uplift, they were covered by the Tethys Ocean. Current plates Main article: List of tectonic plates Depending on how they are defined, there are usually seven or eight ""major"" plates: African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian.

The latter is sometimes subdivided into the Indian and Australian plates. There are dozens of smaller plates, the seven largest of which are the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine Sea, and Scotia. The current motion of the tectonic plates is today determined by remote sensing satellite data sets, calibrated with ground station measurements.

Other celestial bodies (planets, moons)The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics.

Earth may be a borderline case, owing its tectonic activity to abundant water [72] (silica and water form a deep eutectic ).

Venus See also: Geology of Venus Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved impact craters have been utilized as a dating method to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods).

Dates derived are dominantly in the range 500 to 750 million years ago, although ages of up to 1,200 million years ago have been calculated.

This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressive thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent. One explanation for Venus's lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.

[73] [74] The Earth's crust is soaked with water, and water plays an important role in the development of shear zones. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water.

However, some researchers [ who?] remain convinced that plate tectonics is or was once active on this planet. Mars See also: Geology of Mars Mars is considerably smaller than Earth and Venus, and there is evidence for ice on its surface and in its crust.

In the 1990s, it was proposed that Martian Crustal Dichotomy was created by plate tectonic processes. [75] Scientists today disagree, and think that it was created either by upwelling within the Martian mantle that thickened the crust of the Southern Highlands and formed Tharsis [76] or by a giant impact that excavated the Northern Lowlands.

[77]Valles Marineris may be a tectonic boundary. [78]Observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999 showed patterns of magnetic striping discovered on this planet. Some scientists interpreted these as requiring plate tectonic processes, such as seafloor spreading. [79] However, their data fail a ""magnetic reversal test"", which is used to see if they were formed by flipping polarities of a global magnetic field.

[80]Icy satellites Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth.

On 8 September 2014, NASA reported finding evidence of plate tectonics on Europa, a satellite of Jupiter—the first sign of subduction activity on another world other than Earth. [81]Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens probe, which landed on Titan on January 14, 2005. [82]Exoplanets On Earth-sized planets, plate tectonics is more likely if there are oceans of water.

However, in 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths [83] [84] with one team saying that plate tectonics would be episodic or stagnant [85] and the other team saying that plate tectonics is very likely on super-earths even if the planet is dry. [72]Consideration of plate tectonics is a part of the search for extraterrestrial intelligence and extraterrestrial life.

platetectonics.com. Archived from the original on 16 June 2010. Retrieved 12 June 2010.^ ""Understanding plate motions"". USGS. Retrieved 12 June 2010.^ Grove, Timothy L.; Till, Christy B.; Krawczynski, Michael J. (8 March 2012).

""The Role of H2O in Subduction Zone Magmatism"". Annual Review of Earth and Planetary Sciences. 40 (40): 413–39. Bibcode: 2012AREPS..40..413G. doi: 10.1146/annurev-earth-042711-105310. Retrieved 14 January 2016.^ Mendia-Landa, Pedro. ""Myths and Legends on Natural Disasters: Making Sense of Our World"". Retrieved 2008-02-05.^ van Dijk 1992, van Dijk & Okkes 1991.^ Holmes, Arthur (1931).

""Radioactivity and Earth Movements"" (PDF). Transactions of the Geological Society of Glasgow. Geological Society of Glasgow: 559–606.^ Tanimoto & Lay 2000.^ Meyerhoff et al. 1996.^ Mallard, Claire; Coltice, Nicolas; Seton, Maria; Müller, R. Dietmar; Tackley, Paul J. (2016). ""Subduction controls the distribution and fragmentation of Earth's tectonic plates"". Nature. 535 (7610): 140–43.

Bibcode: 2016Natur.535..140M. doi: 10.1038/nature17992. ISSN 0028-0836.^ Spence 1987, White & Mc Kenzie 1989.^ a b Conrad & Lithgow-Bertelloni 2002.^ Spence 1987, White & Mckenzie 1989, Segev 2002.^ ""Alfred Wegener (1880–1930)"". University of California Museum of Paleontology.^ Neith, Katie (April 15, 2011). ""Caltech Researchers Use GPS Data to Model Effects of Tidal Loads on Earth's Surface"".

Caltech. Archived from the original on October 19, 2011. Retrieved August 15, 2012.^ a b Ricard, Y. (2009). ""2. Physics of Mantle Convection"". In David Bercovici; Gerald Schubert. Treatise on Geophysics: Mantle Dynamics. 7. Elsevier Science. p. 36. ISBN 978-0-444-53580-1.^ a b Glatzmaier, Gary A. (2013). Introduction to Modeling Convection in Planets and Stars: Magnetic Field, Density Stratification, Rotation.

Princeton University Press. p. 149. ISBN 978-1-4008-4890-4.^ van Dijk 1992, van Dijk & Okkes 1990.^ Moore 1973.^ Bostrom 1971.^ Scoppola et al. 2006.^ Torsvik et al. 2010.^ [""Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling"", Rowley et al, Science Advances 23 Dec 2016: Vol.

2, no. 12, e1601107, doi: 10.1126/sciadv.1601107]^ a b Wegener 1929.^ Hughes 2001a.^ Wegener 1966, Hughes 2001b.^ a b Runcorn 1956.^ Carey 1956.^ see for example the milestone paper of Lyman & Fleming 1940.^ Korgen 1995, Spiess & Kuperman 2003.^ Kious & Tilling 1996.^ Frankel 1987.^ Joly 1909.^ Thomson 1863.^ Wegener 1912.^ ""Pioneers of Plate Tectonics"".

The Geological Society. Retrieved 23 March 2018.^ Stein & Wysession 2009, p. 26^ Carey 1956; see also Quilty 2003.^ Holmes 1928; see also Holmes 1978, Frankel 1978.^ Lippsett 2001, Lippsett 2006.^ Heezen 1960.^ Dietz 1961.^ Hess 1962.^ Mason & Raff 1961, Raff & Mason 1961.^ Vine & Matthews 1963.^ See summary in Heirzler, Le Pichon & Baron 1966^ Wilson 1963.^ Wilson 1965.^ Wilson 1966.^ Morgan 1968.^ Le Pichon 1967.^ Mc Kenzie & Parker 1967.^ Casadevall, Arturo; Fang, Ferric C. (1 March 2016).

""Revolutionary Science"". m Bio. 7 (2): e00158–16. doi: 10.1128/m Bio.00158-16.^ Moss & Wilson 1998.^ Condie 1997.^ Lliboutry 2000.^ Kranendonk, V.; Martin, J.

(2011). ""Onset of Plate Tectonics"". Science. 333 (6041): 413–14. Bibcode: 2011Sci...333..413V. doi: 10.1126/science.1208766. PMID 21778389.^ Plate Tectonics May Have Begun a Billion Years After Earth's Birth Pappas, S Live Science report of PNAS research 21 Sept 2017^ Torsvik, Trond Helge.

""Reconstruction Methods"". Retrieved 18 June 2010.^ a b Torsvik 2008.^ Butler 1992.^ Scotese, C. R. (2002-04-20). ""Climate History"". Paleomap Project. Retrieved 18 June 2010.^ Zhao 2002, 2004^ a b Valencia, O'Connell & Sasselov 2007.^ Kasting 1988.^ Bortman, Henry (2004-08-26).

""Was Venus alive? ""The Signs are Probably There "" "". Astrobiology Magazine. Retrieved 2008-01-08.^ Sleep 1994.^ Zhong & Zuber 2001.^ Andrews-Hanna, Zuber & Banerdt 2008.^ Wolpert, Stuart (August 9, 2012). ""UCLA scientist discovers plate tectonics on Mars"". Yin, An. UCLA. Retrieved August 13, 2012.^ Connerney et al.

1999, Connerney et al. 2005^ Harrison 2000.^ Dyches, Preston; Brown, Dwayne; Buckley, Michael (8 September 2014). ""Scientists Find Evidence of 'Diving' Tectonic Plates on Europa"". NASA. Retrieved 8 September 2014.^ Soderblom et al. 2007.^ Valencia, Diana; O'Connell, Richard J. (2009). ""Convection scaling and subduction on Earth and super-Earths"".

Earth and Planetary Science Letters. 286 (3–4): 492–502. Bibcode: 2009E&PSL.286..492V. doi: 10.1016/j.epsl.2009.07.015.^ van Heck, H. J.; Tackley, P. J. (2011). ""Plate tectonics on super-Earths: Equally or more likely than on Earth"". Earth and Planetary Science Letters. 310 (3–4): 252–61. Bibcode: 2011E&PSL.310..252V. doi: 10.1016/j.epsl.2011.07.029.^ C. O'Neill, A. Lenardic Geological consequences of super-sized Earths Geophysical Research Letters 34: L19204 doi: 10.1029/2007GL030598^ Stern, Robert J. (July 2016). ""Is plate tectonics needed to evolve technological species on exoplanets?"". Geoscience Frontiers. 7 (4): 573–580. doi: 10.1016/j.gsf.2015.12.002. Cited books Butler, Robert F. (1992). ""Applications to paleogeography"".

Paleomagnetism: Magnetic domains to geologic terranes (PDF). Blackwell. ISBN 0-86542-070-X. Archived from the original (PDF) on 17 August 2010. Retrieved 18 June 2010. Carey, S. W. (1958). ""The tectonic approach to continental drift"". In Carey, S. W. Continental Drift – A symposium, held in March 1956.

Hobart: Univ. of Tasmania. pp. 177–363. Expanding Earth from pp. 311–49. Condie, K. C. (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 282. ISBN 978-0-7506-3386-4. Retrieved 2010-06-18. Foulger, Gillian R. (2010). Plates vs Plumes: A Geological Controversy. Wiley-Blackwell. ISBN 978-1-4051-6148-0. Frankel, H. (1987). ""The Continental Drift Debate"". In H. T. Engelhardt Jr; A. L. Caplan.

Scientific Controversies: Case Studies in the Resolution and Closure of Disputes in Science and Technology. Cambridge University Press. ISBN 978-0-521-27560-6. Hancock, Paul L.; Skinner, Brian J.; Dineley, David L. (2000). The Oxford Companion to The Earth. Oxford University Press. ISBN 0-19-854039-6. Hess, H. H. (November 1962).

""History of Ocean Basins"" (PDF). In A. E. J. Engel; Harold L. James; B. F. Leonard. Petrologic studies: a volume to honor of A. F. Buddington. Boulder, CO: Geological Society of America. pp. 599–620. Holmes, Arthur (1978). Principles of Physical Geology (3 ed.). Wiley. pp. 640–41. ISBN 0-471-07251-6. Joly, John (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History.

London: Archibald Constable. p. 36. ISBN 1-4021-3577-7. Kious, W. Jacquelyne; Tilling, Robert I. (February 2001) [1996]. ""Historical perspective"". This Dynamic Earth: the Story of Plate Tectonics (Online ed.). U. S. Geological Survey. ISBN 0-16-048220-8. Retrieved 2008-01-29. Abraham Ortelius in his work Thesaurus Geographicus... suggested that the Americas were 'torn away from Europe and Africa... by earthquakes and floods...

The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]. 'Lippsett, Laurence (2006). ""Maurice Ewing and the Lamont-Doherty Earth Observatory"". In William Theodore De Bary; Jerry Kisslinger; Tom Mathewson. Living Legacies at Columbia. Columbia University Press. pp. 277–97. ISBN 0-231-13884-9. Retrieved 2010-06-22. Little, W.; Fowler, H. W.; Coulson, J. (1990). Onions C. T., ed. The Shorter Oxford English Dictionary: on historical principles.

II (3 ed.). Clarendon Press. ISBN 978-0-19-861126-4. Lliboutry, L. (2000). Quantitative geophysics and geology. Springer. p. 480. ISBN 978-1-85233-115-3. Retrieved 2010-06-18. Mc Knight, Tom (2004). Geographica: The complete illustrated Atlas of the world. New York: Barnes and Noble Books. ISBN 0-7607-5974-X. Meissner, Rolf (2002). The Little Book of Planet Earth. New York: Copernicus Books. p. 202. ISBN 978-0-387-95258-1. Meyerhoff, Arthur Augustus; Taner, I.; Morris, A. E. L.; Agocs, W. B.; Kamen-Kaye, M.; Bhat, Mohammad I.; Smoot, N. Christian; Choi, Dong R. (1996).

Donna Meyerhoff Hull, ed. Surge tectonics: a new hypothesis of global geodynamics. Solid Earth Sciences Library. 9. Springer Netherlands. p. 348. ISBN 978-0-7923-4156-7. Moss, S. J.; Wilson, M. E. J. (1998). ""Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo"" (PDF). In Hall R; Holloway JD. Biogeography and Geological Evolution of SE Asia (PDF). Leiden, The Netherlands: Backhuys. pp. 133–63. ISBN 90-73348-97-8. Archived from the original on 2008-02-16. Oreskes, Naomi, ed. (2003).

Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview. ISBN 0-8133-4132-9. Read, Herbert Harold; Watson, Janet (1975). Introduction to Geology. New York: Halsted. pp. 13–15. ISBN 978-0-470-71165-1. OCLC 317775677. Schmidt, Victor A.; Harbert, William (1998). ""The Living Machine: Plate Tectonics"".

Planet Earth and the New Geosciences (3 ed.). p. 442. ISBN 0-7872-4296-9. Archived from the original on 2010-01-24. Retrieved 2008-01-28. Schubert, Gerald; Turcotte, Donald L.; Olson, Peter (2001). Mantle Convection in the Earth and Planets. Cambridge: Cambridge University Press. ISBN 0-521-35367-X. Stanley, Steven M. (1999). Earth System History. W. H. Freeman. pp. 211–28. ISBN 0-7167-2882-6. Stein, Seth; Wysession, Michael (2009). An Introduction to Seismology, Earthquakes, and Earth Structure.

Chichester: John Wiley & Sons. ISBN 978-1-4443-1131-0. Sverdrup, H. U., Johnson, M. W. and Fleming, R. H. (1942). The Oceans: Their physics, chemistry and general biology. Englewood Cliffs: Prentice-Hall. p. 1087. Thompson, Graham R. & Turk, Jonathan (1991). Modern Physical Geology. Saunders College Publishing. ISBN 0-03-025398-5. Torsvik, Trond Helge; Steinberger, Bernhard (December 2006). ""Fra kontinentaldrift til manteldynamikk"" [From Continental Drift to Mantle Dynamics].

Geo (in Norwegian). 8: 20–30. Archived from the original on 23 July 2011. Retrieved 22 June 2010., translation: Torsvik, Trond Helge; Steinberger, Bernhard (2008). ""From Continental Drift to Mantle Dynamics"" (PDF). In Trond Slagstad; Rolv Dahl Gråsteinen. Geology for Society for 150 years – The Legacy after Kjerulf. 12. Trondheim: Norges Geologiske Undersokelse. pp. 24–38. Archived from the original (PDF) on 2011-07-23 [Norwegian Geological Survey, Popular Science]. Turcotte, D. L.; Schubert, G. (2002). ""Plate Tectonics"". Geodynamics (2 ed.). Cambridge University Press. pp. 1–21. ISBN 0-521-66186-2. Wegener, Alfred (1929). Die Entstehung der Kontinente und Ozeane (4 ed.).

Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. ISBN 3-443-01056-3. Wegener, Alfred (1966). The origin of continents and oceans. Biram John (translator). Courier Dover. p. 246. ISBN 0-486-61708-4. Winchester, Simon (2003). Krakatoa: The Day the World Exploded: August 27, 1883. Harper Collins. ISBN 0-06-621285-5. Cited articles Andrews-Hanna, Jeffrey C.; Zuber, Maria T.; Banerdt, W. Bruce (2008).

""The Borealis basin and the origin of the martian crustal dichotomy"". Nature. 453 (7199): 1212–15. Bibcode: 2008Natur.453.1212A. doi: 10.1038/nature07011. PMID 18580944. Blacket, P. M. S.; Bullard, E.; Runcorn, S. K., eds. (1965). A Symposium on Continental Drift, held in 28 October 1965. Philosophical Transactions of the Royal Society A.

258. The Royal Society of London. p. 323. Bostrom, R. C. (31 December 1971). ""Westward displacement of the lithosphere"". Nature. 234 (5331): 536–38. Bibcode: 1971Natur.234..536B. doi: 10.1038/234536a0. Connerney, J. E. P.; Acuña, M. H.; Wasilewski, P. J.; Ness, N. F.; Rème H.; Mazelle C.; Vignes D.; Lin R. P.; Mitchell D. L.; Cloutier P. A.

(1999). ""Magnetic Lineations in the Ancient Crust of Mars"". Science. 284 (5415): 794–98. Bibcode: 1999Sci...284..794C. doi: 10.1126/science.284.5415.794. PMID 10221909. Connerney, J. E. P.; Acuña, M. H.; Ness, N. F.; Kletetschka, G.; Mitchell D. L.; Lin R. P.; Rème H. (2005).

""Tectonic implications of Mars crustal magnetism"". Proceedings of the National Academy of Sciences. 102 (42): 14970–175. Bibcode: 2005PNAS..10214970C. doi: 10.1073/pnas.0507469102. PMC 1250232. PMID 16217034. Conrad, Clinton P.; Lithgow-Bertelloni, Carolina (2002). ""How Mantle Slabs Drive Plate Tectonics"". Science. 298 (5591): 207–09. Bibcode: 2002Sci...298..207C. doi: 10.1126/science.1074161. PMID 12364804.

Archived from the original on September 20, 2009. Dietz, Robert S. (June 1961). ""Continent and Ocean Basin Evolution by Spreading of the Sea Floor"". Nature. 190 (4779): 854–57. Bibcode: 1961Natur.190..854D. doi: 10.1038/190854a0.van Dijk, Janpieter; Okkes, F. W. Mark (1990).

""The analysis of shear zones in Calabria; implications for the geodynamics of the Central Mediterranean"". Rivista Italiana di Paleontologia e Stratigrafia. 96 (2–3): 241–70.van Dijk, J. P.; Okkes, F. W. M. (1991). ""Neogene tectonostratigraphy and kinematics of Calabrian Basins: implications for the geodynamics of the Central Mediterranean"". Tectonophysics. 196: 23–60. Bibcode: 1991Tectp.196...23V. doi: 10.1016/0040-1951 (91)90288-4.van Dijk, Janpieter (1992).

""Late Neogene fore-arc basin evolution in the Calabrian Arc (Central Mediterranean). Tectonic sequence stratigraphy and dynamic geohistory. With special reference to the geology of Central Calabria"". Geologica Ultraiectina. 92: 288. Archived from the original on 2013-04-20.

Frankel, Henry (July 1978). ""Arthur Holmes and continental drift"". The British Journal for the History of Science. 11 (2): 130–50. doi: 10.1017/S0007087400016551. JSTOR 4025726. Harrison, C. G. A. (2000). ""Questions About Magnetic Lineations in the Ancient Crust of Mars"".

Science. 287 (5453): 547a. doi: 10.1126/science.287.5453.547a. Heezen, B. (1960). ""The rift in the ocean floor"". Scientific American. 203 (4): 98–110. Bibcode: 1960Sci Am.203d..98H. doi: 10.1038/scientificamerican1060-98. Heirtzler, James R.; Le Pichon, Xavier; Baron, J. Gregory (1966). ""Magnetic anomalies over the Reykjanes Ridge"". Deep-Sea Research. 13 (3): 427–32. Bibcode: 1966DSROA..13..427H. doi: 10.1016/0011-7471 (66)91078-3.

Holmes, Arthur (1928). ""Radioactivity and Earth movements"". Transactions of the Geological Society of Glasgow. 18: 559–606. Hughes, Patrick (8 February 2001). ""Alfred Wegener (1880–1930): A Geographic Jigsaw Puzzle"". On the shoulders of giants. Earth Observatory, NASA. Retrieved 2007-12-26. ... on January 6, 1912, Wegener... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography.

Hughes, Patrick (8 February 2001). ""Alfred Wegener (1880–1930): The origin of continents and oceans"".

On the Shoulders of Giants. Earth Observatory, NASA. Retrieved 2007-12-26. By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it Pangaea (all lands),... Kasting, James F. (1988). ""Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus"". Icarus. 74 (3): 472–94. Bibcode: 1988Icar...74..472K. doi: 10.1016/0019-1035 (88)90116-9. PMID 11538226.

Korgen, Ben J. (1995). ""A voice from the past: John Lyman and the plate tectonics story"" (PDF). Oceanography. The Oceanography Society. 8 (1): 19–20. doi: 10.5670/oceanog.1995.29. Archived from the original (PDF) on 2007-09-26. Lippsett, Laurence (2001). ""Maurice Ewing and the Lamont-Doherty Earth Observatory"". Living Legacies. Retrieved 2008-03-04. Lovett, Richard A (24 January 2006). ""Moon Is Dragging Continents West, Scientist Says"".

National Geographic News. Lyman, J.; Fleming, R. H. (1940). ""Composition of Seawater"". Journal of Marine Research. 3: 134–46. Mason, Ronald G.; Raff, Arthur D. (1961). ""Magnetic survey off the west coast of the United States between 32°N latitude and 42°N latitude"". Bulletin of the Geological Society of America. 72 (8): 1259–66. Bibcode: 1961GSAB...72.1259M. doi: 10.1130/0016-7606 (1961)72 [1259: MSOTWC]2.0. CO;2. ISSN 0016-7606. Mc Kenzie, D.; Parker, R. L. (1967). ""The North Pacific: an example of tectonics on a sphere"". Nature. 216 (5122): 1276–1280. Bibcode: 1967Natur.216.1276M. doi: 10.1038/2161276a0. Moore, George W. (1973). ""Westward Tidal Lag as the Driving Force of Plate Tectonics"". Geology. 1 (3): 99–100. Bibcode: 1973Geo.....1...99M. doi: 10.1130/0091-7613 (1973)1<99: WTLATD>2.0. CO;2. ISSN 0091-7613. Morgan, W. Jason (1968). ""Rises, Trenches, Great Faults, and Crustal Blocks"" (PDF). Journal of Geophysical Research. 73 (6): 1959–182. Bibcode: 1968JGR....73.1959M. doi: 10.1029/JB073i006p01959. Le Pichon, Xavier (15 June 1968). ""Sea-floor spreading and continental drift"". Journal of Geophysical Research.

73 (12): 3661–97. Bibcode: 1968JGR....73.3661L. doi: 10.1029/JB073i012p03661. Quilty, Patrick G.; Banks, Maxwell R. (2003). ""Samuel Warren Carey, 1911–2002"". Biographical memoirs. Australian Academy of Science. Archived from the original on 2010-12-21.

Retrieved 2010-06-19. This memoir was originally published in Historical Records of Australian Science (2003) 14 (3). Raff, Arthur D.; Mason, Roland G. (1961). ""Magnetic survey off the west coast of the United States between 40°N latitude and 52°N latitude"". Bulletin of the Geological Society of America. 72 (8): 1267–70. Bibcode: 1961GSAB...72.1267R. doi: 10.1130/0016-7606 (1961)72 [1267: MSOTWC]2.0.

CO;2. ISSN 0016-7606. Runcorn, S. K. (1956). ""Paleomagnetic comparisons between Europe and North America"". Proceedings, Geological Association of Canada. 8 (1088): 7785. Bibcode: 1965RSPTA.258....1R. doi: 10.1098/rsta.1965.0016. Scalera, G. & Lavecchia, G. (2006). ""Frontiers in earth sciences: new ideas and interpretation"".

Annals of Geophysics. 49 (1). doi: 10.4401/ag-4406 (inactive 2017-01-16). Scoppola, B.; Boccaletti, D.; Bevis, M.; Carminati, E.; Doglioni, C. (2006). ""The westward drift of the lithosphere: A rotational drag?"". Geological Society of America Bulletin. 118: 199–209. Bibcode: 2006GSAB..118..199S. doi: 10.1130/B25734.1. Segev, A (2002). ""Flood basalts, continental breakup and the dispersal of Gondwana: evidence for periodic migration of upwelling mantle flows (plumes)"" (PDF).

EGU Stephan Mueller Special Publication Series. 2: 171–91. doi: 10.5194/smsps-2-171-2002. Retrieved 5 August 2010. Sleep, Norman H. (1994). ""Martian plate tectonics"" (PDF). Journal of Geophysical Research. 99: 5639.

Bibcode: 1994JGR....99.5639S. doi: 10.1029/94JE00216. Soderblom, Laurence A.; Tomasko, Martin G.; Archinal, Brent A.; Becker, Tammy L.; Bushroe, Michael W.; Cook, Debbie A.; Doose, Lyn R.; Galuszka, Donna M.; Hare, Trent M.; Howington-Kraus, Elpitha; Karkoschka, Erich; Kirk, Randolph L.; Lunine, Jonathan I.; Mc Farlane, Elisabeth A.; Redding, Bonnie L.; Rizk, Bashar; Rosiek, Mark R.; See, Charles; Smith, Peter H. (2007). ""Topography and geomorphology of the Huygens landing site on Titan"". Planetary and Space Science. 55 (13): 2015–24. Bibcode: 2007P&SS...55.2015S. doi: 10.1016/j.pss.2007.04.015. Spence, William (1987). ""Slab pull and the seismotectonics of subducting lithosphere"" (PDF). Reviews of Geophysics. 25 (1): 55–69. Bibcode: 1987Rv Geo..25...55S. doi: 10.1029/RG025i001p00055. Spiess, Fred; Kuperman, William (2003). ""The Marine Physical Laboratory at Scripps"" (PDF). Oceanography. The Oceanography Society. 16 (3): 45–54. doi: 10.5670/oceanog.2003.30. Archived from the original (PDF) on 2007-09-26. Tanimoto, Toshiro; Lay, Thorne (7 November 2000). ""Mantle dynamics and seismic tomography"". Proceedings of the National Academy of Sciences. 97 (23): 12409–110. Bibcode: 2000PNAS...9712409T. doi: 10.1073/pnas.210382197. PMC 34063. PMID 11035784. Thomson, W (1863). ""On the secular cooling of the earth"". Philosophical Magazine. 4 (25): 1–14. doi: 10.1080/14786446308643410. Torsvik, Trond H.; Steinberger, Bernhard; Gurnis, Michael; Gaina, Carmen (2010). ""Plate tectonics and net lithosphere rotation over the past 150 My"" (PDF). Earth and Planetary Science Letters. 291: 106–12. Bibcode: 2010E&PSL.291..106T. doi: 10.1016/j.epsl.2009.12.055. Archived from the original (PDF) on 16 May 2011. Retrieved 18 June 2010. Valencia, Diana; O'Connell, Richard J.; Sasselov, Dimitar D (November 2007). ""Inevitability of Plate Tectonics on Super-Earths"". Astrophysical Journal Letters. 670 (1): L45–L48. ar Xiv: 0710.0699. Bibcode: 2007Ap J...670L..45V. doi: 10.1086/524012. Vine, F. J.; Matthews, D. H. (1963). ""Magnetic anomalies over oceanic ridges"". Nature. 199 (4897): 947–949. Bibcode: 1963Natur.199..947V. doi: 10.1038/199947a0. Wegener, Alfred (6 January 1912). ""Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage"" (PDF). Petermanns Geographische Mitteilungen. 63: 185–95, 253–56, 305–09. Archived from the original (PDF) on 5 July 2010. White, R.; Mc Kenzie, D. (1989). ""Magmatism at rift zones: The generation of volcanic continental margins and flood basalts"". Journal of Geophysical Research. 94: 7685–729. Bibcode: 1989JGR....94.7685W. doi: 10.1029/JB094i B06p07685. Wilson, J. T. (8 June 1963). ""Hypothesis on the Earth's behaviour"". Nature. 198 (4884): 849–65. Bibcode: 1963Natur.198..925T. doi: 10.1038/198925a0. Wilson, J. Tuzo (July 1965). ""A new class of faults and their bearing on continental drift"" (PDF). Nature. 207 (4995): 343–47. Bibcode: 1965Natur.207..343W. doi: 10.1038/207343a0. Archived from the original (PDF) on August 6, 2010. Wilson, J. Tuzo (13 August 1966). ""Did the Atlantic close and then re-open?"" (PDF). Nature. 211 (5050): 676–81. Bibcode: 1966Natur.211..676W. doi: 10.1038/211676a0. [ permanent dead link]Zhen Shao, Huang (1997). ""Speed of the Continental Plates"". The Physics Factbook. Archived from the original on 2012-02-05. Zhao, Guochun, Cawood, Peter A., Wilde, Simon A., and Sun, M. (2002). ""Review of global 2.1–1.8 Ga orogens: implications for a pre-Rodinia supercontinent"". Earth-Science Reviews. 59: 125–62. Bibcode: 2002ESRv...59..125Z. doi: 10.1016/S0012-8252 (02)00073-9. Zhao, Guochun, Sun, M., Wilde, Simon A., and Li, S. Z. (2004). ""A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup"". Earth-Science Reviews. 67: 91–123. Bibcode: 2004ESRv...67...91Z. doi: 10.1016/j.earscirev.2004.02.003. Zhong, Shijie; Zuber, Maria T. (2001). ""Degree-1 mantle convection and the crustal dichotomy on Mars"" (PDF). Earth and Planetary Science Letters. 189: 75–84. Bibcode: 2001E&PSL.189...75Z. doi: 10.1016/S0012-821X (01)00345-4. External links The Wikibook Historical Geology has a page on the topic of: Plate tectonics: overview Wikimedia Commons has media related to Plate tectonics. This Dynamic Earth: The Story of Plate Tectonics. USGS. Understanding Plate Tectonics. USGS. An explanation of tectonic forces. Example of calculations to show that Earth Rotation could be a driving force. Bird, P. (2003); An updated digital model of plate boundaries. Map of tectonic plates. MORVEL plate velocity estimates and information. C. De Mets, D. Argus, & R. Gordon. Plate Tectonics on In Our Time at the BBC. Videos Khan Academy Explanation of evidence750 million years of global tectonic activity. Movie. Multiple videos of plate tectonic movements Quartz December 31, 2015 [ show]v t e Earth [ show]v t e Tectonic plates [ show]v t e Structure of the Earth [ show]v t e Geologic principles and processes [ show]v t e Elements of nature Authority control GND: 4046317-5 NDL: 00569219Categories: Plate tectonics Geodynamics Geology theories Seismology "

A destructive margin where pre-existing crust is destroyed (subduction zones) Found at active continental margins where plates move toward each other and often associated with volcanic activity Turbidity Current Deposit sediment at the base of continental slopes which accumulate, forming graded bedding called deep-sea fans.

Plate tectonics I

"21 terms carolinewo1Plate tectonics ILearn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Like this study set?

Create a free account to save it. Create a free account Maybe later Sort Constructive Margin Line where two plates move away from eachother as new crust is formed (MOR)Destructive Margin Line where two plates meet and one is subducted as crust is destroyed (Volcanoes/Mountain Ranges form)Trailing Edge Associated with passive continental margin where plates are divergent (e.g.

Florida's east coast)Continental Shelf A flooded extension of the continent where continental margins drop below sea level Ocean Trench The surface expression of a descending plate

The fans accumulate and form the continental rise (ONLY FOUND IN PASSIVE MARGINS)Pillow Basalt During formation of oceanic crust caused by the forward motion of lava flows: As the molten material below the ocean surface cools quickly to solid, the still-liquid, hot material moving forward beneath creates protuberances in the crust called ""pillow basalts""Leading Edge Associated with active continental margin with plates are convergent.

(e.g. California's coast)Graded Bedding Formed by deposits of sediment down continental slopes by the turbidity current which eventually forms the incline called continental rise, found at passive continental margins Continental Slope Marks the seaward edge of the continental shelf -boundary between continental crust and oceanic crust Rift Valley Form at location of divergent crust activity -first stage of divergence >Molten material rises to surface, thinning the crust until it begins to separate and move outward >The activity eventually produces the ocean basin with the origin of divergence being a mid-ocean ridge Black Smoker Seamount Interactions between seawater and oceanic crust As oceanic water circulates downward into the fractured crust at the seamount, the water heats, interacting with ions of various metals, ultimately precipitating them out into dark clouds called ""black smokers""Hot Spot Result from intraplate activity associated with mantle plumes (plumes of magma rise from mantle beneath crust...ith a volcano, y'all) -may result in new land masses >ex: Hawaiian Islands formed above hot spot What are the 3 major plate boundary types and major geologic features associated with each?

Convergent: Move toward each other (destroy crust) -ocean trenches, volcanoes, mountains Divergent: Move away from each other (new crust) -Ocean ridges, rift valleys, pillow basalts, black smokers Transform: Move parallel to each other Identify 2 types of continental margins and the major geologic features associated with each Passive: Where plates move away from each other, creating new crust -Assoc'd with continental shelves where continental slope drops only slightly into point of continental rise Active: Plates move towards each other, destroying crust -Assoc'd with continental shelves where continental slope drops steeply into an ocean trench How are the following terms related?

-active continental margin -convergent plate boundary -leading plate edge All elements/descriptions of location where two plates move toward each other and crust is destroyed. How are the following terms related? -passive continental margin -divergent plate boundary -trailing plate edge All elements/descriptions of location where two plates move away from each other and new crust is formed. What are the 3 types of convergent plate boundaries Ocean-Continental Ocean-Ocean Continental-Continental What is a hot spot?

A good example would be... A location of movement of a plate over mantle plume. Columnar magma rises through center of Earth, escaping the crust, and forming new land masses.

A good example is the Hawaiian Islands. They done that thing. Ocean Ridge Characteristics Longest topographical feature on Earth's crust Through all major oceans 20% of Earth's surface Oceanic-Continental Convergence Crust is destroyed as denser oceanic slab is forced beneath continental crust. Mixing of basalt and granite generates the MOST explosive volcanic activity (called a continental volcanic arc)Advertisement Upgrade to remove ads Features Quizlet Live Quizlet Learn Diagrams Flashcards Mobile Help Sign up Help Center Honor Code Community Guidelines Students Teachers About Company Press Jobs Privacy Terms Follow us Language© 2018 Quizlet Inc. "

Convergent boundaries are the place where two tectonic plates converge (i.e. two plates move toward each other). These zones tend to be where compressional stresses are active and this results in thrust or reverse faults being common. Converging plate boundaries are of two types: Subduction boundaries occur where oceanic lithosphere is pushed beneath continental or oceanic lithospheres. Where two plates converge at an oceanic trench a subduction boundary is formed as cold oceanic lithospheres are pushed back down into the mantle.

Earthquakes: tectonic plates

"Earthquakes: tectonic plates The Earth is formed of four concentric layers that have very different physical and chemical properties: inner core, outer core, mantle and crust. The centre or inner core is the hottest part of planet Earth. A solid mass of iron and nickel this centre has temperatures up to 5,500 degrees Celsius.

With this extraordinary heat the inner core functions as engine for the planet: Surrounding the outer core is the outer core, a liquid layer of iron and nickel, extremely hot, this layer has similar temperatures to the inner core: The widest section of the Earth is the mantle with an approximate diameter of 2,900 km.

The mantle is composed of semi-molten rock called magma. The upper parts of the mantle are hard rock but deeper within the rock is softer and beginning to melt: The outer layer of the planet is known as the crust.

It is a solid rock layer which averages about 60 kilometers in thickness. Image: The structure of the Earth The crust and the upper mantle form a cold, strong layer known as the ""lithosphere"" (from the Greek for ""rocky"" sphere) floating on the inner mantle.

The lithosphere is fragmented into a dozen of huge, irregularly shaped pieces, called tectonic plates, which are in constant motion and slide over, under and past each other on top of the partly molten inner layer.

There are two kinds of plates: oceanic crust (i.e. the plates under the ocean) and continental crust. Image: Tectonic plates There are 7 large and many small moving plates. The plates have a depth of 50 miles and on average they move only a few inches a year relative to one another.

Coastlines and plate boundaries do not always align. Although the upper mantle is solid rock, it slowly flows in a convection current because of heat dissipating from the core. Convection currents in the mantle cause the plates to move several centimeters a year in different directions.

Image: The convection currents of the Earth If you look at the convection currents, you'll see that:as the hotter Mantle rises cooler material sinks;the material rises and spreads out while the crust splits and diverges;the plates converge and subduct as the material sinks. Stress is exerted the plates as they move and those around them. They may collide, sink, or pull apart as the plates scrape boundaries creating stress which results in strain setting energy free.

Subduction occurs when plates collide and one is drawn beneath another. This process can take thousands of years. The collision of plates creates mountains as rock layers are forced upward. As the plates diverge lava pushes through the mantle, cools and forms a new section of crust. Plates moving slowly along side each other create friction and intense heat as they slide resulting in volcanic activity if the rock melts.

Earthquakes can occur if the plate slips sliding away or towards each other. In some cases this is a gradual movement. Sometimes the plates lock together unable to release the energy accumulated which builds up in the rock.

When this energy elevates to the elastic limit of the rocks, they will break free causing the ground to shake. This usually occurs when two plates either ride over, or slide against each other, and the material at the edge of the tectonic plates deforms and ruptures at its weakest point. Thus the strain energy stored within the plate is released in the form of vibrations. Plate Boundaries Plate Boundaries are categorized into three types:1.

Divergent boundaries (also called spreading zones)There are places on earth where two plates are separating or spreading apart, such as at oceanic ridges. Rift valleys and faults occur when the lithosphere is under tensional stress. At spreading zones, new magma comes up from the mantle, pushing two plates apart and adding new material at their edges.

Spreading zones are usually found in oceans along with mid-ocean ridges. For example, the North American and Eurasian plates are spreading apart along the mid-Atlantic ridge. As the new material flows out of the ridge, it pushes the existing ground floor out, until it eventually sinks under another plate, which leads us into a different type of boundary.

Earthquakes with low Richter magnitudes along boundaries with normal fault motion tend to be shallow focus. These quakes can have focal depths of less than 20km. This indicates the brittle lithosphere must be thin along the diverging plate boundaries.

Image: Magma up swelling from deep within the Earth occurs when divergent plates spread apart2. Transform boundaries (also called transform faults)These are found where plates slide past one another. The San Andreas Fault is an example of a transform -fault plate boundary along the north western Mexican and California coast.

Earthquakes along transform boundaries show strike-slip motion on the faults, they form fairly straight linear patterns and tend to be shallow focus earthquakes with depths usually less than about 100 km. Richter magnitudes could be large. Image: The Fault of San Andreas As seen in the image above, the trees (they look like small dots) in the aerial view of San Andreas fault have been offset by the slipping of the plates. The North American Plate to the right and the Pacific Plate to the left.3. Convergent boundaries (also called transform faults)

Convergent boundaries are the place where two tectonic plates converge (i.e. two plates move toward each other). These zones tend to be where compressional stresses are active and this results in thrust or reverse faults being common. Converging plate boundaries are of two types: Subduction boundaries occur where oceanic lithosphere is pushed beneath continental or oceanic lithospheres. Where two plates converge at an oceanic trench a subduction boundary is formed as cold oceanic lithospheres are pushed back down into the mantle.

This happens because the oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. In this case, one plate overrides, or ""subducts"" the other, pushing it slowly downward into the mantle where it melts to form magma.

A subducted lithosphere remains cold and brittle as it descends and can fracture under compressional stress. These fractures generate earthquakes that define a zone of quakes at increasing focal depth under the overriding plate. This zone is called the Benioff Zone. Depths of up to 700km are reached in the Benioff Zone. Examples of subduction zones are found along the northwest coast of the United States, Mexico, western Canada, southern Alaska, South America, Central America, Japan, Philippines, Caribbean Islands and the Aleutian Islands.

Image: Earthquakes hazards risks Collisional boundaries where two plates of continental lithosphere collide result in fold-thrust mountain belts. The continental crust is squashed together as the plates push together and is forced upwards. This is called folding. Fold mountains are created by this process of folding. Where two continental plates converge and push towards each other fold mountains can also be formed.

This is how mountain ranges such as the Himalayas and the Alps were formed. Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km. Examples are found along the Himalayan Belt into China, along the Northern edge of the Mediterranean Sea through Black Sea and Caspian Sea into Iraq and Iran.

Image: A view of the Himalayas from Gorak Shep Convergent boundary zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active volcanoes. Image: Convergent plates collide into each other In general, where an oceanic and a continental plate collide, the denser oceanic plate will be forced under (subduction) the other. Back To Top "

The great majority of earthquakes occur at tectonic plate boundaries. The majority of great earthquakes (magnitude 8.0 or larger) are megathrust earthquakes that occur at convergent plate boundaries, also called destructive margins or subduction zones.

Earthquakes - 2001-2015

"Description This animation shows every recorded earthquake in sequence as they occurred from January 1, 2001, through December 31, 2015, at a rate of 30 days per second. The earthquake hypocenters first appear as flashes then remain as colored circles before shrinking with time so as not to obscure subsequent earthquakes.

The size of the circle represents the earthquake magnitude while the color represents its depth within the earth. At the end of the animation it will first show all quakes in this 15-year period. Next, it will show only those earthquakes greater than magnitude 6.5, the smallest earthquake size known to make a tsunami.

Finally it will only show those earthquakes with magnitudes of magnitude 8.0 or larger, the ""great"" earthquakes most likely to pose a tsunami threat when they occur under the ocean or near a coastline and when they are shallow within the earth (less than 100 km or 60 mi.

deep). This time period includes some remarkable events. Several large earthquakes caused devastating tsunamis, including 9.1 magnitude in Sumatra (26 December 2004), 8.1 magnitude in Samoa (29 September 2009), 8.8 magnitude in Chile (27 February 2010), and 9.0 magnitude off of Japan (11 March 2011). Like most earthquakes these events occurred at plate boundaries, and truly large events like these tend to occur at subduction zones where tectonic plates collide.

Other, much smaller earthquakes also occur away from plate boundaries such as those related to volcanic activity in Hawaii or those related to wastewater injection wells in Oklahoma. You can view a You Tube version of this animation here. Notable Features

For an earthquake to pose a tsunami hazard it needs to vertically move the seafloor; therefore it needs to be large (typically 8.0 or larger), under or near the ocean, and shallow within the earth (less than 100 km).

During the 15-year period covered by this animation 20 earthquakes had a magnitude of 8.0 or larger: June 23, 2001, MW = 8.4, near coast of southern Peru September 25, 2003, MW = 8.3, Hokkaido, Japan December 23, 2004, MW = 8.1, north of Macquarie Island (south of New Zealand)December 26, 2004, MW = 9.1, northern Sumatra and Andaman Islands March 28, 2005, MW = 8.6, northern Sumatra, Indonesia May 3, 2006, MW = 8.0, Tonga November 15, 2006, MW = 8.3, Kuril Islands, Russia January 13, 2007, MW = 8.1, east of Kuril Islands, Russia April 1, 2007, MW = 8.1, Solomon Islands August 15, 2007, MW = 8.0, near coast of central Peru September 12, 2007, MW = 8.4, southern Sumatra, Indonesia September 29, 2009, MW = 8.1, Samoa Islands February 27, 2010, MW = 8.8, offshore of central Chile March 11, 2011, MW = 9.1, near the east coast of Honshu, Japan April 11, 2012, MW = 8.6, off the west coast of northern Sumatra, Indonesia April 11, 2012, MW = 8.2, off the west coast of northern Sumatra, Indonesia February 6, 2013, MW = 8.0, west of Lata, Solomon Islands May 24, 2013, MW = 8.3, Sea of Okhotsk, Russia April 1, 2014, MW = 8.2, northern Chile September 16, 2015, MW = 8.3, central Chile Builder 3Details Media Preview View Interactive Sphere Directory FTP Link Categories Land: Plate Tectonics Dataset Source NOAA Pacific Tsunami Warning Center, USGS National Earthquake Information Center, NASA Blue Marble GEBCODataset Contacts Nathan Becker , Leon R. Geschwind Dataset Visualization Developers Nathan Becker , Leon R. Geschwind Keywords Natural Disasters, Earthquakes, Tsunamis, Plate Tectonics, Major Events "

ers Answers Best Answer: The great majority of seismicity on the planet occurs at plate boundaries. Where plate pull apart slide by each other or collide, there is tectonic activity manifested as earthquakes. Volcanism is associated with two of the plate boundary types: divergent and convergent margins. Where two plates containing continental crust at their margins collide, there is little or no volcanism (such as at the Himalaya).

Which plate boundaries can produce volcanoes?

Science & Mathematics Earth Sciences & Geology Which plate boundaries can produce volcanoes? Transform, Divergent, and/or Convergent? Also, what ones can produce earthquakes, which one creates sea-floor, and which one destroys sea-floor? It has to be out of the 3 choices. Thanks. Follow 2 answ

Occasionally, plate boundaries where plates are mostly sliding by each other can experience small amounts of volcanism as well if there is a component of extension across this boundary.

Darkangel · 4 years ago0 0 Comment Asker's ratingdivergent and certain convergent boundaries called subduction zones Source (s):.. Punk Rock and Minerals · 4 years ago0 0 Comment Maybe you would like to learn more about one of these? Online education an option? Explore Digital Home Security Earn your MBA online! Need Home Equity Loan Options?

All three margins are seismically active, with volcanic activity at constructive and destructive margins. Some plate margins exhibit features of more than one of the three main types and are known as combined plate margins. See PLATE TECTONICS; and SEA-FLOOR SPREADING.

plate margin

"plate margin A Dictionary of Earth Sciences © A Dictionary of Earth Sciences 1999, originally published by Oxford University Press 1999.plate margin ( plate boundary) The boundary of one of the plates that form the upper layer (the lithosphere) and together cover the surface of the Earth. Plate margins are characterized by a combination of tectonic and topographic features: oceanic ridges, Benioff zones, young fold mountains, and transform faults.

Plate margins are of three main types: ( a) constructive margins where newly created lithosphere is being added to plates which are moving apart at oceanic ridges; ( b) convergent margins which can be either destructive margins, where one plate is carried down into the mantle, beneath the bordering plate, at a subduction zone, or a collision zone, where two island arcs or continents, or an arc and a continent, are colliding; or ( c) conservative margins, where two plates are moving in opposite directions to each other along a transform fault.

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e. Passive Continental Margins - A passive continental margin occurs in the interior of plate, far away from any plate boundary. Present examples of passive continental margins are the Atlantic coast of North and South America, Europe, and Africa.

"EENS 1110Physical Geology Tulane University Prof. Stephen A. Nelson Oceans and Coasts This page last updated on 06-Nov-2015The Oceans Cover about 71% of the surface of the Earth. The greatest ocean depth of 11,035 m occurs in the Mariana Trench Have an average depth of 3,800 m. Have a present volume of about 1.35 billion cubic kilometers, but the volume fluctuates with the growth and melting of glacial ice.

Help to regulate transfer of mass and energy between biosphere, lithosphere and atmosphere.40 to 50% of the world's population live within 100 km of a coast. The Oceans exist because of differences in lithosphere as reflected by isostasy. Continental lithosphere “floats higher” on the asthenosphere because the embedded continental crust has a lower density.

Oceanic lithosphere “floats deeper” in the asthenosphere because is it is denser. The ocean basins collect water because they are “lower.”The Ocean Floor The ocean floor was very much unknown until the late 1800s when the first scientific expeditions were undertaken.

Our knowledge greatly expanded during and after World War II. Bathymetry was mapped, and oceanic ridges and trenches were discovered, This was accomplished through sonar soundings of ocean depth and submarine exploration of the deep oceans.

Later drilling of the sea floor for the collection of samples was undertaken. It was a better understanding of the ocean floor which led to the theory of Plate Tectonics. The most important bathymetric features of the sea floor are Continental Shelf, Slope, and Rise Abyssal Plains Oceanic ridges Oceanic Trenches The bathymetry of the sea floor reflects tectonics.

The Continental shelf is underlain by thinning continental crust. The continental slope and rise are transitional between crustal types, and the abyssal plain is underlain by mafic oceanic crust. Oceanic ridges are diverging plate boundaries where new oceanic lithosphere is formed and oceanic trenches are converging plate boundaries where oceanic lithosphere is subducted. Because oceanic lithosphere may get subducted, the age of the ocean basins is relatively young. The oldest oceanic crust occurs farthest away from a ridge.

In the Atlantic Ocean, the oldest oceanic crust occurs next to the North American and African continents and is Jurassic in age (see figure 4.9 in your text).

In the Pacific Ocean, the oldest crust is also Jurassic in age, and occurs off the coast of Japan. Because the oceanic ridges are areas of young crust, there is very little sediment accumulation on the ridges. Sediment thickness increases in both directions away of the ridge, and is thickest where the oceanic crust is the oldest.

Sediment on the abyssal plain is mainly fined grained sediment (clay size) that was input into the oceans by streams and winds from the continents. The accumulation of the remains of silica secreting planktonic organisms like radiolaria and diatoms, has produced chert and the accumulation of the remains of foraminifera has produced biogenic limestones.

Besides the oceanic ridges, oceanic islands and seamounts occur in the ocean basins. These are mostly volcanic islands that were formed above hot spots. The volcanoes formed over the hot spot and after the volcano goes extinct, it is eroded to sea level. Continued cooling and subsidence submerges the island to form seamounts (also called guyots). Examples include: the Hawaiian Islands and the Emperor Seamount chain and others in both the Atlantic and Pacific Oceans. Where the continents meet the oceans, the continental margins are characterized as being of one of two types: Passive or Activ

No current deformation is taking place along these margins because they are not close to plate boundaries. The passive continental margins developed as a result of rifting of a former larger continent. A passive continental margin is characterized by a broad continental shelf overlying thinning continental lithosphere. The shelf is made of relatively shallow oceanic sediments that have been shed by the continents. Active Continental Margins- Continental convergent margins occur where the margin of the continent coincides with a convergent plate boundary.

E xamples of a current active continental margins occur along the Pacific coast of South America and in the Cascade Mountains of the western U. S. An active continental margin is characterized by a narrow continental shelf, again composed of sediments shed from the continents. Submarine canyons crosscut the continental shelves. These are associated with large rivers from the continents. Erosion carved the canyons during times when sea-level was lower than at present. The submerged canyons funnel sediments to deeper water producing submarine fans where the canyons empty onto the continental rise.

Ocean Water Salinity, a measure of amount of dissolved ions in the oceans, ranges between 33 and 37 parts per thousand. The dissolved ions have been concentrated in seawater as a result of chemical weathering (Na, Ca, Mg, S, K, Br, and HCO 3) and degassing of the mantle by volcanic activity (Cl & S).

Seawater would contain higher concentrations of dissolved ions if some were not removed by chemical precipitation, plants and animals, and absorption onto clay minerals. Salinity varies in the oceans (see figure 18.9 in your text) because surface waters evaporate, rain and stream water is added, and ice forms or thaws. Salinity is higher in mid-latitude oceans because evaporation exceeds precipitation Salinity is higher in restricted areas of the oceans like the Mediterranean and Red Seas (up to 41 parts per thousand). Salinity is lower near the equator because precipitation is higher. Salinity is low near the mouths of major rivers because of input of fresh water.

The temperature of surface seawater varies with latitude, from near 0 o C near the poles to 29 o C near the equator.

But restricted areas can have temperatures up to 37 o C. (See figure 18.9 in your text. )Properties of seawater also vary with depth. The density and salinity of seawater increase with depth. Temperature decreases with depth. Ocean Circulation Oceanic circulation is three dimensional.

Most visible to humans are the surface ocean currents that are mainly driven by the wind. Vertical currents and deep ocean currents are driven by upwelling and downwelling near the coasts and differences in density, temperature and salinity between the surface waters and the deep ocean waters. Surface Ocean currents are result of drift of the upper 50 to 100 m of the ocean due to drag by wind. Thus, surface ocean currents generally follow the same patterns as atmospheric circulation with the exception that atmospheric currents continue over the land surface while ocean currents are deflected by the land.

The surface currents have the following properties: Because of the Coriolis effect, circulation is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. In each hemisphere cooler waters from higher latitudes circulate toward the equator where they are warmed and circulate back toward the poles.

As surface waters approach the coast, they have to push the water down in order to make room for more water to come in.

This results in downwelling currents. If surface waters move away from the coast, water from below rises to replace the water removed, resulting in upwelling. In addition to surface circulation, seawater also circulates vertically as a result of changes in density controlled by changing salinity and temperature (see figures 18.12 in your text).

Such circulation, because it controlled by both temperature differences and differences in salinity of the water, is called thermohaline circulation. Ocean Tides Tides are due to the gravitational attraction of moon and to a lesser extent, the sun on the Earth. Because the moon is closer to the Earth than the sun, it has a larger effect and causes the Earth to bulge toward the moon.

At the same time, a bulge occurs on the opposite side of the Earth due to inertial forces (this is explained well in the book on pages 670-671, if you are interested).

These bulges remain stationary while Earth rotates. The tidal bulges result in a rhythmic rise and fall of ocean surface, which is not noticeable to someone on a boat at sea, but is magnified along the coasts. Usually there are two high tides and two low tides each day, and thus a variation in sea level as the tidal bulge passes through each point on the Earth's surface.

Along most coasts the range is about 2 m, but in narrow inlets tidal currents can be strong and fast and cause variations in sea level up to 16 m. Because the Sun also exerts a gravitational attraction on the Earth, there are also monthly tidal cycles that are controlled by the relative position of the sun and moon.

The highest high tides occur when the Sun and the moon are on the same side of the Earth (new moon) or on opposite sides of the Earth (full moon).

These are called Spring Tides. The lowest high tides occur when the Sun and the moon are not opposed relative to the Earth (quarter moons). These highest high tides become important to coastal areas during hurricane season and you always hear dire predications of what might happen if the storm surge created by the hurricane arrives at the same time as the highest high tides.

Ocean Waves Waves are generated by winds that blow over the surface of oceans. Wave height, length, and period depend on wind speed, wind duration, and distance of travel (fetch). In a wave, water travels in circular loops. But since the surface is the area affected, the diameter of the loops decreases with depth. The Diameters of loops at the surface is equal to wave height (h). (See animation at http://www.youtube.com/watch?v=7y PTa8qi5X8)Wavelength (L) = distance to complete one cycle Wave Period (P) = time required to complete on cycle.

Wave Velocity (V) = wavelength/wave period (L/P). Wave Base Motion of waves is only effective at moving water to depth equal to one half of the Wavelength (L/2).

Water deeper than L/2 does not move. Thus, waves cannot erode the bottom or move sediment in water deeper than L/2. This depth is called wave base. In the Pacific Ocean, wavelengths up to 600 m have been observed, thus water deeper than 300m will not feel passage of wave.

But outer parts of continental shelves average 200 m depth, so considerable erosion can take place out to the edge of the continental shelf with such long wavelength waves. When waves approach shore, the water depth decreases and the wave will start feeling bottom. Because of friction, the wave velocity (= L/P) decreases, but its period (P) remains the same Thus, the wavelength (L) will decrease.

Furthermore, as the wave ""feels the bottom"", the circular loops of water motion change to elliptical shapes, as loops are deformed by the bottom. As the wavelength (L) shortens, the wave height (h) increases. Eventually the steep front portion of wave cannot support the water as the rear part moves over, and the wave breaks.

This results in turbulent water of the surf, where incoming waves meet back flowing water. Waves that crash onto the beach are called breakers. Wave energy is dissipated by turbulence, which creates frothy white water in the surf zone. A surge of water (swash) rushes up the beach face.

Gravity pulls the backwash down the slope of the beach. (See video at http://www.youtube.com/watch?v=2_N7C9vdp V8)Rip currents form where water is channeled back into the ocean. Wave Refraction Waves generally do not approach shoreline parallel to shore. Instead some parts of waves feel the bottom before other parts, resulting in wave refraction or bending. Wave energy can thus be concentrated on headlands, to form cliffs. Headlands erode faster than bays because the wave energy gets concentrated at headlands Wave Erosion Rigorous erosion of sea floor takes place in surf zone, i.e.

between shoreline and breakers. Waves break at depths between 1 and 1.5 times wave height. Thus for 6m tall waves, rigorous erosion of sea floor can take place in up to 9 m of water. Waves can also erode by abrasion and flinging rock particles against one another or against rocks along the coastline. Coastal Sediment Transport Coastlines are zones along which water is continually making changes. Waves can both erode rock & deposit sediment. Because of the continuous nature of ocean currents & waves, energy is constantly being expended along coastlines & they are thus dynamically changing systems, even over short (human) time scales.

Longshore currents - Since most waves arrive at the shoreline at an angle even after refraction. Such waves have a velocity oriented in the direction perpendicular to the wave crests (V w ), but this velocity can be resolved into a component perpendicular to the shore (Vp) and a component parallel to the shore (V L ).

The component parallel to the shore can move sediment and is called the longshore current. Beach drift - is due to waves approaching at angles to beach, but retreating perpendicular to the shore line.

This results in the swash of the incoming wave moving the sand up the beach in a direction perpendicular to the incoming wave crests and the backwash moving the sand down the beach perpendicular to the shoreline.

Thus, with successive waves, the sand will move along a zigzag path along the beach. Offshore Transport and Sorting Particles picked up by wave motion move down slope, but the deeper the water, the less energy is involved in wave motion, so smaller and smaller particles are moved farther off shore. This results in size sorting of sediment, with grain size decreasing away from coast. (see video at http://www.youtube.com/watch?v=2_N7C9vdp V8 ). Coastal Deposits and Landforms Coastlines represent a balance between wave energy and sediment supply.

If wave energy and sediment supply are constant, then a steady state is reached. If any one of these factors change, then shoreline will adjust. For example, winter storms may increase wave energy, if sediment supply is constant, fine grained beach sand may be carried offshore resulting in pebble beaches or cobble beaches.

Due to input of sediment from rivers, marine deltas may form, due to beach and longshore drift such features as spits, bay barriers, and tombolos may form. Coast represents the boundary between sea and land. When waves hit the coast, they can erode by breaking up rocks into finer particles and abrading other rocks by flinging rocks, sand and water against them.

Over time, the effects can be large. Sediment is moved and redeposited to increase the size of continental shelves. Beaches occur where sand is deposited along the shoreline. A beach can be divided into a foreshore zone, which is equivalent to the swash zone, and backshore zone, which is commonly separated from the foreshore by a distinct ridge, called a berm .

Behind the backshore may be a zone of cliffs, marshes, or sand dunes. Sandy beaches are the most common. The sand tends to be very well sorted and well rounded. The sorting is a reflection of the wave energy reaching the beach - the waves are energetic enough to carry away the finer grained sediment, but not energetic enough to carry in the coarser grained sediment.

Beach deposits usually show cross-bedding. Sediment compositions reflect the local geology. Quartz is the most common, although calcite is common where the coastal rocks are made of limestone or where there are offshore carbonate reefs.

In other environments, only minerals that are resistant to chemical weathering occur. In temperate climates, energy reaching coast is seasonal. Winter storms bring high energy – wash sand offshore, forming a narrow gravel beach. In the summer, less energy reaches the beach, sand returns, and creates a wider beach (See figure 18.20 in your text). Rocky Coasts - Where wave action has not had time to lower the coastline to sea level, a rocky coast may occur.

Because of the resistance to erosion, a wave cut bench and wave cut cliff develops. If subsequent uplift of the wave-cut bench occurs, it may be preserved above sea level as a marine terrace. The cliff may retreat by undercutting and resulting mass-wasting processes. In areas where differential erosion takes place, the undercutting may initially produces sea caves. If sea caves from opposite sides of a rocky headland meet, then a sea arch may form. Eventual weakening of the sea arch may result in its collapse to form a sea stack .

Because cliffed shorelines are continually attacked by the erosive & undercutting action of waves, they are susceptible to frequent mass-wasting processes which make the tops of these cliffs unstable areas for construction. Depositional Features along coasts. Deltas -- Deltas form where sediment supply is greater than ability of waves to remove sediment. An example is the Mississippi River Delta, which is composed of several lobes that were deposited within the last several thousand years.

Erosion of the older delta lobes has taken place due to subsidence, sea level rise, and lack of new sediment being supplied to the delta because of the human-made levee system. Spits - elongated deposits of sand or gravel that projects from the land into open water. Spits usually form at the mouth of a bay due to long shore current and beach drift.

Generally they curve inward towards the bay due to refraction of the waves around the mouth of the bay. Bay Mouth Bars - if a spit extends across a bay, it is called a bay mouth bar. Exchange of water between the bay and the ocean is accomplished through the groundwater system.

Tombolos - a spit that connects the mainland to an offshore island is called a tombolo. Barrier Islands - A barrier island is a long narrow ridge of sand just offshore running parallel to the coast. Separating the island and coast is a narrow channel of water called a lagoon. Most barrier islands were built during after the last glaciation as a result of sea level rise.

Barrier islands are constantly changing. They grow parallel to the coast by beach drift and longshore drift, and they are eroded by storm surges that often cut them into smaller islands.

Reefs and Atolls - Reefs consist of colonies of organisms, like corals, which secrete calcium carbonate. Since these organisms can only live in warm waters and need sunlight to survive, reefs only form in shallow tropical seas. In the deeper oceans reefs can build up on the margins of volcanic islands, but only do so after the volcanoes have become extinct. After the volcanism ceases, the volcanic island begins to erode and also begins to subside, due to the weight of newly added material.

As the island subsides, the reefs continue to grow upward. Eventually, the original volcanic island subsides and is eroded below sea level. But, the reefs trap sediment and a circular or annular island, called an atoll, forms (see figures 18.27 in your text). Estuaries - River valleys flooded by marine water are estuaries. They are characterized by mixing of fresh and salt water.

Modern estuaries are related to glaciation. During glacial periods, when sea level was lower, rivers carved canyons along the coasts. When the glacial ice melted and returned water to the oceans, the resulting rise in sea-level flooded these canyons.

Tidal Flats - Form in intertidal zones lacking strong waves. They are common behind barrier islands or in estuaries. The sediment consists of thinly laminated sand and mud often showing ripple marks. Fjords - Flooded U-shaped valleys carved by glaciers. They form spectacular bedrock-bounded troughs. Notable examples are found in Norway. British Columbia, and New Zealand.

Coastal Variability The shape of coast is controlled mainly by tectonic forces and climate, both of which act to determine the elevation of the coast. Plate tectonic setting governs the style of coastline. In general, along passive margins,broad low lying coastal plains dominate.

Along active margins, uplifted rocky coasts dominate. Sea level changes can be local or global. Rise of fall of local sea level occurs as a result of tectonic forces or isostatic forces. Global sea level changes, however, are referred to as eustatic sea level changes. Eustatic sea level change is controlled by: The changes in the volume of water in the oceans, which can occur as a result of changing climate.

During cold periods, called glacial periods, much water is tied up in glacial ice on the continents and sea level falls. During warm periods, called interglacial periods, the glacial ice melts and returns to the oceans, thus raising sea level.

Warmer climates also mean warmer water in the oceans. Since the volume of water is higher at higher temperatures, this also contributes to a rise in sea level. Changing the shape of the oceans can also result in eustatic sea level rise. This might occur if volcanic output on the sea floor or at oceanic ridges increases substantially, thus raising the floor of the oceans.

Emergent coasts result from local tectonic or isostatic uplift or from a drop in eustatic sea level. Emergent coasts are characterized by rocky coasts with sea cliffs and raised wave cut benches (marine terraces).

The west coast of the U. S. is a an emergent coast and in this case is due to a recent episode of uplift of the land relative to the sea by tectonic forces.

The coast of New England is also an emergent case, but in the case, the rise of the land surface has been due to removal of glacial ice which had depressed the land during the last glaciation.

Upon removal of the ice by melting at the end of the last glaciation, the land has been rising. Submergent coasts result from either subsidence along the coast due to tectonic forces or eustatic sea level rise. These are characterized by gentle shorelines, flooded river valleys (estuaries and fjords), and barrier islands. Much of the East Coast and Gulf Coast of the U. S. has these characteristics which are attributed to the rise of eustatic sea level since last glaciation.

Coastal Problems Sea Level Rise - Sea level is presently rising and the rate of sea level rise may increase due to to melting of continental ice sheets like now cover Greenland and Antarctica.

Human habitation of low-lying coastlines may be in jeopardy in the near future. Storms - great storms such as hurricanes or other winter storms can cause erosion of the coastline at much higher rate than normal. During such storms beaches can erode rapidly and heavy wave action can cause rapid undercutting and mass-wasting events of cliffs along the coast.

Tsunami - a tsunami is a giant sea wave generated by an earthquake. Such waves travel at speeds up to 950 km/hr, have wavelengths up to 200 m, and upon approaching a shallow coastline, can have wave heights up to 30 m. Such waves have great potential to wipe out cities located along coastlines. Landslides - on coasts with cliffs, the main erosive force of the waves is concentrated at the base of the cliffs.

As the waves undercut the cliffs, they may become unstable and mass-wasting processes like landslides will result.

Massive landslides can also generate tsunami. Pollution - includes garbage, especially plastic containers, sewage, agricultural runoff rich in nitrates, and oil spills, all of which affect the natural habitats that include the base of the food chain, and aesthetics of the coast, and the protection from storms that the coastal environment offers. Protection from Shoreline Erosion Sea cliffs, since they are susceptible to landslides due to undercutting, and barrier islands and beaches, since they are made of unconsolidated sand and gravel, are difficult to protect from the action of the waves.

Human construction can attempt to prevent erosion, but human engineering cannot always protect against abnormal conditions. Humans construct such things as sea walls, breakwaters, and groins in an attempt to slow coastal erosion, but sometimes other problems are caused by these engineering feats.

For example, construction of groin (a wall built perpendicular to the shoreline) can trap sand and prevent beach drift and longshore drift from supplying sand to areas down current along the coastline.

These down current areas may then erode, causing other problems. Examples of questions on this material that could be asked on an exam. Define the following: (a) continental shelf, (b) continental slope, (c) abyssal plain, (d) passive continental margin, (e) active continental margin, (f) salinity, (g) wave base, (h) wave refraction, (i) spit, (j) tombolo, (k) fjord, (l) atoll, (m) estuary.

What are the main factors responsible for oceanic circulation? Explain how longshore current and beach drift operates along a coast. What are the main characteristics of beaches in terms of sediment type, sediment composition, and sedimentary structures? How does wave energy affect these? Describe how the following features of rocky coasts form (a) wave cut benches, (b) wave-cut cliffs, (c) marine terraces, (d) sea caves, (e) sea arches, (f) sea stacks. Why are waves effective as erosional agents? What can cause eustatic sea level changes? What are emergent coasts and submergent coasts, what causes each to form, and what are the characteristics of each? Discuss the problems that affect coastal environments. Return to EENS 1110 Page "

ca. Active Margins occur where oceanic lithosphere is being subducted beneath the edge of the continent. the result is a relatively narrow margin that consists of highly deformed sediment that was scraped from the decending lithospheric slab and plastered against the margin of the overriding continent.

Earth Science Ch. 13

22 terms scassidy1Earth Science Ch. 13Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Like this study set?

Create a free account to save it. Create a free account Maybe later Sort Oceanography the science that draws on the methods and knowledge of geology, chemistry, physics and biology to study all aspects of the world ocean.

Bathmetry the measurement of oceans depth and charting of the shape or topography of the ocean floor,.continental margins two kinds, passive and active. Passive Margins found along most of the coastal areas surrounding the atlantic ocean, includes the east coast of north and south america as well as the coastal areas of western europe and afri

Continental shelf a gently sloping submerged surface extending from a shoreline toward the deep ocean basin. Contintental slope relatively steep zone that is compared with the shelf that marks the boundary between continental crust and ocean crust.

Continental Rise where the slope drops to about 1/3rd degree or 6 meters per kilometer. or 32 feet per mile. Deep sea fan a comb shaped deposit at the base of the continental slope, caused by turbitity currents that transports sediment. Submarine canyons are cut into the continental slope that may extend across the entire continental rise to the deep ocean basin.

Turbitity currents are downsloping movements of dense sediment laden water. Deep ocean trenches long relatively narrow troughs that are the deepest part of the ocean. Volcanic island arc Trenches often paralleled by an arc shaped row of active volcanoes. Continental arc. those making up portions of the andes and cascades mountains, located parallel trenches and lie adjacent to trenches. Deep ocean basin between the continental margin and the oceanic ridge lies the deep ocean basin. The size of this region is almost 30% of the earth's surface.

Roughly comparable to the percentage of land currently above sea level. Abyssal Plains deep incredibly flat features, infact the these regions are likely the most level places on earth.

they are found off the coast of argentina. Sea mounts these may rise hundreds of meters above the surrounding topograph. It is estimated that over a million exist. Middle ocean ridge Same thing as oceanic ridge. Long well developed divergent plate boundary, sea floor is elevated forming a broad line swell called the ocean ridge. knowledge of the ridge comes from soundings of the ocean floor and floor samples from deep sea drilling, visual inspections from deep submersibles.

And even first hand inspection of peices of ocean floor that have been thrust on dry land. Rift Valleys Along the axis of some segments of the ocean ridge system are deep down faulted structures because of their striking similarity to the rift valleys found in east africa.

Terrigenous Land to produce biogenious life. Hydrogenous Water to produce biogenous life. Gas Hydrates Usually compact chemical structures made of water and natural gas. Advertisement Upgrade to remove ads Features Quizlet Live Quizlet Learn Diagrams Flashcards Mobile Help Sign up Help Center Honor Code Community Guidelines Students Teachers About Company Press Jobs Privacy Terms Follow us Language© 2018 Quizlet Inc.

In plate tectonics, a convergent boundary, also known as a destructive plate boundary, is a region of active deformation where two or more tectonic plates or fragments of the lithosphere are near the end of their life cycle.

Convergent boundary

From Wikipedia, the free encyclopedia (Redirected from Convergent Plate Boundary)navigation search This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2016) ( Learn how and when to remove this template message)

This is in contrast to a constructive plate boundary (also known as a mid-ocean ridge or spreading center). As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near destructive boundaries, where subduction zones or an area of continental collision (depending on the nature of the plates involved) occurs. The subducting plate in a subduction zone is normally oceanic crust, and moves beneath the other plate, which can be made of either oceanic or continental crust.

During collisions between two continental plates, large mountain ranges, such as the Himalayas are formed.

In other regions, a divergent boundary or transform faults may be present. Oceanic-continental Continental-continental Oceanic-oceanic Contents [ hide ]1 Mid-ocean ridges2 Subduction zones3 Volcanic arcs4 Back arc basins5 Oceanic trenches6 Examples7 See also8 References Mid-ocean ridges [ edit]See also: Earth's internal heat budget The reason for the existence of convergent boundaries are the mid-ocean ridges that were only beginning to be understood with the acceptance of the theory of plate tectonics in the mid-1960s.

The radioactive decay of elements within the rocks in the interior of the earth generates large amounts of heat that moves magma to the surface of the crust in a process called mantle convection. This results in new oceanic crust or sea floor being formed that slowly moves away from the ridges (sometimes called spreading centers) towards the subduction zones.

In other words, new sea floor originates from the upwelling rock as it emerges from the depths of the mantle and it slowly cools as it is transported over the millennia to the collision or subduction zones where it is consumed or recycled. [1]Subduction zones [ edit]See also: Forearc and Wadati–Benioff zone Large, shallow earthquakes on subduction zone interfaces are the largest types of earthquakes. They accounted for 90% of the total seismic moment released for the period 1900–1989, but these events usually occur offshore and far from population centers.

Non-subduction interface earthquakes also occur in the region of the trench including shallow upper plate shocks, deeper events within the downgoing slab, and events that are related to outer trench swell. [2]Volcanic arcs [ edit]A volcanic arc is formed on the continental plate, above the location of the downgoing oceanic slab. The volcanic arc is the surface expression of the magma that is generated by hydrous melting of the mantle above the downgoing slab.

Hydrated minerals (e.g., phlogopite, lawsonite, amphibole) within the oceanic lithosphere become unstable at certain depths due to increased temperature and pressure, causing the crystal structure of the hydrated minerals break down and release water. The buoyant fluids then rise into the asthenosphere, where they lower the melting temperature of the mantle and cause partial melting. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases.

When two plates with oceanic crust converge, they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it.

The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife).

A deep oceanic trench is located in front of such arcs where the descending slab dips downward, such as the Mariana Trench near the Mariana Islands. Back arc basins [ edit]See also: Back-arc region Some convergent margins have zones of active seafloor spreading behind the island arc, known as back-arc basins. When one plate is composed of oceanic lithosphere and the other is composed of continental lithosphere, the denser oceanic plate is subducted, often forming an orogenic belt and associated mountain range.

This type of convergent boundary is similar to the Andes or the Cascade Range in North America. Oceanic trenches [ edit]An oceanic trench is found where the denser plate is subducted underneath the other plate. There is water in the rocks of the oceanic plate (because they are underwater), and as this plate moves further down into the subduction zone, much of the water contained in the plate is squeezed out when the plate begins to subduct.

However, the recrystallization of ocean floor rocks, such as serpentinite, which are unstable in the upper mantle, recrystallize into olivine, causing dehydration through loss of hydroxyl groups. This addition of water to the mantle causes partial melting of the mantle, generating magma, which then rises, and which normally results in volcanoes. This normally happens at a certain depth, about 70 to 80 miles below the Earth's surface, and so volcanoes are formed fairly close to, but not right next to, the trench.

Examples [ edit]The collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.

The collision between the Australian Plate and the Pacific Plate that formed the Southern Alps in New Zealand Subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands. Subduction of the Nazca Plate beneath the South American Plate to form the Andes. Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate, forming the complex New Zealand to New Guinea subduction/transform boundaries.

Collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey. Mariana Trench Subduction of the Juan de Fuca Plate beneath the North American Plate to form the Cascade Range. See also [ edit]List of tectonic plates List of tectonic plate interactions References [ edit]^ Bolt, B.

(2005), Earthquakes: 2006 Centennial Update – The 1906 Big One (Fifth ed. ), W. H. Freeman and Company, pp. 146–150, ISBN 978-0716775485^ Yeats, R. S.; Sieh, K. E.; Allen, C. R. (1997). The Geology of Earthquakes.

Oxford University Press. pp. 371–378. ISBN 978-0-19-507827-5. [ hide]v t e Physical oceanography Waves Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Iribarren number Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunamimegatsunami Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow waterone-dimensional Saint-Venant equations shallow water equations Wind wavemodel Circulation Atmospheric circulation Baroclinity Boundary current Coriolis force Coriolis–Stokes force Craik–Leibovich vortex force Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño–Southern Oscillation General circulation model Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Modular Ocean Model Ocean dynamics Ocean gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulationshutdown Upwelling Whirlpool World Ocean Circulation Experiment Tides Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline Landforms Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano Plate tectonics Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc Ocean zones Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash Sea level Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level rise World Geodetic System Acoustics Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics Satellites Jason-1 Jason-2 (Ocean Surface Topography Mission) Jason-3Related Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas Category Commons Categories: Plate tectonics

n. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquake s and volcano es. Oceanic crust created by seafloor spreading in the East Pacific Rise, for instance, may become part of the Ring of Fire, the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific ocean basin.

seafloor spreading

Encyclopedic Entry Vocabulary Earth's newest crust is created at sites of seafloor spreading—red sites on this map. Map courtesy NOAATriple Junctions Seafloor spreading and rift valleys are common features at “triple junctions.” Triple junctions are the intersection of three divergent plate boundaries. The triple junction is the central point where three cracks (boundaries) split off at about 120° angles from each other. In the Afar Triple Junction, the African, Somali, and Arabian plates are splitting from each other.

The Great Rift Valley and Red Sea (a major site of seafloor spreading) are the result of plate tectonics in the Afar Triple Junction.

Seafloor spreading is a geologic process in which tectonic plate s—large slabs of Earth's lithosphere —split apart from each other. Seafloor spreading and other tectonic activity processes are the result of mantle convection. Mantle convection is the slow, churn ing motion of Earth’s mantle.

Convection current s carry heat from the lower mantle and core to the lithosphere. Convection currents also “recycle” lithospheric materials back to the mantle. Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense.

The less-dense material rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks. Hot magma fueled by mantle convection bubbles up to fill these fracture s and spills onto the crust.

This bubbled-up magma is cooled by frigid seawater to form igneous rock. This rock ( basalt) becomes a new part of Earth’s crust. Mid-Ocean Ridges Seafloor spreading occurs along mid-ocean ridge s—large mountain range s rising from the ocean floor.

The Mid-Atlantic Ridge, for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate. The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate. The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate.

Seafloor spreading is not consistent at all mid-ocean ridges. Slowly spreading ridges are the sites of tall, narrow underwater cliff s and mountains. Rapidly spreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading center.

It spreads 2-5 centimeters (.8-2 inches) every year and forms an ocean trench about the size of the Grand Canyon. The East Pacific Rise, on the other hand, is a fast spreading center. It spreads about 6-16 centimeters (3-6 inches) every year.

There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop!The newest, thinnest crust on Earth is located near the center of mid-ocean ridge—the actual site of seafloor spreading. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. Geomagnetic Reversals The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century.

Basalt, the once- molten rock that makes up most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometer s to measure the magnetism of the ocean floor in the 1950s. What they discovered was that the magnetism of the ocean floor around mid-ocean ridges was divided into matching “stripes” on either side of the ridge. The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma is cooling.

Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern. Geographic Features Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, more dense, and more thick. Eventually, older oceanic crust encounters a tectonic boundary with continental crust. In some cases, oceanic crust encounters an active plate margi

In other cases, oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transition s from oceanic lithosphere to continental lithosphere.

Passive margins are not sites of fault s or subduction zone s. Thick layers of sediment overlay the transitional crust of a passive margin.

The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate (on the east coast of North America) or the Eurasian plate (on the west coast of Europe). New geographic features can be created through seafloor spreading.

. Ocean‐continent convergence occurs when oceanic crust is subducted under continental crust. This forms an active continental margin between the subduction zone and the edge of the continent. The leading edge of the continental plate is usually studded with steep andesitic mountain ranges.

Types of Plate Boundaries

Types of Plate Boundaries Convergent boundaries. Plates may converge directly or at an angle.

Three types of convergent boundaries are recognized: continent‐continent, ocean‐continent, and ocean‐ocean. Continent‐continent convergence results when two continents collide. The continents were separated at one time by oceanic crust that was progressively subducted under one of the continents.

The continent overlying the subduction zone will develop a magmatic arc until the ocean floor becomes so narrow that the continents collide. Because the continents are less dense than the oceanic crust, they will not be pulled down the subduction zone.

One continent may override the other for a short distance, but the two continents eventually become welded together along a geologically complex suture zone that represents the original line of collision.

The crust is thickened along the suture zone, resulting in isostatic uplift, mountain‐building, and thrust faulting

Some convergent margins have zones of active seafloor spreading behind the island arc, known as back-arc basins. When one plate is composed of oceanic lithosphere and the other is composed of continental lithosphere, the denser oceanic plate is subducted, often forming an orogenic belt and associated mountain range.

Convergent boundary

From Wikipedia, the free encyclopedianavigation search This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2016) ( Learn how and when to remove this template message)In plate tectonics, a convergent boundary, also known as a destructive plate boundary, is a region of active deformation where two or more tectonic plates or fragments of the lithosphere are near the end of their life cycle.

This is in contrast to a constructive plate boundary (also known as a mid-ocean ridge or spreading center).

As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near destructive boundaries, where subduction zones or an area of continental collision (depending on the nature of the plates involved) occurs. The subducting plate in a subduction zone is normally oceanic crust, and moves beneath the other plate, which can be made of either oceanic or continental crust.

During collisions between two continental plates, large mountain ranges, such as the Himalayas are formed.

In other regions, a divergent boundary or transform faults may be present. Oceanic-continental Continental-continental Oceanic-oceanic Contents [ hide ]1 Mid-ocean ridges2 Subduction zones3 Volcanic arcs4 Back arc basins5 Oceanic trenches6 Examples7 See also8 References Mid-ocean ridges [ edit]See also: Earth's internal heat budget The reason for the existence of convergent boundaries are the mid-ocean ridges that were only beginning to be understood with the acceptance of the theory of plate tectonics in the mid-1960s.

The radioactive decay of elements within the rocks in the interior of the earth generates large amounts of heat that moves magma to the surface of the crust in a process called mantle convection. This results in new oceanic crust or sea floor being formed that slowly moves away from the ridges (sometimes called spreading centers) towards the subduction zones.

In other words, new sea floor originates from the upwelling rock as it emerges from the depths of the mantle and it slowly cools as it is transported over the millennia to the collision or subduction zones where it is consumed or recycled. [1]Subduction zones [ edit]See also: Forearc and Wadati–Benioff zone Large, shallow earthquakes on subduction zone interfaces are the largest types of earthquakes. They accounted for 90% of the total seismic moment released for the period 1900–1989, but these events usually occur offshore and far from population centers.

Non-subduction interface earthquakes also occur in the region of the trench including shallow upper plate shocks, deeper events within the downgoing slab, and events that are related to outer trench swell. [2]Volcanic arcs [ edit]A volcanic arc is formed on the continental plate, above the location of the downgoing oceanic slab. The volcanic arc is the surface expression of the magma that is generated by hydrous melting of the mantle above the downgoing slab.

Hydrated minerals (e.g., phlogopite, lawsonite, amphibole) within the oceanic lithosphere become unstable at certain depths due to increased temperature and pressure, causing the crystal structure of the hydrated minerals break down and release water. The buoyant fluids then rise into the asthenosphere, where they lower the melting temperature of the mantle and cause partial melting. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases.

When two plates with oceanic crust converge, they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it.

The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep oceanic trench is located in front of such arcs where the descending slab dips downward, such as the Mariana Trench near the Mariana Islands. Back arc basins [ edit]See also: Back-arc region

This type of convergent boundary is similar to the Andes or the Cascade Range in North America. Oceanic trenches [ edit]An oceanic trench is found where the denser plate is subducted underneath the other plate. There is water in the rocks of the oceanic plate (because they are underwater), and as this plate moves further down into the subduction zone, much of the water contained in the plate is squeezed out when the plate begins to subduct.

However, the recrystallization of ocean floor rocks, such as serpentinite, which are unstable in the upper mantle, recrystallize into olivine, causing dehydration through loss of hydroxyl groups. This addition of water to the mantle causes partial melting of the mantle, generating magma, which then rises, and which normally results in volcanoes.

This normally happens at a certain depth, about 70 to 80 miles below the Earth's surface, and so volcanoes are formed fairly close to, but not right next to, the trench. Examples [ edit]The collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.

The collision between the Australian Plate and the Pacific Plate that formed the Southern Alps in New Zealand Subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands. Subduction of the Nazca Plate beneath the South American Plate to form the Andes. Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate, forming the complex New Zealand to New Guinea subduction/transform boundaries.

Collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey. Mariana Trench Subduction of the Juan de Fuca Plate beneath the North American Plate to form the Cascade Range.

See also [ edit]List of tectonic plates List of tectonic plate interactions References [ edit]^ Bolt, B. (2005), Earthquakes: 2006 Centennial Update – The 1906 Big One (Fifth ed. ), W. H. Freeman and Company, pp. 146–150, ISBN 978-0716775485^ Yeats, R. S.; Sieh, K. E.; Allen, C. R. (1997). The Geology of Earthquakes.

A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise is known as a passive continental margin. This type of margin experiences little, if any, volcanic or earthquake activity. The build-up of sediment is the primary activity affecting a passive margin.

Continental Margin

"Continental Margin Follow City-Data.com founder on our Forum or @Lech Mazur Photo by: Kwest They are the drowned edges of continents. Covered by the oceans, continental margins are part of the same crust (thin, solid outermost layer of Earth) that forms the continents.

They are covered in sediments that have been eroded from dry land. (Erosion is the gradual wearing away of Earth surfaces through the action of wind and water.) Lying between the deep ocean basins and the above-water land areas, continental margins account for 11 percent of Earth's surface.

The shape of the land The continental margin is the submerged outer edge of a continent. It is generally divided into two sections: the continental shelf and the continental slope. The continental shelf is the region that extends seaward from the shoreline to a sharp drop-off that marks the beginning of the continental slope. That drop-off is known as the continental shelf break. Continental shelves vary in width from 3 to 930 miles (5 to 1,500 kilometers).

The average width worldwide is about 40 miles (64 kilometers). The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Narrow shelves are found off the western coasts of North and South America. Continental shelves along the coasts of the United States cover an area of about 891,000 square miles (2,307,690 square kilometers). Continental shelves are normally gently sloping, with an average seaward slope of about 0.1 degree.

They tend to have the same topography or surface features that dominate the adjacent land. Whether a coastal area is mountainous, dominated by low hills, or flat, the continental shelf next to it will be similarly shaped. The average depth of the continental shelf at the shelf break is about 430 feet (131 meters) below the surface of the ocean. Basic composition of continental margins, which include the continental shelf, continental slope, and continental rise.

In contrast to the continental shelf, the continental slope is generally narrow in width, ranging from 6.2 to 62 miles (10 to 100 kilometers).

Relatively steeper than the shelf, the slope angles down to the ocean basin at an average of 4 degrees; in some areas, the angle may be as much as 25 degrees. The steepness of a slope often reflects the steepness of the nearby coastal area. Slopes along mountainous coasts are steeper than those along flat coasts.

In general, the steepest slopes tend to be found in the Pacific Ocean and the least steep slopes in the Atlantic and Indian Oceans.

The continental slope marks the transition between continental crust and oceanic crust. Continental crust is composed mostly of granite, whereas oceanic crust is mostly basalt. (Although they differ in composition, both are types of igneous rock, which forms when magma cools and solidifies. Granite forms when magma with a high silica content cools slowly deep beneath Earth's surface; basalt forms when magma with a low silica content cools quickly outside of or very near Earth's surface.

)The most distinctive features of continental slopes are submarine canyons. Typically, these are steep-walled, V-shaped canyons that may be thousands of feet deep. The incline of a canyon is normally related to that of the slope: steep slopes have short, steep canyons, while broader slopes have longer, shallower canyons. The deepest of the submarine canyons easily rival the size of the Grand Canyon on the Colorado River in Arizona. Sometimes, submarine canyons begin at the outer edge of the continental shelf. Most often, they form down-current from major rivers that flow into an ocean.

Geologists believe that some submarine canyons may have been carved during a time in Earth's past when sea levels were lower than at present, and rivers were able to flow out to the edge of the shelf. Other canyons may have formed when earthquakes created faults or cracks along slopes that were subsequently eroded into the canyons. Continental margin: Words to Know Accretionary wedge: A mass of sediment and oceanic rock that is transferred from an oceanic plate to the edge of the less dense plate under which it is subducting.

Active continental margin: A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope that ends in a deep trench instead of a continental rise; it is marked by earthquake and volcanic activity.

Asthenosphere: The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock. Continental drift: The hypothesis proposed by Alfred Wegener that the continents are not stationary, but have moved across the surface of Earth over time. Continental margin: The submerged outer edge of a continent, composed of the continental shelf and the continental slope.

Continental rise: The gently sloping, smooth-surfaced, thick accumulation of sediment at the base of certain continental slopes. Continental shelf: The gently sloping region of the continental margin that extends seaward from the shoreline to the continental shelf break.

Continental shelf break: The outer edge of the continental shelf at which there is a sharp drop-off to the steeper continental slope. Continental slope: The steeply sloping region of the continental margin that extends from the continental shelf break downward to the ocean basin.

Convection current: The circular movement of a gas or liquid between hot and cold areas. Crust: The thin, solid outermost layer of Earth. Erosion: The gradual wearing away of Earth surfaces through the action of wind and water. Lithosphere: The rigid uppermost section of the mantle combined with the crust.

Mantle: The thick, dense layer of rock that lies beneath Earth's crust. Passive continental margin: A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise; it is marked by a lack of earthquake and volcanic activity.

Plates: Large sections of Earth's lithosphere separated by deep fault zones. Plate tectonics: The geologic theory that Earth's crust is composed of rigid plates that ""float"" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.

Seafloor spreading: The process by which new oceanic crust is formed by the upwelling of magma at mid-ocean ridges, resulting in the continuous lateral movement of existing oceanic crust. Subduction zone: A region where two plates come together and the edge of one plate slides beneath the other.

Submarine canyon: A steep-walled, V-shaped canyon that is cut into the rocks and sediments of the continental slope and, sometimes, the outer continental shelf. Trench: A long, deep, narrow depression on the ocean basin with relatively steep sides. Turbidity current: A turbulent mixture of water and sediment that flows down a continental slope under the influence of gravity.

Geologists know that underwater currents called turbidity currents have eroded most submarine canyons.

These turbulent mixtures of water and sediment develop near the continental shelf break when some event, such as an earthquake or a violent storm, triggers their formation. Denser (heavier) than the surrounding water, the currents are pulled downward by gravity. Flowing down the continental slope like an avalanche at up to 50 miles (80 kilometers) per hour, the sediment-laden currents surge through the canyons, scouring their sides.

When the currents reach the ocean basin, they slow, and the sediment they carry falls to the bottom in a fanlike deposit, much like an alluvial fan that forms in a desert environment when water flows out of a canyon onto a plain or flat area.

(For further information on alluvial fans, see the Dune and other desert features chapter. )At the base of certain continental slopes is a gently sloping, smooth-surfaced, thick accumulation of sediment from land that had been transported to the shelf and then down the slope. This transition between the continental slope and the ocean basin is known as the continental rise.

Some sources include the continental rise as being a major section of the continental margin, along with the shelf and slope. Since it does not occur on all continental margins around the planet, it is best considered a characteristic of the margin in certain areas. Continental rises are well-developed around Antarctica and in the Atlantic and Indian Oceans. They are hardly found in the Pacific. When they are present, continental rises vary in width from 62 to 620 miles (100 to 1,000 kilometers).

A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise is known as a passive continental margin. This type of margin experiences little, if any, volcanic or earthquake activity. The build-up of sediment is the primary activity affecting a passive margin.

Because these margins are found along the east coasts of North and South America and the west coasts of Europe and Africa, they are also known as ""Atlantic-type"" margins.

The continental margins along India and Antarctica are other examples of passive margins. A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope is known as an active continental margin. Instead of ending in a continental rise, the continental slope of this type of margin often plunges into a deep-ocean trench, which may be filled with sediment.

Earthquake and volcanic activity are prominent here. Since active continental margins occur along many coasts of the Pacific Ocean, these types of margins are also known as ""Pacific-type"" margins. Forces and changes: Construction and destruction The two types of continental margins, passive and active, tell about the geologic history of Earth and the activities that are continually affecting its surface.

The continents are not stationary, but move about the planet's surface. That movement varies, but in general is extremely slow, only about 2 inches (5 centimeters) per year.

Over millions of years, however, the continents have made their way along on an endless journey across the planet's surface, repeatedly crashing into or breaking away from one another. A Burial and Dumping Ground Continental shelves have not always been covered by water. During Earth's history, changes in sea level have alternately exposed, then covered portions of the shelves.

Scientists estimate that during the last glacial period over 10,000 years ago, much of Earth's water was trapped in the polar ice sheets. The level of the oceans may have dropped as much as 350 feet (107 meters) below the current level. At times of low sea level, land plants and animals, including humans and their ancestors, lived on the shelves.

Evidence of this lies in their remains that are often found there in the present day. Twelve-thousand-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States. Because water above continental shelves is not that deep, sunlight is able to penetrate, helping plants grow.

This, in turn, leads to a rich web of sea life. Most commercial fishing takes place in these waters. Extensive deposits of oil, natural gas, minerals, and other natural resources lie beneath continental shelves. The economic benefit of the fish and natural resources is important to many nations, which claim territorial ownership of the continental shelves adjacent to their land areas. Many political disagreements have arisen because of this. In spite of their desire to reap the benefits contained over and in the shelves, many nations illegally dump much of their waste in the ocean over these areas.

The drifting continents In 1915 German geophysicist Alfred Wegener (1880–1930) published a book in which he presented geological evidence that all the continents had once been joined together in a supercontinent he called Pangaea (pronounced pan-JEE-ah; from the Greek words meaning ""all lands"").

Wegener suggested that the Atlantic Ocean and the Indian Ocean formed when this supercontinent broke apart and the continents drifted away from each other. He called his hypothesis continental drift. (A hypothesis is an educated guess, while a theory is a principle supported by extensive scientific evidence and testing.

)Wegener formed his hypothesis after he had observed that the present-day continental margins along some of the continents seemed to fit together like pieces in a jigsaw puzzle: eastern South America with western Africa, eastern North America with western Europe, and India and Antarctica with eastern Africa. When the continents are linked, some of their geological features, such as mountain ranges and mineral deposits, also match.

In addition, related species of land animals are often present on both side of the present-day oceans. Wegener argued further that if the continents had drifted, they would have passed through various climate zones.

This explains how evidence of past glaciers could be found in the Sahara desert region and why fossil coral reefs appear north of the Arctic Circle. The theory of plate tectonics What Wegener lacked, however, was a convincing explanation as to what moved the continents along the surface.

Evidence to support his hypothesis did not come until the early 1960s when geologists developed the theory of plate tectonics (from the Greek word tekton, meaning ""builder""). A revolutionary idea, it helps geologists and others understand how Earth has changed over long periods of time. Changes in the positions and features of the continents and oceans have had a profound effect on everything from global climate to the evolution of life.

Simply, the theory states that the surface of the planet is broken into sections—some large, some small—called tectonic plates. As these plates drift slowly over Earth, they slide past, collide with, and move away from each other. The boundaries where the plates meet and interact are called plate margins. What moves the plates along occurs within the planet.

Geologists divide Earth into three distinct layers: the crust, the mantle, and the core. Each layer has its own unique properties and composition.

As already mentioned, the crust is the thin shell of rock that covers Earth. Two types of crust exist: continental crust, which underlies the continents, and oceanic crust, which underlies the oceans. Varying in thickness, the crust is thickest below land and thinnest below the oceans. Underneath the crust is the mantle, which is separated into two layers: The uppermost part of the mantle is solid.

Along with the overlying crust, it forms what is called the lithosphere (pronounced LITH-uh-sfeer). It is the brittle lithosphere that has broken into the tectonic plates. Under the lithosphere is the part of the mantle known as the asthenosphere (pronounced as-THEN-uh-sfeer). Beneath Earth's surface, temperature and pressure increase with increasing depths. Rock in the asthenosphere is hot Cross-section of Earth's interior, with the solid inner core, molten outer core, mantle, and crust.enough to fold, stretch, compress, and flow very slowly without fracturing.

It is puttylike in its consistency, or what geologists call ""plastic."" The rigid tectonic plates ""float"" on the more dense, flowing asthenosphere.

The core, lying at the center of the planet, is divided into a liquid outer layer and a solid inner layer. Made up of the metallic elements iron and nickel, the core is almost five times as dense as rock on Earth's surface.

Temperatures in the core are estimated to exceed 9,900°F (5,482°C), creating extreme heat energy. Moving the plates This heat energy moves the tectonic plates across the planet's surface. It is carried to the area beneath the plates by convection currents, which act similar to the currents produced in a pot of boiling liquid on a hot stove.

When a liquid in a pot begins to boil, it turns over and over. Liquid heated at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense (lighter).

Once at the surface, the heated material cools and becomes dense once more. It then sinks back down to the bottom to become reheated. This continuous motion of heated material rising, cooling, and sinking forms the circular convection currents.

Like a gigantic furnace, the core heats the mantle rock immediately above it. Expanding and becoming less dense, the heated rock slowly rises through cooler, denser mantle rock. When the heated rock reaches the lithosphere, it moves along its base, exerting dragging forces on the tectonic plates.

This causes the plates to move. In the process, the heated rock begins to lose heat. Cooling and becoming denser, the rock then sinks back toward the core, where it will be heated once more. It takes an estimated 200 million years for heated mantle rock to make the circular trip from the core to the lithosphere and back again.

As the plates move, they interact in several different ways. Where they diverge, or move away from each other, new lithospheric rock is created as magma rises up through the crust at the plate boundary and gradually begins to cool.

Many boundaries of diverging plates are on the floors of the oceans. They are known as mid-ocean ridges, and the process of new ocean crust forming at the ridges is known as seafloor spreading. Where plates converge, or move into one another, either they crumple up and compress or one plate slides beneath the other.

When two continental (land) plates converge, the crust bends and breaks from the collision, forming complex mountain ranges and very high plateaus. When a continental plate and an oceanic plate converge, the oceanic plate (which is thinner, yet denser) bends and plunges at an angle into the asthenosphere beneath the continental plate. As it does so, its leading edge begins to melt because of high temperature and pressure in the mantle.

This forms thick, flowing magma (molten rock beneath the planet's surface). Less dense than the rock that surrounds it deep underground, the magma rises toward Earth's surface, forcing its way through weakened layers of rock. In most instances, the magma collects in underground reservoirs called magma chambers. It remains there until enough pressure builds up to eject it onto the planet's surface through vents called volcanoes. (For further information, see the Volcano chapter.) The process of one plate sinking beneath another is known as subduction, and the area where it occurs is known as a subduction zone.

At about 10 percent of the plate boundaries, the plates neither diverge nor converge; instead, they transform or slide past one another.

On land, such a boundary is called a slip-strike boundary. A classic example of such a boundary is the San Andreas Fault system, an area of cracks in Earth's surface that occurs at the boundary between two tectonic plates, which extends down the coast of California.

Passive continental margins The passive continental margins of eastern North America, eastern South America, western Africa, and western Europe began to form about 200 million years ago when Pangaea began to break up. The rift or crack that caused them to split, known as the Mid-Atlantic Ridge, now lies on

Two types of continental margins exist: active margins and passive margins. Active margins form primarily along the boundaries of plates that are actively converging. Passive margins currently exist in the middle of plates, not at plate boundaries.

Cases of continental margins that are on or very close to a plate margin. Most continental margins around the Pacific are active. Key characteristics of active continental margins include the activity of volcanoes and earthquakes, narrow continental shelves, large coastal mountains, and much more.

"Definitions Back Home Types of continental margins - active and passive A continental margin is the edge of a continent.

What makes a continent a ""continent""? Geologically speaking, a continent is a continuous piece of crust composed on ""continental"" rocks — rocks such as diorite, andesite, granite, rhyolite, etc. Continents make up one of the two types of crust that occur on Earth — oceanic and continental crusts.

Continental crust is thicker than oceanic crust. A ""continental"" margin isn't the same thing as a ""plate"" margin. Look at the eastern edge of North America. Is it located in the same place as the eastern edge of the North American tectonic plate?

How about the western edge of North America? A continental margin is the edge of a continent: A plate margin is the edge of a plate. Continents and plates are not the same thing. Continental margins are relevant because many people live near coasts.

Plate margins are relevant becuase that's where plate interact. Image source - USGSThe edge of most continents extends beyond the coast. Look at Florida. Shallow marine areas less than 200 m below sea level extend far beyond the coast of the Florida peninsula.

Zoom Image source - GEBCO Seafloor less than 200 mbsl (meters below sea level Seafloor greater than 200 mbsl (meters below sea level Two kinds of continental margins Continents are areas of continental crust. Look at the thickness of crust underlying North America compared to the Pacific Ocean. Which has thicker crust - most continental or oceanic areas? How thick is the continental crust under continental shelf areas? Zoom Image source - USGSActive margins

Passive margins Cased of continental margins that are well within the interiors of a plate and far away from plate margins. Most continental margins around the Atlantic are passive. Key characteristics of passive continental margins includes the absence of volcanoes and much earthquake activity, wide continental shelves, no large coastal mountains, and much more.

Questions for thought What kind of continental margin is - the eastern edge of North America? - the western edge of North America? How wide is the continental shelf on - the eastern edge of North America? - the western edge of North America?

Which kind of continental margin tends to have wider continental shelves, active or divergent? Which kind of continental margin tends to rimmed with large mountain ranges? Which kind of continental margin tends to associated with the places where the world's largest rivers meet the oceans?

Which kind of margin experiences greater earthquakes and volcanism? Where is the westernmost edge of the continent of North America near San Diego? Is San Diego near a passive or active continental margin? "

um. Andean-Type Convergent Zone Mountain building along continental margins that involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust. Results in the formation of a continental volcanic arc and related tectonic features inland of the continental margin.

Chapter 9: Mountain Building

350 terms Bob_Grove_Science Chapter 9: Mountain Building Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Like this study set? Create a free account to save it. Create a free account Maybe later Sort Deformation refers to all changes in the original shape and/or size of a rock body.

Elastic Deformation rock's first response to deformation. Changes that are recoverable Ductile Deformation Once the elastic limit of a rock is surpassed, it flows. Occurs at depths where temperatures and pressures are high. Brittle Deformation Once the elastic limit of a rock is surpassed, it fractures. Occurs near the surface where temperatures and pressures are low. Folds A series of wavelike undulations.

The result of compressional forces. Anticline Folds most commonly formed by the upfolding, or arching, of rock layers. One of the two most common folds, the other being a syncline. Syncline A downfold, or trough. One of the two most common folds, the other being an anticline. Symmetrical folds with limbs that are mirror imagees of each other. Asymmetrical folds with limbs that are not symmetrical. Overturned folds with one limb tilted beyond the vertical. Monoclines Large, step-like folds in otherwise horizontal sedimentary strata.

Dome Broad upwarps in basement rock that deform the overlying cover of sedimentary strata and produces a circular or elongated structure. Basins The structure produced by downwarping. Faults fractures in the crust along which appreciable displacement has taken place. Dip-Slip Faults Faults in which the movement is primarily parallel to the inclination, or dip of the fault surface. Fault Scarps Long, low cliffs produced by vertical displacements along dip-slip faults. Hanging Wall The rock surface that is immediately above the fault.

Footwall The rock surface immediately below the fault. Normal Faults Dip-slip faults with the hanging wall block moving down relative to the footwall block. Reverse and Thrust Faults Dip-slip faults in which the hanging wall block moves up relative to the footwall block.

Strike-Slip Faults Faults in which the dominant displacement is horizontal and parallel to the trend, or strike, of the fault surface. Transform Fault Major strike-slip faults that cut through the lithosphere and accommodate motion between two large crustal plates.

Joints Fractures along which no appreciable displacement has occurred. Columnar Joints Joints formed from igneous rocks cooling and developing shrinkage fractures that produce elongated, pillarlike columns. Orogenesis The name for the processes that collectively produce a mountain belt. Passive Continental Margin not plate boundary but a part of the same plate as the adjoining oceanic crust. Active Continental Margin At some point the continental margin becomes active. A subduction zone forms and the deformation process begins.

Terrane Any crustal feature that has a geologic history distinct from that of adjoining terranes. Accretionary Wedge The chaotic accumulation of sedimentary and metamorphic rocks with occasional scraps of ocean crust. The Himalayas The youngest collision mountains on Earth and are still rising. Isostasy The concept of a floating crust in gravitational balance. Isostatic Adjustment The process of establishing a new level of gravitational equilibri

Accretionary Wedge a large wedge-shaped mass that accumulates in subduction zones; sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block. Active Continental Margin usually narrow and consisting of highly deformed sediment; occur where oceanic lithosphere is being subducted beneath the margin of a continent. Anticline a fold in sedimentary strata that resembles an arch. Basin a circular downfolded structure.

Brittle Failure/Brittle Deformation the loss on strength of a material usually in the form of sudden fracturing. Core inner-most layer of Earth based on composition; thought to be largely iron and nickel alloy with minor amounts of oxygen, silicon, and sulfur.

Crust very thin outermost layer of Earth. Deformation general term for the process of folding, faulting, shearing, compression, or extension of rocks. Dip-Slip Fault a fault in which the movement is parallel to the dip of the fault. Dome a roughly circular upfolded structure similar to an anticline.

Ductile Deformation a type of solid-state flow that produces a change in the size and shape of a rock body without fracturing it; occurs at depths where temps and confining pressures are high.

Fault a break in a rock mass along which movement has occured. Fault-Block Mountains a mountain formed by the displacement of rock along a fault. Fault Scarp a cliff created by movement along a fault; represents the exposed surface of the fault prior to modification by weathering and erosion.

Fold bent rock layer or series of layers that were originally horizontal and subsequently deformed. Footwall Block rock surface below a fault. Graben a valley formed by the downward displacement of a fault-bounded block. Hanging Wall Block rock surface immediately above a fault. Horst an elongated, uplifted block of crust bounded by faults.

Liquefaction phenomenon sometime associated with earthquakes in which soils and other unconsolidated materials containing abundant water are turned into a fluid-like mass that is not capable of supporting buildings. Lithosphere outer layer of Earth containing crust and upper mantle. Lower Mantle part of the mantle that extends from the core-mantle boundary to a depth of 66o km; also called the Mesosphere.

Mantle the 2900 km (1800 mi) thick layer of earth located below the crust. Normal Fault fault in which the rock above the fault plane has moved down relative to the rock below.

Orogenesis processes that collectively result in the formation of mountains. Outer Core layer beneath the mantle about 2200km (1364 mi) thick that has the properties of a liquid. Passive Continental Margin margin that consists of a continental shelf, continental slope, and continental rise; not associated with plate boundaries therefore experience little volcanism and earthquakes.

Reverse Fault a fault in which the material above the fault plane moves up in relation to the material below. Richter Scale scale of earthquake magnitude based on the motion of a seismograph. Seismic Waves form of elastic energy released during and earthquake that causes vibrations in the materials that transmit them.. Strike-Slip Fault a fault along which the movement is horizontal.

Syncline linear downfold in sedimentary strata; opposite of anticline. Terrane crustal block bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks.

Thrust Fault low-angle reverse fault. Transform Fault major strike-slip fault that cuts through the lithosphere and accommodates motion between to plates.deformation all changes in the original form and/or size of a rock bodycrustal deformation occurs along plate marginstemperature and confining pressure factor that influences strength of a rockrock type factor that influences strength of a rockmineral composition factor that influences strength of a rocktime ie marble benches sag factor that influences strength of a rockfold rock bent into series of wavesfold result from compressional forces which shorten and thicken the crustanticline upfolded or arched rock layers, type of foldsyncline down folded rock layerssymmetrical anticline/syncline limbs are mirror imagesasymmetrical anticline/syncline limbs are not mirror imagesoverturned anticline/syncline one limb is tilted beyond the vertical (recumbant)plunging folds where folds die outmonoclines large step-like folds in horizontal sedimentary strataductile deformation flows; high temp and pressurebrittle deformation fractures; near surface; low temps and pressureelastic deformation changes are recoverabledome circular; slightly elongated; upwarped displacement of rocksdome oldest rocks in coredome black hills of south dakotabasin circular; slightly elongated; downwarped displacement of rocksbasin youngest rocks in corebasin michiganfaults fractures (breaks) in rocks along which movement as taken placedip-slip fault movement along inclination of fault plane; vertical displacementhanging wall rock above the fault surfacefault scarps low cliffsnormal fault hanging wall moves down; prevalent at spreading centers; caused by tensional forcesreverse fault hanging wall moves up; caused by strong compressional stresses; greater than 45 degreesthrust fault hanging wall moves up; caused by strong compressional stresses; less than 45 degreesstrike-slip fault dominant displacement is horizontal and parallel to the trend or striketransform fault large strike-slip fault that cuts through the lithosphere; associated with plate boundariessan andreas transform faultright lateral bottom moves rightleft lateral bottom moves leftjoints fractures (breaks) in rocks along which no movement has taken placemountain building occurs at convergent boundariesandean type mountain building ocean plate converges with a content plate and is pushed beneath it; subduction zone forms; deformation process beginsandean type mountain building continental volcanic arc formsocean/ocean boundary aleutian mountainsocean/land boundary andes mountainsland/land boundary india mountainsmountain belts where two plates with continental crust convergemountain belts india and eurasian plate collision; himalayan mountains and tibetan plateauaccreted terrances to attachcontinental accretion small crustal fragments collide with and accrete to continental marginsterranes accreted crustal blockscontinental accretion occurred along the pacific coast; western margin of north americaisostasy concept of floating crust in gravitational balanceisostatic adjustment when weight is removed from the crust, crustal uplifting occurswave cut platforms high above sea level evidence for crustal uplift Mountain Building at Subduction Zones - Island Arcs 2 ocean plates converge - 1 subducts beneath the other making volcanic islands Active Volcanic arcs include the Mariana, Kurile, Tonga, Aleutian Active Volcanic arcs are currently Creating Volcanoes Old Volcanic Arcs include J, LA, P Japan, Lesser Antilles, Philippines Ocean to Ocean older subducts eventually is an ocean continent margin Mountain Building at subduction zones - Andean-Type Margins: _____ converges with a _____ creating a _______ Ocean plate continent continental volcanic arc Stage 1 of Andean-Type margins before subduction continental margin is ______ and lots of ________ accumulates on ____,___, ___ passive sediment shelf, slope and rise Stage 2 of Andean-Type margins subduction zone forms and margin becomes active Stage 2 Conti Continental Volcanic Arc are at the _________ and _______ are at a depth.

They are eventually ____ to become _______ surface plutons uplifted exposed igneous mountains The Sierra Nevadas are mostly _____ and ____ and most ______ of _____ was erased by _____ igneous metamorphic evidence volcanoes erosion In Stage 2, ________ from the ocean floor and ________ are bulldozed onto the _______ creating a _________ or ______ Sediment scrapes, accretionary wedge, continent, collection that gets plastered onto coast, accreted The Costal range of Ca is mostly sedimentary and metamorphic Mountain building can be due to 2 things Collisions and fault-blocks Accretted Terrances forms of land that can collide with continents Accreted Terrances other continents either ________ subducts so the _________ and _____ into a ______ plate, crust shortens, thickens mountain range Terrances small crustal fragments do not subduct well so they get accretted onto continentexamples of Terrances are Island arc, micro-continents and oceanic plateaus Oceanic plateaus basaltic lava spilling onto the ocean floor at a hot spot Continental Collisions example Himalayas - 45 mya India collided with Asia The tectonic forces that created the Himalayas were __ and ____ and ___ the rock great folded faulted the rock Crustal thickening caused above also caused ______ to become ______ and experience high _____ and ______ producing ______ that intruded further _______ above lower layers - deeply buried - pressure - temperature - magma bodies - deforming layers Appalachian Orogeny were created when Pangaea came together Fault-Block Mountains are an example of continental rifting - extension can produce mountainsmost mountain building results from collisions - compression resulting in thrust folds and faults T or F can build mountains with faults TBasin and Range province is fault blocks T or F TEroded material are in grobensis it possible to uplift Fault Block Mountains?

Y or N YSierra Nevada in CA are also tilted blocks North American Cordillera is a combination of mountain ranges See more

In plate tectonics, a convergent boundary, also known as a destructive plate boundary, is a region of active deformation where two or more tectonic plates or fragments of the lithosphere are near the end of their life cycle. This is in contrast to a constructive plate boundary (also known as a mid-ocean ridge or spreading center

Convergent boundary

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). As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near destructive boundaries, where subduction zones or an area of continental collision (depending on the nature of the plates involved) occurs.

The subducting plate in a subduction zone is normally oceanic crust, and moves beneath the other plate, which can be made of either oceanic or continental crust. During collisions between two continental plates, large mountain ranges, such as the Himalayas are formed.

In other regions, a divergent boundary or transform faults may be present. Oceanic-continental Continental-continental Oceanic-oceanic Contents [ hide ]1 Mid-ocean ridges2 Subduction zones3 Volcanic arcs4 Back arc basins5 Oceanic trenches6 Examples7 See also8 References Mid-ocean ridges [ edit]See also: Earth's internal heat budget The reason for the existence of convergent boundaries are the mid-ocean ridges that were only beginning to be understood with the acceptance of the theory of plate tectonics in the mid-1960s.

The radioactive decay of elements within the rocks in the interior of the earth generates large amounts of heat that moves magma to the surface of the crust in a process called mantle convection. This results in new oceanic crust or sea floor being formed that slowly moves away from the ridges (sometimes called spreading centers) towards the subduction zones.

In other words, new sea floor originates from the upwelling rock as it emerges from the depths of the mantle and it slowly cools as it is transported over the millennia to the collision or subduction zones where it is consumed or recycled. [1]Subduction zones [ edit]See also: Forearc and Wadati–Benioff zone Large, shallow earthquakes on subduction zone interfaces are the largest types of earthquakes. They accounted for 90% of the total seismic moment released for the period 1900–1989, but these events usually occur offshore and far from population centers.

Non-subduction interface earthquakes also occur in the region of the trench including shallow upper plate shocks, deeper events within the downgoing slab, and events that are related to outer trench swell. [2]Volcanic arcs [ edit]A volcanic arc is formed on the continental plate, above the location of the downgoing oceanic slab. The volcanic arc is the surface expression of the magma that is generated by hydrous melting of the mantle above the downgoing slab.

Hydrated minerals (e.g., phlogopite, lawsonite, amphibole) within the oceanic lithosphere become unstable at certain depths due to increased temperature and pressure, causing the crystal structure of the hydrated minerals break down and release water. The buoyant fluids then rise into the asthenosphere, where they lower the melting temperature of the mantle and cause partial melting. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases.

When two plates with oceanic crust converge, they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it.

The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife).

A deep oceanic trench is located in front of such arcs where the descending slab dips downward, such as the Mariana Trench near the Mariana Islands. Back arc basins [ edit]See also: Back-arc region Some convergent margins have zones of active seafloor spreading behind the island arc, known as back-arc basins. When one plate is composed of oceanic lithosphere and the other is composed of continental lithosphere, the denser oceanic plate is subducted, often forming an orogenic belt and associated mountain range.

This type of convergent boundary is similar to the Andes or the Cascade Range in North America. Oceanic trenches [ edit]An oceanic trench is found where the denser plate is subducted underneath the other plate. There is water in the rocks of the oceanic plate (because they are underwater), and as this plate moves further down into the subduction zone, much of the water contained in the plate is squeezed out when the plate begins to subduct.

However, the recrystallization of ocean floor rocks, such as serpentinite, which are unstable in the upper mantle, recrystallize into olivine, causing dehydration through loss of hydroxyl groups. This addition of water to the mantle causes partial melting of the mantle, generating magma, which then rises, and which normally results in volcanoes. This normally happens at a certain depth, about 70 to 80 miles below the Earth's surface, and so volcanoes are formed fairly close to, but not right next to, the trench.

Examples [ edit]The collision between the Eurasian Plate and the Indian Plate that is forming the Himalayas.

The collision between the Australian Plate and the Pacific Plate that formed the Southern Alps in New Zealand Subduction of the northern part of the Pacific Plate and the NW North American Plate that is forming the Aleutian Islands. Subduction of the Nazca Plate beneath the South American Plate to form the Andes. Subduction of the Pacific Plate beneath the Australian Plate and Tonga Plate, forming the complex New Zealand to New Guinea subduction/transform boundaries.

Collision of the Eurasian Plate and the African Plate formed the Pontic Mountains in Turkey. Mariana Trench Subduction of the Juan de Fuca Plate beneath the North American Plate to form the Cascade Range.

See also [ edit]List of tectonic plates List of tectonic plate interactions References [ edit]^ Bolt, B. (2005), Earthquakes: 2006 Centennial Update – The 1906 Big One (Fifth ed. ), W. H. Freeman and Company, pp. 146–150, ISBN 978-0716775485^ Yeats, R. S.; Sieh, K. E.; Allen, C. R. (1997). The Geology of Earthquakes.

Oxford University Press. pp. 371–378. ISBN 978-0-19-507827-5. [ hide]v t e Physical oceanography Waves Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Iribarren number Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunamimegatsunami Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow waterone-dimensional Saint-Venant equationsshallow water equations Wind wavemodel Circulation Atmospheric circulation Baroclinity Boundary current Coriolis force Coriolis–Stokes force Craik–Leibovich vortex force Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño–Southern Oscillation General circulation model Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Modular Ocean Model Ocean dynamics Ocean gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulationshutdown Upwelling Whirlpool World Ocean Circulation Experiment Tides Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline Landforms Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano Plate tectonics Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc Ocean zones Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash Sea level Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level rise World Geodetic System Acoustics Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics Satellites Jason-1 Jason-2 (Ocean Surface Topography Mission) Jason-3Related Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas Category Commons Categories: Plate tectonics

Passive plate margins endure very little constructive or destructive movement. Active plate margins describe subduction zones and other boundaries where movement is regular and common. Cause of earthquakes.

The difference between Passive and Active Plate Margins ...?

Science & Mathematics Earth Sciences & Geology The difference between Passive and Active Plate Margins ...? What is the difference between Passive and Active plate margins? Follow 6 answers Answers Relevance Rating Newest Oldest

Best Answer: An active margin is a tectonic plate boundary, a passive margin is a compositional transition within a tectonic plate. As you probably know the theory of plate tectonics posits the outer layer of the Earth consist of plates which move relative to each other driven by convection in the Mantle.

The boundaries of the these plates are active plate margins and they can be convergent (Andes), divergent (Mid atlantic ridge) or conservative (San Andreas fault) Passive margins are not tectonic plate boundaries. Instead they occur within active plates. They mark where crust transfers from oceanic to continental or vice versa.

An example is the Eastern seaboard of the USA. Source (s): MA MSC Geology Pete · 9 years ago0 0 Comment This Site Might Help You. RE: The difference between Passive and Active Plate Margins ...? What is the difference between Passive and Active plate margins? Source (s):difference passive active plate margins: https://tr.im/YX3Pm Anonymous · 3 years ago0 0 Comment Passive Margins Source (s):https://shrinks.im/a9g PWfleischman · 1 year ago0 0 Comment Active Margin Source (s):https://shorte.im/bbxb9Erika · 1 year ago0 0 Comment For the best answers, search on this site https://shorturl.im/ayc Ip An active continental margin is found on the leading edge of the continent where it is crashing into an oceanic plate.

An excellent example is the west coast of South America. Active margins are commonly the sites of tectonic activity: earthquakes, volcanoes, mountain building, and the formation of new igneous rock. Because of the mountainous terrain, most of the rivers are fairly short, and the continental shelf is narrow to non-existent, dropping off quickly into the depths of the subduction trench.

Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal and the earth's weathering and erosional processes are winning. This leads to lots of low-relief (flat) land extending both directions from the beach, long river systems, and the accumulation of thick piles of sedimentary debris on the relatively wide continental shelves. The Amazon River, whose source is in the Andes Mountains (the active margin) drains east across the interior of South America to the coast, where it enters the Atlantic Ocean and deposits the tremendous volume of sedimentary materials it eroded from the continent. Leisha · 2 years ago0 0 Comment

Plate tectonics, or continental drift, which is the study of the Earth's crust, consists of volcanic and seismic activity around its margins that sometimes leads to earthquakes, volcanoes, and development of mountains.

Bright Hub

Plate Tectonics Effects Climate Changewritten by: Raunekk•edited by: Donna Cosmato •updated: 6/14/2010There are a few phenomena which, though not man-made, affect the climate of the atmosphere to a great extent.

Plate tectonics movement is one such effect that has been affecting climate change for a long time now. The article describes how does the movement of plate tectonics affect climate change? Introduction Going back to geography classes, we learned the surface of the Earth is composed of different large plates, which keep moving at a rate of approximately 3 centimeters yearly.

These plates shift in various directions due to the convection currents that are below them. However, the radioactive decay found in the core of the Earth is the primary force behind the heat generated to move these plates.

So how does the movement of plate tectonics affect climate change? Let us find out. Effects on Climate

Generally, plate tectonics causes a change in the ocean currents and in turn generates more heat. When these tectonic plates shift, the creation of the volcanic eruptions increases the degree of carbon dioxide (CO2) and sulfur dioxide (SO2) in the atmosphere. This is what leads to a rise in the temperature levels globally.

Higher concentrations of SO2 could mean the Earth is becoming warmer at four times the rate of CO2 concentration.

On the contrary, if the move is insignificant, the temperature levels record a fall. A new dimension revealed by these studies also hints to the increased heat leading to a shift in the tectonic plates. Thus, we can conclude that plate tectonic movement and change in climate go hand in hand. See the complete Bright Hub Guide to Weather Systems »About 200 million years ago, the present seven continents were a single piece of land known as Pangea.

With the change in the tectonic plates, this larger land drifted to form different smaller land areas. Climate is one of the systems adversely affected by these movements. For instance, a larger land area is likely to follow a strong seasonal pattern of climate.

Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.

Plate tectonics

"From Wikipedia, the free encyclopedia (Redirected from Continental plate)navigation search The tectonic plates of the world were mapped in the second half of the 20th century.

Diagram of the internal layering of the Earth showing the lithosphere above the asthenosphere (not to scale)Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός ""pertaining to building"") [1] is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth 's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century.

The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s. The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform.

Earthquakes, volcanic activity, mountain -building, and oceanic trench formation occur along these plate boundaries (or faults ). The relative movement of the plates typically ranges from zero to 100 mm annually. [2]

In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe. [3]Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle.

Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography (the ridge is a topographic high) and density changes in the crust (density increases as newly formed crust cools and moves away from the ridge).

At subduction zones the relatively cold, dense crust is ""pulled"" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.

The division is based on differences in mechanical properties and in the method for the transfer of heat. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient.

This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like ( visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/year ( Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year ( Nazca Plate; about as fast as hair grows).

[4] The driving mechanism behind this movement is described below. Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust ( sial from silicon and aluminium ).

Average oceanic lithosphere is typically 100 km (62 mi) thick; [5] its thickness is a function of its age: as time passes, it conductively cools and subjacent cooling mantle is added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed.

For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively.

[6] Continental lithosphere is typically about 200 km thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents.

The location where two plates meet is called a plate boundary. Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and widely known today.

These boundaries are discussed in further detail below. Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation [7] and to mantle plumes. As explained above, tectonic plates may include continental crust or oceanic crust, and most plates contain both.

For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans.

The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents.

Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements ("" mafic "") than continental crust ("" felsic "").

[8] As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate ), while continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).

Types of plate boundaries Main article: List of tectonic plate interactions Three types of plate boundaries exist, [9] with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: [10] [11]Transform boundary Divergent boundary Convergent boundary Transform boundaries (Conservative) occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed.

The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer).

Transform faults occur across a spreading center. Strong earthquakes can occur along a fault. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion. Divergent boundaries (Constructive) occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin.

As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of mid-ocean ridges (e.g., the Mid-Atlantic Ridge and East Pacific Rise ), and continent-to-continent rifting (such as Africa's East African Rift and Valley and the Red Sea), are examples of divergent boundaries.

Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision. At zones of ocean-to-continent subduction (e.g. the Andes mountain range in South America, and the Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent.

Earthquakes trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate is heated it releases volatiles, mostly water from hydrous minerals, into the surrounding mantle. The addition of water lowers the melting point of the mantle material above the subducting slab, causing it to melt.

The magma that results typically leads to volcanism. [12] At zones of ocean-to-ocean subduction (e.g. Aleutian islands, Mariana Islands, and the Japanese island arc ), older, cooler, denser crust slips beneath less dense crust. This motion causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called ""foreland basins"".

Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic continental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.

Plate boundary zones occur where the effects of the interactions are unclear, and the boundaries, usually occurring along a broad belt, are not well defined and may show various types of movements in different episodes. Driving forces of plate motion Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. The vectors show direction and magnitude of motion. It has generally been accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere.

Dissipation of heat from the mantle is acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming.

The current view, though still a matter of some debate, asserts that as a consequence, a powerful source of plate motion is generated due to the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens.

The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.

[13] Although subduction is thought to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among scientists.

One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movement, as some patterns may be explained by more than one mechanism.

[14] In short, the driving forces advocated at the moment can be divided into three categories based on the relationship to the movement: mantle dynamics related, gravity related (mostly secondary forces), and earth rotation related.

Driving forces related to mantle dynamics Main article: Mantle convection For much of the last quarter century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through the asthenosphere.

This theory was launched by Arthur Holmes and some forerunners in the 1930s [15] and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged a static Earth without moving continents up until the major breakthroughs of the early sixties.

Two- and three-dimensional imaging of Earth's interior ( seismic tomography) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density is mantle convection from buoyancy forces. [16]How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics.

Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There are essentially two main types of forces that are thought to influence plate motion: friction and gravity. Basal drag (friction): Plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere.

Slab suction (gravity): Plate motion driven by local convection currents that exert a downward pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Lately, the convection theory has been much debated, as modern techniques based on 3D seismic tomography still fail to recognize these predicted large scale convection cells; therefore, alternative views have been proposed: In the theory of plume tectonics developed during the 1990s, a modified concept of mantle convection currents is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas, which find their roots in the early 1930s with the so-called ""fixistic"" ideas of the European and Russian Earth Science Schools, find resonance in the modern theories which envisage hot spots or mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators).

Modern theories that continue building on the older mantle doming concepts and see plate movements as a secondary phenomena are beyond the scope of this article and are discussed elsewhere (for example on the Plume tectonics article).

Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below the Earth's crust, which then provide basal friction to the lithosphere. This theory, called ""surge tectonics"", became quite popular in geophysics and geodynamics during the 1980s and 1990s.

[17] Recent research, based on three-dimensional computer modeling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere. [18]Driving forces related to gravity Forces related to gravity are usually invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above.

Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges. [19] As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load.

The result is a slight lateral incline with increased distance from the ridge axis. This force is regarded as a secondary force and is often referred to as "" ridge push "".

This is a misnomer as nothing is ""pushing"" horizontally and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature.

Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate which produces a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges, and mantle plumes and hot spots, which are postulated to impinge on the underside of tectonic plates.

Slab-pull: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere.

Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this current understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches.

[20] Recent models indicate that trench suction plays an important role as well. However, the fact that the North American Plate is nowhere being subducted, although it is in motion, presents a problem. The same holds for the African, Eurasian, and Antarctic plates. Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them. This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism.

This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin. [21]Driving forces related to Earth rotation This section needs additional citations for verification.

Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2015) ( Learn how and when to remove this template message)Alfred Wegener, being a meteorologist, had proposed tidal forces and centrifugal forces as the main driving mechanisms behind continental drift; however, these forces were considered far too small to cause continental motion as the concept was of continents plowing through oceanic crust.

[22] Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929. However, in the plate tectonics context (accepted since the seafloor spreading proposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see below) during the early 1960s), the oceanic crust is suggested to be in motion with the continents which caused the proposals related to Earth rotation to be reconsidered.

In more recent literature, these driving forces are: Tidal drag due to the gravitational force the Moon (and the Sun) exerts on the crust of the Earth [23]Global deformation of the geoid due to small displacements of the rotational pole with respect to the Earth's crust;Other smaller deformation effects of the crust due to wobbles and spin movements of the Earth rotation on a smaller time scale.

Forces that are small and generally negligible are: The Coriolis force [24] [25]The centrifugal force, which is treated as a slight modification of gravity [24] [25]: 249For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and the geographical latitudinal and longitudinal grid of the Earth itself.

Ironically, these systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to the Earth equator and axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, ""fixist theories"").

Later studies (discussed below on this page), therefore, invoked many of the relationships recognized during this pre-plate tectonics period to support their theories (see the anticipations and reviews in the work of van Dijk and collaborators). [26]Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possible principle driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond the discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model.

In 1973, George W. Moore [27] of the USGS and R. C. Bostrom [28] presented evidence for a general westward drift of the Earth's lithosphere with respect to the mantle. He concluded that tidal forces (the tidal lag or ""friction"") caused by the Earth's rotation and the forces acting upon it by the Moon are a driving force for plate tectonics.

As the Earth spins eastward beneath the moon, the moon's gravity ever so slightly pulls the Earth's surface layer back westward, just as proposed by Alfred Wegener (see above). In a more recent 2006 study, [29] scientists reviewed and advocated these earlier proposed ideas. It has also been suggested recently in Lovett (2006) that this observation may also explain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on the planet.

In a recent paper, [30] it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific Ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces).

In the same paper the authors admit, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.

One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates.

The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.

[20] This view however has been contradicted by a recent study which found that the actual motions of the Pacific Plate and other plates associated with the East Pacific Rise do not correlate mainly with either slab pull or slab push, but rather with a mantle convection upwelling whose horizontal spreading along the bases of the various plates drives them along via viscosity-related traction forces. [31] The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics.

Development of the theory Further information: Timeline of the development of tectonophysics Summary Detailed map showing the tectonic plates with their movement vectors.

In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 book The Origin of Continents and Oceans [32] and the scientific debate started that would end up fifty years later in the theory of plate tectonics. [33] Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was called Pangea later on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to ""icebergs"" of low density granite floating on a sea of denser basalt.

[34] Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast, and from the matching of the rock formations along these edges.

Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris and Gangamopteris, and the therapsid or mammal-like reptile Lystrosaurus, all widely distributed over South America, Africa, Antarctica, India, and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in the southern hemisphere. The South African Alex du Toit put together a mass of such information in his 1937 publication Our Wandering Continents, and went further than Wegener in recognising the strong links between the Gondwana fragments.

But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around.

Distinguished scientists, such as Harold Jeffreys and Charles Schuchert, were outspoken critics of continental drift. Despite much opposition, the view of continental drift gained support and a lively debate started between ""drifters"" or ""mobilists"" (proponents of the theory) and ""fixists"" (opponents).

During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove.

One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since the mid–nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time.

Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called ""polar wander"" (see apparent polar wander ), i.e., it was assumed that the north pole location had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own ""polar wander path"".

During the late 1950s it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956, [35] and by Warren Carey in a symposium held in March 1956.

[36]The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on the bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology [37] which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals, published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley.

The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology.

Continental drift Further information: Continental drift In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what is called the geosynclinal theory.

Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time. Alfred Wegener in Greenland in the winter of 1912–13. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves —have similar shapes and seem to have once fitted together.

[39]Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid Earth made these various proposals difficult to accept. [40]The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth.

[41] This had previously been estimated by its cooling rate under the assumption that the Earth's surface radiated like a black body. [42] Those calculations had implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years.

Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.

Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick.

Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology. However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift.

Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930. [44]Floating continents, paleomagnetism, and seismicity zones Global earthquake epicenters, 1963–1998. Most earthquakes occur in narrow belts that correspond to the locations of lithospheric plate boundaries.

Map of earthquakes in 2016As it was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt, the prevailing concept during the first half of the twentieth century was that there were two types of crust, named ""sial"" (continental type crust) and ""sima"" (oceanic type crust).

Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that a layer of basalt (sial) underlies the continental rocks. However, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath.

The concept that mountains had ""roots"" was confirmed by George B. Airy a hundred years later, during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by the mid-1950s, the question remained unresolved as to whether mountain roots were clenched in surrounding basalt or were floating on it like an iceberg.

During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in specific areas, most notably along the oceanic trenches and spreading ridges.

By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth.

These zones later became known as Wadati–Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States.

The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) [45] to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.

Meanwhile, debates developed around the phenomena of polar wander. Since the early debates of continental drift, scientists had discussed and used evidence that polar drift had occurred because continents seemed to have moved through different climatic zones during the past.

Furthermore, paleomagnetic data had shown that the magnetic pole had also shifted during time. Reasoning in an opposite way, the continents might have shifted and rotated, while the pole remained relatively fixed.

The first time the evidence of magnetic polar wander was used to support the movements of continents was in a paper by Keith Runcorn in 1956, [35] and successive papers by him and his students Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.

This was immediately followed by a symposium in Tasmania in March 1956. [46] In this symposium, the evidence was used in the theory of an expansion of the global crust. In this hypothesis the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth.

Certainly there is no evidence that the moon has expanded in the past 3 billion years; other work would soon show that the evidence was equally in support of continental drift on a globe with a stable radius. During the thirties up to the late fifties, works by Vening-Meinesz, Holmes, Umbgrove, and numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory.

In particular, the English geologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convection currents within the mantle might be the driving force.

[47] Often, these contributions are forgotten because: At the time, continental drift was not accepted. Some of these ideas were discussed in the context of abandoned fixistic ideas of a deforming globe without continental drift or an expanding Earth. They were published during an episode of extreme political and economic instability that hampered scientific communication. Many were published by European scientists and at first not mentioned or given little credit in the papers on sea floor spreading published by the American researchers in the 1960s.

Mid-oceanic ridge spreading and convection Further information on Mid-ocean ridge: Seafloor spreading In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution 's research vessel Atlantis and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents.

They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions. [48]The new data that had been collected on the ocean basins also showed particular characteristics regarding the bathymetry. One of the major outcomes of these datasets was that all along the globe, a system of mid-oceanic ridges was detected.

An important conclusion was that along this system, new ocean floor was being created, which led to the concept of the "" Great Global Rift "". This was described in the crucial paper of Bruce Heezen (1960), [49] which would trigger a real revolution in thinking. A profound consequence of seafloor spreading is that new crust was, and still is, being continually created along the oceanic ridges. Therefore, Heezen advocated the so-called "" expanding Earth "" hypothesis of S. Warren Carey (see above).

So, still the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth? In reality, this question had been solved already by numerous scientists during the forties and the fifties, like Arthur Holmes, Vening-Meinesz, Coates and many others: The crust in excess disappeared along what were called the oceanic trenches, where so-called ""subduction"" occurred.

Therefore, when various scientists during the early sixties started to reason on the data at their disposal regarding the ocean floor, the pieces of the theory quickly fell into place. The question particularly intrigued Harry Hammond Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U. S. Coast and Geodetic Survey who first coined the term seafloor spreading.

Dietz and Hess (the former published the same idea one year earlier in Nature, [50] but priority belongs to Hess who had already distributed an unpublished manuscript of his 1962 article by 1960) [51] were among the small handful who really understood the broad implications of sea floor spreading and how it would eventually agree with the, at that time, unconventional and unaccepted ideas of continental drift and the elegant and mobilistic models proposed by previous workers like Holmes.

In the same year, Robert R. Coats of the U. S. Geological Survey described the main features of island arc subduction in the Aleutian Islands.

His paper, though little noted (and even ridiculed) at the time, has since been called ""seminal"" and ""prescient"". In reality, it actually shows that the work by the European scientists on island arcs and mountain belts performed and published during the 1930s up until the 1950s was applied and appreciated also in the United States.

If the Earth's crust was expanding along the oceanic ridges, Hess and Dietz reasoned like Holmes and others before them, it must be shrinking elsewhere. Hess followed Heezen, suggesting that new oceanic crust continuously spreads away from the ridges in a conveyor belt–like motion. And, using the mobilistic concepts developed before, he correctly concluded that many millions of years later, the oceanic crust eventually descends along the continental margins where oceanic trenches – very deep, narrow canyons – are formed, e.g.

along the rim of the Pacific Ocean basin. The important step Hess made was that convection currents would be the driving force in this process, arriving at the same conclusions as Holmes had decades before with the only difference that the thinning of the ocean crust was performed using Heezen's mechanism of spreading along the ridges.

Hess therefore concluded that the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is ""consumed"" in the trenches (like Holmes and others, he thought this was done by thickening of the continental lithosphere, not, as now understood, by underthrusting at a larger scale of the oceanic crust itself into the mantle), new magma rises and erupts along the spreading ridges to form new crust.

In effect, the ocean basins are perpetually being ""recycled,"" with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously.

Thus, the new mobilistic concepts neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks. Magnetic striping Seafloor magnetic striping. A demonstration of magnetic striping. (The darker the color is, the closer it is to normal polarity)Further information: Vine–Matthews–Morley hypothesis Beginning in the 1950s, scientists like Victor Vacquier, using magnetic instruments ( magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor.

This finding, though unexpected, was not entirely surprising because it was known that basalt—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral ( magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor.

When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time. As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns.

When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra -like pattern: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping, and was published by Ron G. Mason and co-workers in 1961, who did not find, though, an explanation for these data in terms of sea floor spreading, like Vine, Matthews and Morley a few years later.

[52]The discovery of magnetic striping called for an explanation. In the early 1960s scientists such as Heezen, Hess and Dietz had begun to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest (see the previous paragraph).

New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust.

This process, at first denominated the ""conveyer belt hypothesis"" and later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges.

Only four years after the maps with the ""zebra pattern"" of magnetic stripes were published, the link between sea floor spreading and these patterns was correctly placed, independently by Lawrence Morley, and by Fred Vine and Drummond Matthews, in 1963, [53] now called the Vine-Matthews-Morley hypothesis.

This hypothesis linked these patterns to geomagnetic reversals and was supported by several lines of evidence: [54]the stripes are symmetrical around the crests of the mid-ocean ridges; at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;the youngest rocks at the ridge crest always have present-day (normal) polarity;stripes of rock parallel to the ridge crest alternate in magnetic polarity (normal-reversed-normal, etc.

), suggesting that they were formed during different epochs documenting the (already known from independent studies) normal and reversal episodes of the Earth's magnetic field.

By explaining both the zebra-like magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis (SFS) quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural ""tape recording"" of the history of the geomagnetic field reversals (GMFR) of the Earth's magnetic field.

Today, extensive studies are dedicated to the calibration of the normal-reversal patterns in the oceanic crust on one hand and known timescales derived from the dating of basalt layers in sedimentary sequences ( magnetostratigraphy) on the other, to arrive at estimates of past spreading rates and plate reconstructions.

Definition and refining of the theory After all these considerations, Plate Tectonics (or, as it was initially called ""New Global Tectonics"") became quickly accepted in the scientific world, and numerous papers followed that defined the concepts: In 1965, Tuzo Wilson who had been a promotor of the sea floor spreading hypothesis and continental drift from the very beginning [55] added the concept of transform faults to the model, completing the classes of fault types necessary to make the mobility of the plates on the globe work out.

[56]A symposium on continental drift was held at the Royal Society of London in 1965 which must be regarded as the official start of the acceptance of plate tectonics by the scientific community, and which abstracts are issued as Blacket, Bullard & Runcorn (1965). In this symposium, Edward Bullard and co-workers showed with a computer calculation how the continents along both sides of the Atlantic would best fit to close the ocean, which became known as the famous ""Bullard's Fit"".

In 1966 Wilson published the paper that referred to previous plate tectonic reconstructions, introducing the concept of what is now known as the "" Wilson Cycle "".

[57]In 1967, at the American Geophysical Union 's meeting, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. [58]Two months later, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions, which marked the final acceptance by the scientific community of plate tectonics. [59]In the same year, Mc Kenzie and Parker independently presented a model similar to Morgan's using translations and rotations on a sphere to define the plate motions. [60]Plate Tectonics Revolution Main article: Plate Tectonics Revolution The Plate Tectonics Revolution was the scientific and cultural change which developed from the acceptance of the plate tectonics theory.

The event was a paradigm shift and scientific revolution. [61]Implications for biogeography Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present-day life found on different continents but having similar ancestors.

[62] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora. Plate reconstruction Main article: Plate reconstruction Reconstruction is used to establish past (and future) plate configurations, helping determine the shape and make-up of ancient supercontinents and providing a basis for paleogeography.

Magnetic stripe patterns provide a reliable guide to relative plate motions going back into the Jurassic period. [67] The tracks of hotspots give absolute reconstructions, but these are only available back to the Cretaceous. [68] Older reconstructions rely mainly on paleomagnetic pole data, although these only constrain the latitude and rotation, but not the longitude.

Combining poles of different ages in a particular plate to produce apparent polar wander paths provides a method for comparing the motions of different plates through time.

[69] Additional evidence comes from the distribution of certain sedimentary rock types, [70] faunal provinces shown by particular fossil groups, and the position of orogenic belts. [68]Formation and break-up of continents The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents.

The supercontinent Columbia or Nuna formed during a period of 2,000 to 1,800 million years ago and broke up about 1,500 to 1,300 million years ago.

[71] The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

The Himalayas, the world's tallest mountain range, are assumed to have been formed by the collision of two major plates. Before uplift, they were covered by the Tethys Ocean. Current plates Main article: List of tectonic plates Depending on how they are defined, there are usually seven or eight ""major"" plates: African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian.

The latter is sometimes subdivided into the Indian and Australian plates. There are dozens of smaller plates, the seven largest of which are the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine Sea, and Scotia.

The current motion of the tectonic plates is today determined by remote sensing satellite data sets, calibrated with ground station measurements. Other celestial bodies (planets, moons)The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics.

Earth may be a borderline case, owing its tectonic activity to abundant water [72] (silica and water form a deep eutectic ).

Dates derived are dominantly in the range 500 to 750 million years ago, although ages of up to 1,200 million years ago have been calculated.

This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages.

While the mechanism of such an impressive thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent. One explanation for Venus's lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.

[73] [74] The Earth's crust is soaked with water, and water plays an important role in the development of shear zones. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water.

However, some researchers [ who?] remain convinced that plate tectonics is or was once active on this planet. Mars See also: Geology of Mars Mars is considerably smaller than Earth and Venus, and there is evidence for ice on its surface and in its crust.

In the 1990s, it was proposed that Martian Crustal Dichotomy was created by plate tectonic processes.

[75] Scientists today disagree, and think that it was created either by upwelling within the Martian mantle that thickened the crust of the Southern Highlands and formed Tharsis [76] or by a giant impact that excavated the Northern Lowlands. [77]Valles Marineris may be a tectonic boundary. [78]Observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999 showed patterns of magnetic striping discovered on this planet.

Some scientists interpreted these as requiring plate tectonic processes, such as seafloor spreading. [79] However, their data fail a ""magnetic reversal test"", which is used to see if they were formed by flipping polarities of a global magnetic field.

[80]Icy satellites Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth.

On 8 September 2014, NASA reported finding evidence of plate tectonics on Europa, a satellite of Jupiter—the first sign of subduction activity on another world other than Earth. [81]Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens probe, which landed on Titan on January 14, 2005. [82]Exoplanets On Earth-sized planets, plate tectonics is more likely if there are oceans of water.

However, in 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths [83] [84] with one team saying that plate tectonics would be episodic or stagnant [85] and the other team saying that plate tectonics is very likely on super-earths even if the planet is dry. [72]Consideration of plate tectonics is a part of the search for extraterrestrial intelligence and extraterrestrial life.

[86]See also Atmospheric circulation Conservation of angular momentum Geological history of Earth – includes timeline of plate configurations Geosyncline concept GPlates, desktop software for the interactive visualization of plate-tectonics List of plate tectonics topics List of submarine topographical features Supercontinent cycle Tectonics References Notes^ Little, Fowler & Coulson 1990.^ Read & Watson 1975.^ Scalera & Lavecchia 2006.^ Zhen Shao 1997, Hancock, Skinner & Dineley 2000.^ Turcotte & Schubert 2002, p. 5.^ Turcotte & Schubert 2002.^ Foulger 2010.^ Schmidt & Harbert 1998.^ Meissner 2002, p. 100.^ ""Plate Tectonics: Plate Boundaries"". platetectonics.com. Archived from the original on 16 June 2010. Retrieved 12 June 2010.^ ""Understanding plate motions"". USGS. Retrieved 12 June 2010.^ Grove, Timothy L.; Till, Christy B.; Krawczynski, Michael J. (8 March 2012).

""The Role of H2O in Subduction Zone Magmatism"". Annual Review of Earth and Planetary Sciences. 40 (40): 413–39. Bibcode: 2012AREPS..40..413G. doi: 10.1146/annurev-earth-042711-105310. Retrieved 14 January 2016.^ Mendia-Landa, Pedro. ""Myths and Legends on Natural Disasters: Making Sense of Our World"". Retrieved 2008-02-05.^ van Dijk 1992, van Dijk & Okkes 1991.^ Holmes, Arthur (1931).

""Radioactivity and Earth Movements"" (PDF). Transactions of the Geological Society of Glasgow. Geological Society of Glasgow: 559–606.^ Tanimoto & Lay 2000.^ Meyerhoff et al. 1996.^ Mallard, Claire; Coltice, Nicolas; Seton, Maria; Müller, R. Dietmar; Tackley, Paul J. (2016).

""Subduction controls the distribution and fragmentation of Earth's tectonic plates"". Nature. 535 (7610): 140–43. Bibcode: 2016Natur.535..140M. doi: 10.1038/nature17992. ISSN 0028-0836.^ Spence 1987, White & Mc Kenzie 1989.^ a b Conrad & Lithgow-Bertelloni 2002.^ Spence 1987, White & Mckenzie 1989, Segev 2002.^ ""Alfred Wegener (1880–1930)"".

University of California Museum of Paleontology.^ Neith, Katie (April 15, 2011). ""Caltech Researchers Use GPS Data to Model Effects of Tidal Loads on Earth's Surface"". Caltech. Archived from the original on October 19, 2011. Retrieved August 15, 2012.^ a b Ricard, Y. (2009). ""2. Physics of Mantle Convection"". In David Bercovici; Gerald Schubert. Treatise on Geophysics: Mantle Dynamics.

7. Elsevier Science. p. 36. ISBN 978-0-444-53580-1.^ a b Glatzmaier, Gary A. (2013). Introduction to Modeling Convection in Planets and Stars: Magnetic Field, Density Stratification, Rotation. Princeton University Press. p. 149. ISBN 978-1-4008-4890-4.^ van Dijk 1992, van Dijk & Okkes 1990.^ Moore 1973.^ Bostrom 1971.^ Scoppola et al. 2006.^ Torsvik et al. 2010.^ [""Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling"", Rowley et al, Science Advances 23 Dec 2016: Vol.

2, no. 12, e1601107, doi: 10.1126/sciadv.1601107]^ a b Wegener 1929.^ Hughes 2001a.^ Wegener 1966, Hughes 2001b.^ a b Runcorn 1956.^ Carey 1956.^ see for example the milestone paper of Lyman & Fleming 1940.^ Korgen 1995, Spiess & Kuperman 2003.^ Kious & Tilling 1996.^ Frankel 1987.^ Joly 1909.^ Thomson 1863.^ Wegener 1912.^ ""Pioneers of Plate Tectonics"".

The Geological Society. Retrieved 23 March 2018.^ Stein & Wysession 2009, p. 26^ Carey 1956; see also Quilty 2003.^ Holmes 1928; see also Holmes 1978, Frankel 1978.^ Lippsett 2001, Lippsett 2006.^ Heezen 1960.^ Dietz 1961.^ Hess 1962.^ Mason & Raff 1961, Raff & Mason 1961.^ Vine & Matthews 1963.^ See summary in Heirzler, Le Pichon & Baron 1966^ Wilson 1963.^ Wilson 1965.^ Wilson 1966.^ Morgan 1968.^ Le Pichon 1967.^ Mc Kenzie & Parker 1967.^ Casadevall, Arturo; Fang, Ferric C. (1 March 2016).

""Revolutionary Science"". m Bio. 7 (2): e00158–16. doi: 10.1128/m Bio.00158-16.^ Moss & Wilson 1998.^ Condie 1997.^ Lliboutry 2000.^ Kranendonk, V.; Martin, J. (2011). ""Onset of Plate Tectonics"". Science. 333 (6041): 413–14. Bibcode: 2011Sci...333..413V. doi: 10.1126/science.1208766. PMID 21778389.^ Plate Tectonics May Have Begun a Billion Years After Earth's Birth Pappas, S Live Science report of PNAS research 21 Sept 2017^ Torsvik, Trond Helge.

""Reconstruction Methods"". Retrieved 18 June 2010.^ a b Torsvik 2008.^ Butler 1992.^ Scotese, C. R. (2002-04-20). ""Climate History"". Paleomap Project. Retrieved 18 June 2010.^ Zhao 2002, 2004^ a b Valencia, O'Connell & Sasselov 2007.^ Kasting 1988.^ Bortman, Henry (2004-08-26).

""Was Venus alive? ""The Signs are Probably There "" "". Astrobiology Magazine. Retrieved 2008-01-08.^ Sleep 1994.^ Zhong & Zuber 2001.^ Andrews-Hanna, Zuber & Banerdt 2008.^ Wolpert, Stuart (August 9, 2012).

""UCLA scientist discovers plate tectonics on Mars"". Yin, An. UCLA. Retrieved August 13, 2012.^ Connerney et al. 1999, Connerney et al. 2005^ Harrison 2000.^ Dyches, Preston; Brown, Dwayne; Buckley, Michael (8 September 2014). ""Scientists Find Evidence of 'Diving' Tectonic Plates on Europa"". NASA. Retrieved 8 September 2014.^ Soderblom et al. 2007.^ Valencia, Diana; O'Connell, Richard J. (2009). ""Convection scaling and subduction on Earth and super-Earths"".

Paleomagnetism: Magnetic domains to geologic terranes (PDF). Blackwell. ISBN 0-86542-070-X. Archived from the original (PDF) on 17 August 2010. Retrieved 18 June 2010. Carey, S. W. (1958). ""The tectonic approach to continental drift"". In Carey, S. W. Continental Drift – A symposium, held in March 1956.

Hobart: Univ. of Tasmania. pp. 177–363. Expanding Earth from pp. 311–49. Condie, K. C. (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 282. ISBN 978-0-7506-3386-4. Retrieved 2010-06-18. Foulger, Gillian R. (2010). Plates vs Plumes: A Geological Controversy. Wiley-Blackwell.

ISBN 978-1-4051-6148-0. Frankel, H. (1987). ""The Continental Drift Debate"". In H. T. Engelhardt Jr; A. L. Caplan. Scientific Controversies: Case Studies in the Resolution and Closure of Disputes in Science and Technology. Cambridge University Press. ISBN 978-0-521-27560-6. Hancock, Paul L.; Skinner, Brian J.; Dineley, David L. (2000).

The Oxford Companion to The Earth. Oxford University Press. ISBN 0-19-854039-6. Hess, H. H. (November 1962). ""History of Ocean Basins"" (PDF). In A. E. J. Engel; Harold L. James; B. F. Leonard. Petrologic studies: a volume to honor of A. F. Buddington.

Boulder, CO: Geological Society of America. pp. 599–620. Holmes, Arthur (1978). Principles of Physical Geology (3 ed.). Wiley. pp. 640–41. ISBN 0-471-07251-6. Joly, John (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History.

London: Archibald Constable. p. 36. ISBN 1-4021-3577-7. Kious, W. Jacquelyne; Tilling, Robert I. (February 2001) [1996]. ""Historical perspective"". This Dynamic Earth: the Story of Plate Tectonics (Online ed.). U. S. Geological Survey. ISBN 0-16-048220-8. Retrieved 2008-01-29. Abraham Ortelius in his work Thesaurus Geographicus... suggested that the Americas were 'torn away from Europe and Africa... by earthquakes and floods...

The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]. 'Lippsett, Laurence (2006). ""Maurice Ewing and the Lamont-Doherty Earth Observatory"". In William Theodore De Bary; Jerry Kisslinger; Tom Mathewson. Living Legacies at Columbia. Columbia University Press. pp. 277–97. ISBN 0-231-13884-9. Retrieved 2010-06-22. Little, W.; Fowler, H. W.; Coulson, J. (1990). Onions C. T., ed. The Shorter Oxford English Dictionary: on historical principles.

II (3 ed.). Clarendon Press. ISBN 978-0-19-861126-4. Lliboutry, L. (2000). Quantitative geophysics and geology. Springer. p. 480. ISBN 978-1-85233-115-3. Retrieved 2010-06-18. Mc Knight, Tom (2004). Geographica: The complete illustrated Atlas of the world. New York: Barnes and Noble Books. ISBN 0-7607-5974-X. Meissner, Rolf (2002). The Little Book of Planet Earth. New York: Copernicus Books. p. 202. ISBN 978-0-387-95258-1. Meyerhoff, Arthur Augustus; Taner, I.; Morris, A. E. L.; Agocs, W. B.; Kamen-Kaye, M.; Bhat, Mohammad I.; Smoot, N. Christian; Choi, Dong R. (1996).

Donna Meyerhoff Hull, ed. Surge tectonics: a new hypothesis of global geodynamics. Solid Earth Sciences Library. 9. Springer Netherlands. p. 348. ISBN 978-0-7923-4156-7. Moss, S. J.; Wilson, M. E. J. (1998). ""Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo"" (PDF). In Hall R; Holloway JD. Biogeography and Geological Evolution of SE Asia (PDF). Leiden, The Netherlands: Backhuys. pp. 133–63. ISBN 90-73348-97-8. Archived from the original on 2008-02-16. Oreskes, Naomi, ed. (2003).

Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview. ISBN 0-8133-4132-9. Read, Herbert Harold; Watson, Janet (1975). Introduction to Geology. New York: Halsted. pp. 13–15.

ISBN 978-0-470-71165-1. OCLC 317775677. Schmidt, Victor A.; Harbert, William (1998). ""The Living Machine: Plate Tectonics"". Planet Earth and the New Geosciences (3 ed.). p. 442. ISBN 0-7872-4296-9. Archived from the original on 2010-01-24. Retrieved 2008-01-28. Schubert, Gerald; Turcotte, Donald L.; Olson, Peter (2001). Mantle Convection in the Earth and Planets. Cambridge: Cambridge University Press. ISBN 0-521-35367-X. Stanley, Steven M. (1999). Earth System History. W. H. Freeman. pp. 211–28. ISBN 0-7167-2882-6. Stein, Seth; Wysession, Michael (2009). An Introduction to Seismology, Earthquakes, and Earth Structure.

Chichester: John Wiley & Sons. ISBN 978-1-4443-1131-0. Sverdrup, H. U., Johnson, M. W. and Fleming, R. H. (1942). The Oceans: Their physics, chemistry and general biology. Englewood Cliffs: Prentice-Hall. p. 1087. Thompson, Graham R. & Turk, Jonathan (1991). Modern Physical Geology. Saunders College Publishing. ISBN 0-03-025398-5. Torsvik, Trond Helge; Steinberger, Bernhard (December 2006). ""Fra kontinentaldrift til manteldynamikk"" [From Continental Drift to Mantle Dynamics].

Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. ISBN 3-443-01056-3. Wegener, Alfred (1966). The origin of continents and oceans. Biram John (translator). Courier Dover. p. 246. ISBN 0-486-61708-4. Winchester, Simon (2003). Krakatoa: The Day the World Exploded: August 27, 1883. Harper Collins. ISBN 0-06-621285-5. Cited articles Andrews-Hanna, Jeffrey C.; Zuber, Maria T.; Banerdt, W. Bruce (2008).

""The Borealis basin and the origin of the martian crustal dichotomy"". Nature. 453 (7199): 1212–15. Bibcode: 2008Natur.453.1212A. doi: 10.1038/nature07011. PMID 18580944. Blacket, P. M. S.; Bullard, E.; Runcorn, S. K., eds.

(1965). A Symposium on Continental Drift, held in 28 October 1965. Philosophical Transactions of the Royal Society A. 258. The Royal Society of London. p. 323. Bostrom, R. C. (31 December 1971). ""Westward displacement of the lithosphere"". Nature. 234 (5331): 536–38. Bibcode: 1971Natur.234..536B. doi: 10.1038/234536a0. Connerney, J. E. P.; Acuña, M. H.; Wasilewski, P. J.; Ness, N. F.; Rème H.; Mazelle C.; Vignes D.; Lin R. P.; Mitchell D. L.; Cloutier P. A. (1999). ""Magnetic Lineations in the Ancient Crust of Mars"". Science. 284 (5415): 794–98. Bibcode: 1999Sci...284..794C.

doi: 10.1126/science.284.5415.794. PMID 10221909. Connerney, J. E. P.; Acuña, M. H.; Ness, N. F.; Kletetschka, G.; Mitchell D. L.; Lin R. P.; Rème H. (2005). ""Tectonic implications of Mars crustal magnetism"". Proceedings of the National Academy of Sciences. 102 (42): 14970–175. Bibcode: 2005PNAS..10214970C. doi: 10.1073/pnas.0507469102. PMC 1250232. PMID 16217034. Conrad, Clinton P.; Lithgow-Bertelloni, Carolina (2002). ""How Mantle Slabs Drive Plate Tectonics"".

Science. 298 (5591): 207–09. Bibcode: 2002Sci...298..207C. doi: 10.1126/science.1074161. PMID 12364804. Archived from the original on September 20, 2009. Dietz, Robert S. (June 1961). ""Continent and Ocean Basin Evolution by Spreading of the Sea Floor"".

Nature. 190 (4779): 854–57. Bibcode: 1961Natur.190..854D. doi: 10.1038/190854a0.van Dijk, Janpieter; Okkes, F. W. Mark (1990). ""The analysis of shear zones in Calabria; implications for the geodynamics of the Central Mediterranean"". Rivista Italiana di Paleontologia e Stratigrafia. 96 (2–3): 241–70.van Dijk, J. P.; Okkes, F. W. M. (1991). ""Neogene tectonostratigraphy and kinematics of Calabrian Basins: implications for the geodynamics of the Central Mediterranean"".

Tectonophysics. 196: 23–60. Bibcode: 1991Tectp.196...23V. doi: 10.1016/0040-1951 (91)90288-4.van Dijk, Janpieter (1992). ""Late Neogene fore-arc basin evolution in the Calabrian Arc (Central Mediterranean). Tectonic sequence stratigraphy and dynamic geohistory. With special reference to the geology of Central Calabria"". Geologica Ultraiectina. 92: 288. Archived from the original on 2013-04-20.

Frankel, Henry (July 1978). ""Arthur Holmes and continental drift"". The British Journal for the History of Science. 11 (2): 130–50. doi: 10.1017/S0007087400016551. JSTOR 4025726. Harrison, C. G. A. (2000). ""Questions About Magnetic Lineations in the Ancient Crust of Mars"". Science. 287 (5453): 547a. doi: 10.1126/science.287.5453.547a. Heezen, B. (1960). ""The rift in the ocean floor"".

Scientific American. 203 (4): 98–110. Bibcode: 1960Sci Am.203d..98H. doi: 10.1038/scientificamerican1060-98. Heirtzler, James R.; Le Pichon, Xavier; Baron, J. Gregory (1966). ""Magnetic anomalies over the Reykjanes Ridge"". Deep-Sea Research. 13 (3): 427–32. Bibcode: 1966DSROA..13..427H. doi: 10.1016/0011-7471 (66)91078-3. Holmes, Arthur (1928). ""Radioactivity and Earth movements"". Transactions of the Geological Society of Glasgow.

18: 559–606. Hughes, Patrick (8 February 2001). ""Alfred Wegener (1880–1930): A Geographic Jigsaw Puzzle"". On the shoulders of giants. Earth Observatory, NASA. Retrieved 2007-12-26. ... on January 6, 1912, Wegener... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography.

Hughes, Patrick (8 February 2001). ""Alfred Wegener (1880–1930): The origin of continents and oceans"".

On the Shoulders of Giants. Earth Observatory, NASA. Retrieved 2007-12-26. By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it Pangaea (all lands),... Kasting, James F. (1988). ""Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus"".

Icarus. 74 (3): 472–94. Bibcode: 1988Icar...74..472K. doi: 10.1016/0019-1035 (88)90116-9. PMID 11538226. Korgen, Ben J. (1995). ""A voice from the past: John Lyman and the plate tectonics story"" (PDF). Oceanography. The Oceanography Society. 8 (1): 19–20. doi: 10.5670/oceanog.1995.29. Archived from the original (PDF) on 2007-09-26. Lippsett, Laurence (2001). ""Maurice Ewing and the Lamont-Doherty Earth Observatory"". Living Legacies. Retrieved 2008-03-04. Lovett, Richard A (24 January 2006). ""Moon Is Dragging Continents West, Scientist Says"". National Geographic News. Lyman, J.; Fleming, R. H. (1940). ""Composition of Seawater"". Journal of Marine Research.

Le Pichon, Xavier (15 June 1968). ""Sea-floor spreading and continental drift"". Journal of Geophysical Research. 73 (12): 3661–97. Bibcode: 1968JGR....73.3661L. doi: 10.1029/JB073i012p03661. Quilty, Patrick G.; Banks, Maxwell R. (2003). ""Samuel Warren Carey, 1911–2002"". Biographical memoirs. Australian Academy of Science. Archived from the original on 2010-12-21.

Retrieved 2010-06-19. This memoir was originally published in Historical Records of Australian Science (2003) 14 (3). Raff, Arthur D.; Mason, Roland G. (1961). ""Magnetic survey off the west coast of the United States between 40°N latitude and 52°N latitude"".

Bulletin of the Geological Society of America. 72 (8): 1267–70. Bibcode: 1961GSAB...72.1267R. doi: 10.1130/0016-7606 (1961)72 [1267: MSOTWC]2.0. CO;2. ISSN 0016-7606. Runcorn, S. K. (1956). ""Paleomagnetic comparisons between Europe and North America"". Proceedings, Geological Association of Canada. 8 (1088): 7785. Bibcode: 1965RSPTA.258....1R. doi: 10.1098/rsta.1965.0016. Scalera, G. & Lavecchia, G. (2006). ""Frontiers in earth sciences: new ideas and interpretation"".

Annals of Geophysics. 49 (1). doi: 10.4401/ag-4406 (inactive 2017-01-16). Scoppola, B.; Boccaletti, D.; Bevis, M.; Carminati, E.; Doglioni, C. (2006). ""The westward drift of the lithosphere: A rotational drag?"". Geological Society of America Bulletin.

118: 199–209. Bibcode: 2006GSAB..118..199S. doi: 10.1130/B25734.1. Segev, A (2002). ""Flood basalts, continental breakup and the dispersal of Gondwana: evidence for periodic migration of upwelling mantle flows (plumes)"" (PDF). EGU Stephan Mueller Special Publication Series. 2: 171–91. doi: 10.5194/smsps-2-171-2002. Retrieved 5 August 2010. Sleep, Norman H. (1994). ""Martian plate tectonics"" (PDF). Journal of Geophysical Research. 99: 5639.

Bibcode: 1994JGR....99.5639S. doi: 10.1029/94JE00216. Soderblom, Laurence A.; Tomasko, Martin G.; Archinal, Brent A.; Becker, Tammy L.; Bushroe, Michael W.; Cook, Debbie A.; Doose, Lyn R.; Galuszka, Donna M.; Hare, Trent M.; Howington-Kraus, Elpitha; Karkoschka, Erich; Kirk, Randolph L.; Lunine, Jonathan I.; Mc Farlane, Elisabeth A.; Redding, Bonnie L.; Rizk, Bashar; Rosiek, Mark R.; See, Charles; Smith, Peter H. (2007).

Two types of continental margins exist: active margins and passive margins. Active margins form primarily along the boundaries of plates that are actively converging. Passive margins currently exist in the middle of plates, not at plate boundaries.

Continental Margin

"Continental Margin Follow City-Data.com founder on our Forum or @Lech Mazur Photo by: Kwest They are the drowned edges of continents. Covered by the oceans, continental margins are part of the same crust (thin, solid outermost layer of Earth) that forms the continents.

They are covered in sediments that have been eroded from dry land. (Erosion is the gradual wearing away of Earth surfaces through the action of wind and water.) Lying between the deep ocean basins and the above-water land areas, continental margins account for 11 percent of Earth's surface.

The shape of the land The continental margin is the submerged outer edge of a continent. It is generally divided into two sections: the continental shelf and the continental slope. The continental shelf is the region that extends seaward from the shoreline to a sharp drop-off that marks the beginning of the continental slope. That drop-off is known as the continental shelf break. Continental shelves vary in width from 3 to 930 miles (5 to 1,500 kilometers).

The average width worldwide is about 40 miles (64 kilometers). The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Narrow shelves are found off the western coasts of North and South America. Continental shelves along the coasts of the United States cover an area of about 891,000 square miles (2,307,690 square kilometers). Continental shelves are normally gently sloping, with an average seaward slope of about 0.1 degree.

They tend to have the same topography or surface features that dominate the adjacent land. Whether a coastal area is mountainous, dominated by low hills, or flat, the continental shelf next to it will be similarly shaped. The average depth of the continental shelf at the shelf break is about 430 feet (131 meters) below the surface of the ocean. Basic composition of continental margins, which include the continental shelf, continental slope, and continental rise.

In contrast to the continental shelf, the continental slope is generally narrow in width, ranging from 6.2 to 62 miles (10 to 100 kilometers).

Relatively steeper than the shelf, the slope angles down to the ocean basin at an average of 4 degrees; in some areas, the angle may be as much as 25 degrees. The steepness of a slope often reflects the steepness of the nearby coastal area. Slopes along mountainous coasts are steeper than those along flat coasts.

In general, the steepest slopes tend to be found in the Pacific Ocean and the least steep slopes in the Atlantic and Indian Oceans.

The continental slope marks the transition between continental crust and oceanic crust. Continental crust is composed mostly of granite, whereas oceanic crust is mostly basalt. (Although they differ in composition, both are types of igneous rock, which forms when magma cools and solidifies. Granite forms when magma with a high silica content cools slowly deep beneath Earth's surface; basalt forms when magma with a low silica content cools quickly outside of or very near Earth's surface.

)The most distinctive features of continental slopes are submarine canyons. Typically, these are steep-walled, V-shaped canyons that may be thousands of feet deep. The incline of a canyon is normally related to that of the slope: steep slopes have short, steep canyons, while broader slopes have longer, shallower canyons. The deepest of the submarine canyons easily rival the size of the Grand Canyon on the Colorado River in Arizona. Sometimes, submarine canyons begin at the outer edge of the continental shelf. Most often, they form down-current from major rivers that flow into an ocean.

Geologists believe that some submarine canyons may have been carved during a time in Earth's past when sea levels were lower than at present, and rivers were able to flow out to the edge of the shelf. Other canyons may have formed when earthquakes created faults or cracks along slopes that were subsequently eroded into the canyons. Continental margin: Words to Know Accretionary wedge: A mass of sediment and oceanic rock that is transferred from an oceanic plate to the edge of the less dense plate under which it is subducting.

Active continental margin: A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope that ends in a deep trench instead of a continental rise; it is marked by earthquake and volcanic activity.

Asthenosphere: The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock. Continental drift: The hypothesis proposed by Alfred Wegener that the continents are not stationary, but have moved across the surface of Earth over time. Continental margin: The submerged outer edge of a continent, composed of the continental shelf and the continental slope.

Continental rise: The gently sloping, smooth-surfaced, thick accumulation of sediment at the base of certain continental slopes. Continental shelf: The gently sloping region of the continental margin that extends seaward from the shoreline to the continental shelf break.

Continental shelf break: The outer edge of the continental shelf at which there is a sharp drop-off to the steeper continental slope. Continental slope: The steeply sloping region of the continental margin that extends from the continental shelf break downward to the ocean basin.

Convection current: The circular movement of a gas or liquid between hot and cold areas. Crust: The thin, solid outermost layer of Earth. Erosion: The gradual wearing away of Earth surfaces through the action of wind and water. Lithosphere: The rigid uppermost section of the mantle combined with the crust.

Mantle: The thick, dense layer of rock that lies beneath Earth's crust. Passive continental margin: A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise; it is marked by a lack of earthquake and volcanic activity.

Plates: Large sections of Earth's lithosphere separated by deep fault zones. Plate tectonics: The geologic theory that Earth's crust is composed of rigid plates that ""float"" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.

Seafloor spreading: The process by which new oceanic crust is formed by the upwelling of magma at mid-ocean ridges, resulting in the continuous lateral movement of existing oceanic crust. Subduction zone: A region where two plates come together and the edge of one plate slides beneath the other.

Submarine canyon: A steep-walled, V-shaped canyon that is cut into the rocks and sediments of the continental slope and, sometimes, the outer continental shelf. Trench: A long, deep, narrow depression on the ocean basin with relatively steep sides. Turbidity current: A turbulent mixture of water and sediment that flows down a continental slope under the influence of gravity.

Geologists know that underwater currents called turbidity currents have eroded most submarine canyons.

These turbulent mixtures of water and sediment develop near the continental shelf break when some event, such as an earthquake or a violent storm, triggers their formation. Denser (heavier) than the surrounding water, the currents are pulled downward by gravity. Flowing down the continental slope like an avalanche at up to 50 miles (80 kilometers) per hour, the sediment-laden currents surge through the canyons, scouring their sides.

When the currents reach the ocean basin, they slow, and the sediment they carry falls to the bottom in a fanlike deposit, much like an alluvial fan that forms in a desert environment when water flows out of a canyon onto a plain or flat area.

(For further information on alluvial fans, see the Dune and other desert features chapter. )At the base of certain continental slopes is a gently sloping, smooth-surfaced, thick accumulation of sediment from land that had been transported to the shelf and then down the slope. This transition between the continental slope and the ocean basin is known as the continental rise.

Some sources include the continental rise as being a major section of the continental margin, along with the shelf and slope. Since it does not occur on all continental margins around the planet, it is best considered a characteristic of the margin in certain areas. Continental rises are well-developed around Antarctica and in the Atlantic and Indian Oceans. They are hardly found in the Pacific. When they are present, continental rises vary in width from 62 to 620 miles (100 to 1,000 kilometers).

A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise is known as a passive continental margin. This type of margin experiences little, if any, volcanic or earthquake activity. The build-up of sediment is the primary activity affecting a passive margin.

Because these margins are found along the east coasts of North and South America and the west coasts of Europe and Africa, they are also known as ""Atlantic-type"" margins.

The continental margins along India and Antarctica are other examples of passive margins. A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope is known as an active continental margin. Instead of ending in a continental rise, the continental slope of this type of margin often plunges into a deep-ocean trench, which may be filled with sediment.

Earthquake and volcanic activity are prominent here. Since active continental margins occur along many coasts of the Pacific Ocean, these types of margins are also known as ""Pacific-type"" margins. Forces and changes: Construction and destruction The two types of continental margins, passive and active, tell about the geologic history of Earth and the activities that are continually affecting its surface.

The continents are not stationary, but move about the planet's surface. That movement varies, but in general is extremely slow, only about 2 inches (5 centimeters) per year.

Over millions of years, however, the continents have made their way along on an endless journey across the planet's surface, repeatedly crashing into or breaking away from one another. A Burial and Dumping Ground Continental shelves have not always been covered by water. During Earth's history, changes in sea level have alternately exposed, then covered portions of the shelves.

Scientists estimate that during the last glacial period over 10,000 years ago, much of Earth's water was trapped in the polar ice sheets. The level of the oceans may have dropped as much as 350 feet (107 meters) below the current level. At times of low sea level, land plants and animals, including humans and their ancestors, lived on the shelves.

Evidence of this lies in their remains that are often found there in the present day. Twelve-thousand-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States. Because water above continental shelves is not that deep, sunlight is able to penetrate, helping plants grow.

This, in turn, leads to a rich web of sea life. Most commercial fishing takes place in these waters. Extensive deposits of oil, natural gas, minerals, and other natural resources lie beneath continental shelves. The economic benefit of the fish and natural resources is important to many nations, which claim territorial ownership of the continental shelves adjacent to their land areas. Many political disagreements have arisen because of this. In spite of their desire to reap the benefits contained over and in the shelves, many nations illegally dump much of their waste in the ocean over these areas.

The drifting continents In 1915 German geophysicist Alfred Wegener (1880–1930) published a book in which he presented geological evidence that all the continents had once been joined together in a supercontinent he called Pangaea (pronounced pan-JEE-ah; from the Greek words meaning ""all lands"").

Wegener suggested that the Atlantic Ocean and the Indian Ocean formed when this supercontinent broke apart and the continents drifted away from each other. He called his hypothesis continental drift. (A hypothesis is an educated guess, while a theory is a principle supported by extensive scientific evidence and testing.

)Wegener formed his hypothesis after he had observed that the present-day continental margins along some of the continents seemed to fit together like pieces in a jigsaw puzzle: eastern South America with western Africa, eastern North America with western Europe, and India and Antarctica with eastern Africa. When the continents are linked, some of their geological features, such as mountain ranges and mineral deposits, also match.

In addition, related species of land animals are often present on both side of the present-day oceans. Wegener argued further that if the continents had drifted, they would have passed through various climate zones.

This explains how evidence of past glaciers could be found in the Sahara desert region and why fossil coral reefs appear north of the Arctic Circle. The theory of plate tectonics What Wegener lacked, however, was a convincing explanation as to what moved the continents along the surface.

Evidence to support his hypothesis did not come until the early 1960s when geologists developed the theory of plate tectonics (from the Greek word tekton, meaning ""builder""). A revolutionary idea, it helps geologists and others understand how Earth has changed over long periods of time. Changes in the positions and features of the continents and oceans have had a profound effect on everything from global climate to the evolution of life.

Simply, the theory states that the surface of the planet is broken into sections—some large, some small—called tectonic plates. As these plates drift slowly over Earth, they slide past, collide with, and move away from each other. The boundaries where the plates meet and interact are called plate margins. What moves the plates along occurs within the planet.

Geologists divide Earth into three distinct layers: the crust, the mantle, and the core. Each layer has its own unique properties and composition.

As already mentioned, the crust is the thin shell of rock that covers Earth. Two types of crust exist: continental crust, which underlies the continents, and oceanic crust, which underlies the oceans. Varying in thickness, the crust is thickest below land and thinnest below the oceans. Underneath the crust is the mantle, which is separated into two layers: The uppermost part of the mantle is solid.

Along with the overlying crust, it forms what is called the lithosphere (pronounced LITH-uh-sfeer). It is the brittle lithosphere that has broken into the tectonic plates. Under the lithosphere is the part of the mantle known as the asthenosphere (pronounced as-THEN-uh-sfeer). Beneath Earth's surface, temperature and pressure increase with increasing depths. Rock in the asthenosphere is hot Cross-section of Earth's interior, with the solid inner core, molten outer core, mantle, and crust.enough to fold, stretch, compress, and flow very slowly without fracturing.

It is puttylike in its consistency, or what geologists call ""plastic."" The rigid tectonic plates ""float"" on the more dense, flowing asthenosphere.

The core, lying at the center of the planet, is divided into a liquid outer layer and a solid inner layer. Made up of the metallic elements iron and nickel, the core is almost five times as dense as rock on Earth's surface.

Temperatures in the core are estimated to exceed 9,900°F (5,482°C), creating extreme heat energy. Moving the plates This heat energy moves the tectonic plates across the planet's surface. It is carried to the area beneath the plates by convection currents, which act similar to the currents produced in a pot of boiling liquid on a hot stove.

When a liquid in a pot begins to boil, it turns over and over. Liquid heated at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense (lighter).

Once at the surface, the heated material cools and becomes dense once more. It then sinks back down to the bottom to become reheated. This continuous motion of heated material rising, cooling, and sinking forms the circular convection currents.

Like a gigantic furnace, the core heats the mantle rock immediately above it. Expanding and becoming less dense, the heated rock slowly rises through cooler, denser mantle rock. When the heated rock reaches the lithosphere, it moves along its base, exerting dragging forces on the tectonic plates.

This causes the plates to move. In the process, the heated rock begins to lose heat. Cooling and becoming denser, the rock then sinks back toward the core, where it will be heated once more. It takes an estimated 200 million years for heated mantle rock to make the circular trip from the core to the lithosphere and back again.

As the plates move, they interact in several different ways. Where they diverge, or move away from each other, new lithospheric rock is created as magma rises up through the crust at the plate boundary and gradually begins to cool.

Many boundaries of diverging plates are on the floors of the oceans. They are known as mid-ocean ridges, and the process of new ocean crust forming at the ridges is known as seafloor spreading. Where plates converge, or move into one another, either they crumple up and compress or one plate slides beneath the other.

When two continental (land) plates converge, the crust bends and breaks from the collision, forming complex mountain ranges and very high plateaus. When a continental plate and an oceanic plate converge, the oceanic plate (which is thinner, yet denser) bends and plunges at an angle into the asthenosphere beneath the continental plate. As it does so, its leading edge begins to melt because of high temperature and pressure in the mantle.

At about 10 percent of the plate boundaries, the plates neither diverge nor converge; instead, they transform or slide past one another.

On land, such a boundary is called a slip-strike boundary. A classic example of such a boundary is the San Andreas Fault system, an area of cracks in Earth's surface that occurs at the boundary between two tectonic plates, which extends down the coast of California.

Passive continental margins The passive continental margins of eastern North America, eastern South America, western Africa, and western Europe began to form about 200 million years ago when Pangaea began to break up. The rift or crack that caused them to split, known as the Mid-Atlantic Ridge, now lies on

Two types of continental margins exist: active margins and passive margins. Active margins form primarily along the boundaries of plates that are actively converging. Passive margins currently exist in the middle of plates, not at plate boundaries.

The continental margins along India and Antarctica are other examples of passive margins. A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope is known as an active continental margin. Instead of ending in a continental rise, the continental slope of this type of margin often plunges into a deep-ocean trench, which may be filled with sediment.

Continental Margin

"Continental Margin Follow City-Data.com founder on our Forum or @Lech Mazur Photo by: Kwest They are the drowned edges of continents. Covered by the oceans, continental margins are part of the same crust (thin, solid outermost layer of Earth) that forms the continents.

They are covered in sediments that have been eroded from dry land. (Erosion is the gradual wearing away of Earth surfaces through the action of wind and water.) Lying between the deep ocean basins and the above-water land areas, continental margins account for 11 percent of Earth's surface.

The shape of the land The continental margin is the submerged outer edge of a continent. It is generally divided into two sections: the continental shelf and the continental slope. The continental shelf is the region that extends seaward from the shoreline to a sharp drop-off that marks the beginning of the continental slope. That drop-off is known as the continental shelf break. Continental shelves vary in width from 3 to 930 miles (5 to 1,500 kilometers).

The average width worldwide is about 40 miles (64 kilometers). The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Narrow shelves are found off the western coasts of North and South America. Continental shelves along the coasts of the United States cover an area of about 891,000 square miles (2,307,690 square kilometers). Continental shelves are normally gently sloping, with an average seaward slope of about 0.1 degree.

They tend to have the same topography or surface features that dominate the adjacent land. Whether a coastal area is mountainous, dominated by low hills, or flat, the continental shelf next to it will be similarly shaped. The average depth of the continental shelf at the shelf break is about 430 feet (131 meters) below the surface of the ocean. Basic composition of continental margins, which include the continental shelf, continental slope, and continental rise.

In contrast to the continental shelf, the continental slope is generally narrow in width, ranging from 6.2 to 62 miles (10 to 100 kilometers).

Relatively steeper than the shelf, the slope angles down to the ocean basin at an average of 4 degrees; in some areas, the angle may be as much as 25 degrees. The steepness of a slope often reflects the steepness of the nearby coastal area. Slopes along mountainous coasts are steeper than those along flat coasts.

In general, the steepest slopes tend to be found in the Pacific Ocean and the least steep slopes in the Atlantic and Indian Oceans.

The continental slope marks the transition between continental crust and oceanic crust. Continental crust is composed mostly of granite, whereas oceanic crust is mostly basalt. (Although they differ in composition, both are types of igneous rock, which forms when magma cools and solidifies. Granite forms when magma with a high silica content cools slowly deep beneath Earth's surface; basalt forms when magma with a low silica content cools quickly outside of or very near Earth's surface.

)The most distinctive features of continental slopes are submarine canyons. Typically, these are steep-walled, V-shaped canyons that may be thousands of feet deep. The incline of a canyon is normally related to that of the slope: steep slopes have short, steep canyons, while broader slopes have longer, shallower canyons. The deepest of the submarine canyons easily rival the size of the Grand Canyon on the Colorado River in Arizona. Sometimes, submarine canyons begin at the outer edge of the continental shelf. Most often, they form down-current from major rivers that flow into an ocean.

Geologists believe that some submarine canyons may have been carved during a time in Earth's past when sea levels were lower than at present, and rivers were able to flow out to the edge of the shelf. Other canyons may have formed when earthquakes created faults or cracks along slopes that were subsequently eroded into the canyons. Continental margin: Words to Know Accretionary wedge: A mass of sediment and oceanic rock that is transferred from an oceanic plate to the edge of the less dense plate under which it is subducting.

Active continental margin: A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope that ends in a deep trench instead of a continental rise; it is marked by earthquake and volcanic activity.

Asthenosphere: The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock. Continental drift: The hypothesis proposed by Alfred Wegener that the continents are not stationary, but have moved across the surface of Earth over time. Continental margin: The submerged outer edge of a continent, composed of the continental shelf and the continental slope.

Continental rise: The gently sloping, smooth-surfaced, thick accumulation of sediment at the base of certain continental slopes. Continental shelf: The gently sloping region of the continental margin that extends seaward from the shoreline to the continental shelf break.

Continental shelf break: The outer edge of the continental shelf at which there is a sharp drop-off to the steeper continental slope. Continental slope: The steeply sloping region of the continental margin that extends from the continental shelf break downward to the ocean basin.

Convection current: The circular movement of a gas or liquid between hot and cold areas. Crust: The thin, solid outermost layer of Earth. Erosion: The gradual wearing away of Earth surfaces through the action of wind and water. Lithosphere: The rigid uppermost section of the mantle combined with the crust.

Mantle: The thick, dense layer of rock that lies beneath Earth's crust. Passive continental margin: A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise; it is marked by a lack of earthquake and volcanic activity.

Plates: Large sections of Earth's lithosphere separated by deep fault zones. Plate tectonics: The geologic theory that Earth's crust is composed of rigid plates that ""float"" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.

Seafloor spreading: The process by which new oceanic crust is formed by the upwelling of magma at mid-ocean ridges, resulting in the continuous lateral movement of existing oceanic crust. Subduction zone: A region where two plates come together and the edge of one plate slides beneath the other.

Submarine canyon: A steep-walled, V-shaped canyon that is cut into the rocks and sediments of the continental slope and, sometimes, the outer continental shelf. Trench: A long, deep, narrow depression on the ocean basin with relatively steep sides. Turbidity current: A turbulent mixture of water and sediment that flows down a continental slope under the influence of gravity.

Geologists know that underwater currents called turbidity currents have eroded most submarine canyons.

These turbulent mixtures of water and sediment develop near the continental shelf break when some event, such as an earthquake or a violent storm, triggers their formation. Denser (heavier) than the surrounding water, the currents are pulled downward by gravity. Flowing down the continental slope like an avalanche at up to 50 miles (80 kilometers) per hour, the sediment-laden currents surge through the canyons, scouring their sides.

When the currents reach the ocean basin, they slow, and the sediment they carry falls to the bottom in a fanlike deposit, much like an alluvial fan that forms in a desert environment when water flows out of a canyon onto a plain or flat area.

(For further information on alluvial fans, see the Dune and other desert features chapter. )At the base of certain continental slopes is a gently sloping, smooth-surfaced, thick accumulation of sediment from land that had been transported to the shelf and then down the slope. This transition between the continental slope and the ocean basin is known as the continental rise.

Some sources include the continental rise as being a major section of the continental margin, along with the shelf and slope. Since it does not occur on all continental margins around the planet, it is best considered a characteristic of the margin in certain areas. Continental rises are well-developed around Antarctica and in the Atlantic and Indian Oceans. They are hardly found in the Pacific. When they are present, continental rises vary in width from 62 to 620 miles (100 to 1,000 kilometers).

A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise is known as a passive continental margin. This type of margin experiences little, if any, volcanic or earthquake activity. The build-up of sediment is the primary activity affecting a passive margin.

Because these margins are found along the east coasts of North and South America and the west coasts of Europe and Africa, they are also known as ""Atlantic-type"" margins.

The continental margins along India and Antarctica are other examples of passive margins. A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope is known as an active continental margin. Instead of ending in a continental rise, the continental slope of this type of margin often plunges into a deep-ocean trench, which may be filled with sediment.

Earthquake and volcanic activity are prominent here. Since active continental margins occur along many coasts of the Pacific Ocean, these types of margins are also known as ""Pacific-type"" margins. Forces and changes: Construction and destruction The two types of continental margins, passive and active, tell about the geologic history of Earth and the activities that are continually affecting its surface.

The continents are not stationary, but move about the planet's surface. That movement varies, but in general is extremely slow, only about 2 inches (5 centimeters) per year.

Over millions of years, however, the continents have made their way along on an endless journey across the planet's surface, repeatedly crashing into or breaking away from one another. A Burial and Dumping Ground Continental shelves have not always been covered by water. During Earth's history, changes in sea level have alternately exposed, then covered portions of the shelves.

Scientists estimate that during the last glacial period over 10,000 years ago, much of Earth's water was trapped in the polar ice sheets. The level of the oceans may have dropped as much as 350 feet (107 meters) below the current level. At times of low sea level, land plants and animals, including humans and their ancestors, lived on the shelves.

Evidence of this lies in their remains that are often found there in the present day. Twelve-thousand-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States. Because water above continental shelves is not that deep, sunlight is able to penetrate, helping plants grow.

This, in turn, leads to a rich web of sea life. Most commercial fishing takes place in these waters. Extensive deposits of oil, natural gas, minerals, and other natural resources lie beneath continental shelves. The economic benefit of the fish and natural resources is important to many nations, which claim territorial ownership of the continental shelves adjacent to their land areas. Many political disagreements have arisen because of this. In spite of their desire to reap the benefits contained over and in the shelves, many nations illegally dump much of their waste in the ocean over these areas.

The drifting continents In 1915 German geophysicist Alfred Wegener (1880–1930) published a book in which he presented geological evidence that all the continents had once been joined together in a supercontinent he called Pangaea (pronounced pan-JEE-ah; from the Greek words meaning ""all lands"").

Wegener suggested that the Atlantic Ocean and the Indian Ocean formed when this supercontinent broke apart and the continents drifted away from each other. He called his hypothesis continental drift. (A hypothesis is an educated guess, while a theory is a principle supported by extensive scientific evidence and testing.

)Wegener formed his hypothesis after he had observed that the present-day continental margins along some of the continents seemed to fit together like pieces in a jigsaw puzzle: eastern South America with western Africa, eastern North America with western Europe, and India and Antarctica with eastern Africa. When the continents are linked, some of their geological features, such as mountain ranges and mineral deposits, also match.

In addition, related species of land animals are often present on both side of the present-day oceans. Wegener argued further that if the continents had drifted, they would have passed through various climate zones.

This explains how evidence of past glaciers could be found in the Sahara desert region and why fossil coral reefs appear north of the Arctic Circle. The theory of plate tectonics What Wegener lacked, however, was a convincing explanation as to what moved the continents along the surface.

Evidence to support his hypothesis did not come until the early 1960s when geologists developed the theory of plate tectonics (from the Greek word tekton, meaning ""builder""). A revolutionary idea, it helps geologists and others understand how Earth has changed over long periods of time. Changes in the positions and features of the continents and oceans have had a profound effect on everything from global climate to the evolution of life.

Simply, the theory states that the surface of the planet is broken into sections—some large, some small—called tectonic plates. As these plates drift slowly over Earth, they slide past, collide with, and move away from each other. The boundaries where the plates meet and interact are called plate margins. What moves the plates along occurs within the planet.

Geologists divide Earth into three distinct layers: the crust, the mantle, and the core. Each layer has its own unique properties and composition.

As already mentioned, the crust is the thin shell of rock that covers Earth. Two types of crust exist: continental crust, which underlies the continents, and oceanic crust, which underlies the oceans. Varying in thickness, the crust is thickest below land and thinnest below the oceans. Underneath the crust is the mantle, which is separated into two layers: The uppermost part of the mantle is solid.

Along with the overlying crust, it forms what is called the lithosphere (pronounced LITH-uh-sfeer). It is the brittle lithosphere that has broken into the tectonic plates. Under the lithosphere is the part of the mantle known as the asthenosphere (pronounced as-THEN-uh-sfeer). Beneath Earth's surface, temperature and pressure increase with increasing depths. Rock in the asthenosphere is hot Cross-section of Earth's interior, with the solid inner core, molten outer core, mantle, and crust.enough to fold, stretch, compress, and flow very slowly without fracturing.

It is puttylike in its consistency, or what geologists call ""plastic."" The rigid tectonic plates ""float"" on the more dense, flowing asthenosphere.

The core, lying at the center of the planet, is divided into a liquid outer layer and a solid inner layer. Made up of the metallic elements iron and nickel, the core is almost five times as dense as rock on Earth's surface.

Temperatures in the core are estimated to exceed 9,900°F (5,482°C), creating extreme heat energy. Moving the plates This heat energy moves the tectonic plates across the planet's surface. It is carried to the area beneath the plates by convection currents, which act similar to the currents produced in a pot of boiling liquid on a hot stove.

When a liquid in a pot begins to boil, it turns over and over. Liquid heated at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense (lighter).

Once at the surface, the heated material cools and becomes dense once more. It then sinks back down to the bottom to become reheated. This continuous motion of heated material rising, cooling, and sinking forms the circular convection currents.

Like a gigantic furnace, the core heats the mantle rock immediately above it. Expanding and becoming less dense, the heated rock slowly rises through cooler, denser mantle rock. When the heated rock reaches the lithosphere, it moves along its base, exerting dragging forces on the tectonic plates.

This causes the plates to move. In the process, the heated rock begins to lose heat. Cooling and becoming denser, the rock then sinks back toward the core, where it will be heated once more. It takes an estimated 200 million years for heated mantle rock to make the circular trip from the core to the lithosphere and back again.

As the plates move, they interact in several different ways. Where they diverge, or move away from each other, new lithospheric rock is created as magma rises up through the crust at the plate boundary and gradually begins to cool.

Many boundaries of diverging plates are on the floors of the oceans. They are known as mid-ocean ridges, and the process of new ocean crust forming at the ridges is known as seafloor spreading. Where plates converge, or move into one another, either they crumple up and compress or one plate slides beneath the other.

When two continental (land) plates converge, the crust bends and breaks from the collision, forming complex mountain ranges and very high plateaus. When a continental plate and an oceanic plate converge, the oceanic plate (which is thinner, yet denser) bends and plunges at an angle into the asthenosphere beneath the continental plate. As it does so, its leading edge begins to melt because of high temperature and pressure in the mantle.

At about 10 percent of the plate boundaries, the plates neither diverge nor converge; instead, they transform or slide past one another.

On land, such a boundary is called a slip-strike boundary. A classic example of such a boundary is the San Andreas Fault system, an area of cracks in Earth's surface that occurs at the boundary between two tectonic plates, which extends down the coast of California.

Passive continental margins The passive continental margins of eastern North America, eastern South America, western Africa, and western Europe began to form about 200 million years ago when Pangaea began to break up. The rift or crack that caused them to split, known as the Mid-Atlantic Ridge, now lies on

Two types of continental margins exist: active margins and passive margins. Active margins form primarily along the boundaries of plates that are actively converging. Passive margins currently exist in the middle of plates, not at plate boundaries.

Active continental margin: A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope that ends in a deep trench instead of a continental rise; it is marked by earthquake and volcanic activity.

Continental Margin

"Continental Margin Follow City-Data.com founder on our Forum or @Lech Mazur Photo by: Kwest They are the drowned edges of continents. Covered by the oceans, continental margins are part of the same crust (thin, solid outermost layer of Earth) that forms the continents.

They are covered in sediments that have been eroded from dry land. (Erosion is the gradual wearing away of Earth surfaces through the action of wind and water.) Lying between the deep ocean basins and the above-water land areas, continental margins account for 11 percent of Earth's surface.

The shape of the land The continental margin is the submerged outer edge of a continent. It is generally divided into two sections: the continental shelf and the continental slope. The continental shelf is the region that extends seaward from the shoreline to a sharp drop-off that marks the beginning of the continental slope. That drop-off is known as the continental shelf break. Continental shelves vary in width from 3 to 930 miles (5 to 1,500 kilometers).

The average width worldwide is about 40 miles (64 kilometers). The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Narrow shelves are found off the western coasts of North and South America. Continental shelves along the coasts of the United States cover an area of about 891,000 square miles (2,307,690 square kilometers). Continental shelves are normally gently sloping, with an average seaward slope of about 0.1 degree.

They tend to have the same topography or surface features that dominate the adjacent land. Whether a coastal area is mountainous, dominated by low hills, or flat, the continental shelf next to it will be similarly shaped. The average depth of the continental shelf at the shelf break is about 430 feet (131 meters) below the surface of the ocean. Basic composition of continental margins, which include the continental shelf, continental slope, and continental rise.

In contrast to the continental shelf, the continental slope is generally narrow in width, ranging from 6.2 to 62 miles (10 to 100 kilometers).

Relatively steeper than the shelf, the slope angles down to the ocean basin at an average of 4 degrees; in some areas, the angle may be as much as 25 degrees. The steepness of a slope often reflects the steepness of the nearby coastal area. Slopes along mountainous coasts are steeper than those along flat coasts.

In general, the steepest slopes tend to be found in the Pacific Ocean and the least steep slopes in the Atlantic and Indian Oceans.

The continental slope marks the transition between continental crust and oceanic crust. Continental crust is composed mostly of granite, whereas oceanic crust is mostly basalt. (Although they differ in composition, both are types of igneous rock, which forms when magma cools and solidifies. Granite forms when magma with a high silica content cools slowly deep beneath Earth's surface; basalt forms when magma with a low silica content cools quickly outside of or very near Earth's surface.

)The most distinctive features of continental slopes are submarine canyons. Typically, these are steep-walled, V-shaped canyons that may be thousands of feet deep. The incline of a canyon is normally related to that of the slope: steep slopes have short, steep canyons, while broader slopes have longer, shallower canyons. The deepest of the submarine canyons easily rival the size of the Grand Canyon on the Colorado River in Arizona. Sometimes, submarine canyons begin at the outer edge of the continental shelf. Most often, they form down-current from major rivers that flow into an ocean.

Geologists believe that some submarine canyons may have been carved during a time in Earth's past when sea levels were lower than at present, and rivers were able to flow out to the edge of the shelf. Other canyons may have formed when earthquakes created faults or cracks along slopes that were subsequently eroded into the canyons. Continental margin: Words to Know Accretionary wedge: A mass of sediment and oceanic rock that is transferred from an oceanic plate to the edge of the less dense plate under which it is subducting.

Asthenosphere: The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock. Continental drift: The hypothesis proposed by Alfred Wegener that the continents are not stationary, but have moved across the surface of Earth over time. Continental margin: The submerged outer edge of a continent, composed of the continental shelf and the continental slope.

Continental rise: The gently sloping, smooth-surfaced, thick accumulation of sediment at the base of certain continental slopes. Continental shelf: The gently sloping region of the continental margin that extends seaward from the shoreline to the continental shelf break.

Continental shelf break: The outer edge of the continental shelf at which there is a sharp drop-off to the steeper continental slope. Continental slope: The steeply sloping region of the continental margin that extends from the continental shelf break downward to the ocean basin.

Convection current: The circular movement of a gas or liquid between hot and cold areas. Crust: The thin, solid outermost layer of Earth. Erosion: The gradual wearing away of Earth surfaces through the action of wind and water. Lithosphere: The rigid uppermost section of the mantle combined with the crust.

Mantle: The thick, dense layer of rock that lies beneath Earth's crust. Passive continental margin: A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise; it is marked by a lack of earthquake and volcanic activity.

Plates: Large sections of Earth's lithosphere separated by deep fault zones. Plate tectonics: The geologic theory that Earth's crust is composed of rigid plates that ""float"" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.

Seafloor spreading: The process by which new oceanic crust is formed by the upwelling of magma at mid-ocean ridges, resulting in the continuous lateral movement of existing oceanic crust. Subduction zone: A region where two plates come together and the edge of one plate slides beneath the other.

Submarine canyon: A steep-walled, V-shaped canyon that is cut into the rocks and sediments of the continental slope and, sometimes, the outer continental shelf. Trench: A long, deep, narrow depression on the ocean basin with relatively steep sides. Turbidity current: A turbulent mixture of water and sediment that flows down a continental slope under the influence of gravity.

Geologists know that underwater currents called turbidity currents have eroded most submarine canyons.

These turbulent mixtures of water and sediment develop near the continental shelf break when some event, such as an earthquake or a violent storm, triggers their formation. Denser (heavier) than the surrounding water, the currents are pulled downward by gravity. Flowing down the continental slope like an avalanche at up to 50 miles (80 kilometers) per hour, the sediment-laden currents surge through the canyons, scouring their sides.

When the currents reach the ocean basin, they slow, and the sediment they carry falls to the bottom in a fanlike deposit, much like an alluvial fan that forms in a desert environment when water flows out of a canyon onto a plain or flat area.

(For further information on alluvial fans, see the Dune and other desert features chapter. )At the base of certain continental slopes is a gently sloping, smooth-surfaced, thick accumulation of sediment from land that had been transported to the shelf and then down the slope. This transition between the continental slope and the ocean basin is known as the continental rise.

Some sources include the continental rise as being a major section of the continental margin, along with the shelf and slope. Since it does not occur on all continental margins around the planet, it is best considered a characteristic of the margin in certain areas. Continental rises are well-developed around Antarctica and in the Atlantic and Indian Oceans. They are hardly found in the Pacific. When they are present, continental rises vary in width from 62 to 620 miles (100 to 1,000 kilometers).

A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise is known as a passive continental margin. This type of margin experiences little, if any, volcanic or earthquake activity. The build-up of sediment is the primary activity affecting a passive margin.

Because these margins are found along the east coasts of North and South America and the west coasts of Europe and Africa, they are also known as ""Atlantic-type"" margins.

The continental margins along India and Antarctica are other examples of passive margins. A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope is known as an active continental margin. Instead of ending in a continental rise, the continental slope of this type of margin often plunges into a deep-ocean trench, which may be filled with sediment.

Earthquake and volcanic activity are prominent here. Since active continental margins occur along many coasts of the Pacific Ocean, these types of margins are also known as ""Pacific-type"" margins. Forces and changes: Construction and destruction The two types of continental margins, passive and active, tell about the geologic history of Earth and the activities that are continually affecting its surface.

The continents are not stationary, but move about the planet's surface. That movement varies, but in general is extremely slow, only about 2 inches (5 centimeters) per year.

Over millions of years, however, the continents have made their way along on an endless journey across the planet's surface, repeatedly crashing into or breaking away from one another. A Burial and Dumping Ground Continental shelves have not always been covered by water. During Earth's history, changes in sea level have alternately exposed, then covered portions of the shelves.

Scientists estimate that during the last glacial period over 10,000 years ago, much of Earth's water was trapped in the polar ice sheets. The level of the oceans may have dropped as much as 350 feet (107 meters) below the current level. At times of low sea level, land plants and animals, including humans and their ancestors, lived on the shelves.

Evidence of this lies in their remains that are often found there in the present day. Twelve-thousand-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States. Because water above continental shelves is not that deep, sunlight is able to penetrate, helping plants grow.

This, in turn, leads to a rich web of sea life. Most commercial fishing takes place in these waters. Extensive deposits of oil, natural gas, minerals, and other natural resources lie beneath continental shelves. The economic benefit of the fish and natural resources is important to many nations, which claim territorial ownership of the continental shelves adjacent to their land areas. Many political disagreements have arisen because of this. In spite of their desire to reap the benefits contained over and in the shelves, many nations illegally dump much of their waste in the ocean over these areas.

The drifting continents In 1915 German geophysicist Alfred Wegener (1880–1930) published a book in which he presented geological evidence that all the continents had once been joined together in a supercontinent he called Pangaea (pronounced pan-JEE-ah; from the Greek words meaning ""all lands"").

Wegener suggested that the Atlantic Ocean and the Indian Ocean formed when this supercontinent broke apart and the continents drifted away from each other. He called his hypothesis continental drift. (A hypothesis is an educated guess, while a theory is a principle supported by extensive scientific evidence and testing.

)Wegener formed his hypothesis after he had observed that the present-day continental margins along some of the continents seemed to fit together like pieces in a jigsaw puzzle: eastern South America with western Africa, eastern North America with western Europe, and India and Antarctica with eastern Africa. When the continents are linked, some of their geological features, such as mountain ranges and mineral deposits, also match.

In addition, related species of land animals are often present on both side of the present-day oceans. Wegener argued further that if the continents had drifted, they would have passed through various climate zones.

This explains how evidence of past glaciers could be found in the Sahara desert region and why fossil coral reefs appear north of the Arctic Circle. The theory of plate tectonics What Wegener lacked, however, was a convincing explanation as to what moved the continents along the surface.

Evidence to support his hypothesis did not come until the early 1960s when geologists developed the theory of plate tectonics (from the Greek word tekton, meaning ""builder""). A revolutionary idea, it helps geologists and others understand how Earth has changed over long periods of time. Changes in the positions and features of the continents and oceans have had a profound effect on everything from global climate to the evolution of life.

Simply, the theory states that the surface of the planet is broken into sections—some large, some small—called tectonic plates. As these plates drift slowly over Earth, they slide past, collide with, and move away from each other. The boundaries where the plates meet and interact are called plate margins. What moves the plates along occurs within the planet.

Geologists divide Earth into three distinct layers: the crust, the mantle, and the core. Each layer has its own unique properties and composition.

As already mentioned, the crust is the thin shell of rock that covers Earth. Two types of crust exist: continental crust, which underlies the continents, and oceanic crust, which underlies the oceans. Varying in thickness, the crust is thickest below land and thinnest below the oceans. Underneath the crust is the mantle, which is separated into two layers: The uppermost part of the mantle is solid.

Along with the overlying crust, it forms what is called the lithosphere (pronounced LITH-uh-sfeer). It is the brittle lithosphere that has broken into the tectonic plates. Under the lithosphere is the part of the mantle known as the asthenosphere (pronounced as-THEN-uh-sfeer). Beneath Earth's surface, temperature and pressure increase with increasing depths. Rock in the asthenosphere is hot Cross-section of Earth's interior, with the solid inner core, molten outer core, mantle, and crust.enough to fold, stretch, compress, and flow very slowly without fracturing.

It is puttylike in its consistency, or what geologists call ""plastic."" The rigid tectonic plates ""float"" on the more dense, flowing asthenosphere.

The core, lying at the center of the planet, is divided into a liquid outer layer and a solid inner layer. Made up of the metallic elements iron and nickel, the core is almost five times as dense as rock on Earth's surface.

Temperatures in the core are estimated to exceed 9,900°F (5,482°C), creating extreme heat energy. Moving the plates This heat energy moves the tectonic plates across the planet's surface. It is carried to the area beneath the plates by convection currents, which act similar to the currents produced in a pot of boiling liquid on a hot stove.

When a liquid in a pot begins to boil, it turns over and over. Liquid heated at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense (lighter).

Once at the surface, the heated material cools and becomes dense once more. It then sinks back down to the bottom to become reheated. This continuous motion of heated material rising, cooling, and sinking forms the circular convection currents.

Like a gigantic furnace, the core heats the mantle rock immediately above it. Expanding and becoming less dense, the heated rock slowly rises through cooler, denser mantle rock. When the heated rock reaches the lithosphere, it moves along its base, exerting dragging forces on the tectonic plates.

This causes the plates to move. In the process, the heated rock begins to lose heat. Cooling and becoming denser, the rock then sinks back toward the core, where it will be heated once more. It takes an estimated 200 million years for heated mantle rock to make the circular trip from the core to the lithosphere and back again.

As the plates move, they interact in several different ways. Where they diverge, or move away from each other, new lithospheric rock is created as magma rises up through the crust at the plate boundary and gradually begins to cool.

Many boundaries of diverging plates are on the floors of the oceans. They are known as mid-ocean ridges, and the process of new ocean crust forming at the ridges is known as seafloor spreading. Where plates converge, or move into one another, either they crumple up and compress or one plate slides beneath the other.

When two continental (land) plates converge, the crust bends and breaks from the collision, forming complex mountain ranges and very high plateaus. When a continental plate and an oceanic plate converge, the oceanic plate (which is thinner, yet denser) bends and plunges at an angle into the asthenosphere beneath the continental plate. As it does so, its leading edge begins to melt because of high temperature and pressure in the mantle.

At about 10 percent of the plate boundaries, the plates neither diverge nor converge; instead, they transform or slide past one another.

On land, such a boundary is called a slip-strike boundary. A classic example of such a boundary is the San Andreas Fault system, an area of cracks in Earth's surface that occurs at the boundary between two tectonic plates, which extends down the coast of California.

Passive continental margins The passive continental margins of eastern North America, eastern South America, western Africa, and western Europe began to form about 200 million years ago when Pangaea began to break up. The rift or crack that caused them to split, known as the Mid-Atlantic Ridge, now lies on

Two types of continental margins exist: active margins and passive margins. Active margins form primarily along the boundaries of plates that are actively converging. Passive margins currently exist in the middle of plates, not at plate boundaries.

). The San Andreas Fault in California is one example. 2. Divergent boundaries occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries. 3. Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones.

Describe the three types of tectonic plate boundaries and motion that occurs at each type of boundaries?

Science & Mathematics Earth Sciences & Geology Describe the three types of tectonic plate boundaries and motion that occurs at each type of boundaries?1 following 3 answers Answers Relevance Rating Newest Oldest Best Answer: Divergent plate boundaries (mid ocean ridges).

Plates are moving away from each other and new ocean floor is being created Convergent plate boundaries (subduction zones) Plates are moving towards each other, with one plate being pulled/pushed down into the athenosphere Transform Boundaries (transform fault) Plates are sliding past each other. Charliemom2012 · 10 years ago12 2 1 comment3 Types Of Plate Boundaries Source (s):https://shrink.im/a8s E3?

· 2 years ago0 0 Comment This Site Might Help You. RE: describe the three types of tectonic plate boundaries and motion that occurs at each type of boundaries?

Source (s):describe types tectonic plate boundaries motion occurs type boundaries: https://biturl.im/IJ8h1Anonymous · 3 years ago0 0 Comment For the best answers, search on this site https://shorturl.im/avc TN convergent boundary: also known as a destructive plate boundary (because of subduction), is an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move toward one another and collide.

As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near convergent boundaries. When two plates move towards one another, they form either a subduction zone or a continental collision.

This depends on the nature of the plates involved. In a subduction zone, the subducting plate, which is normally a plate with oceanic crust, moves beneath the other plate, which can be made of either oceanic or continental crust. During collisions between two continental plates, large mountain ranges, such as the Himalayas are formed.

divergent boundary: (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges.

Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill. transform boundary: also known as conservative plate boundary since these faults neither create nor destroy lithosphere, is a type of fault whose relative motion is predominantly horizontal in either sinistral or dextral direction. Furthermore, transform faults end abruptly and are connected on both ends to other faults, ridges, or subduction zones.

[1] While most transform faults are hidden in the deep oceans where they form a series of short zigzags accommodating seafloor spreading (see graphic at right), the best-known (and most destructive) are those on land at the margins of tectonic plates. Transform faults are the only type of strike-slip fault that can be classified as a plate boundary.? · 2 years ago0 0 Comment1. Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer

The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism.

Examples of this are the Andes mountain range in South America and the Japanese island arc. Source (s):http://en.wikipedia.org/wiki/Plate_tecto... Cupcake Queen · 10 years ago3 2 Commentwhats the difference between a seismonmeter and a seismographmelanie · 4 years ago0 3 Comment Maybe you would like to learn more about one of these?

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Mountain building can occur in a number of ways at a convergent margin: mountains may rise as a consequence of the subduction process itself, by the accretion of small crustal fragments (which, along with linear island chains and oceanic ridges, are known as terranes), or by the collision of two large continents.

Plate tectonics

Plate tectonics, theory dealing with the dynamics of Earth ’s outer shell—the lithosphere —that revolutionized Earth sciences by providing a uniform context for understanding mountain-building processes, volcanoes, and earthquakes as well as the evolution of Earth’s surface and reconstructing its past continents and oceans. Earth's tectonic plates Map showing Earth's major tectonic plates with arrows depicting the directions of plate movement.

Encyclopædia Britannica, Inc. The concept of plate tectonics was formulated in the 1960s. According to the theory, Earth has a rigid outer layer, known as the lithosphere, which is typically about 100 km (60 miles) thick and overlies a plastic (moldable, partially molten) layer called the asthenosphere. The lithosphere is broken up into seven very large continental- and ocean-sized plates, six or seven medium-sized regional plates, and several small ones.

These plates move relative to each other, typically at rates of 5 to 10 cm (2 to 4 inches) per year, and interact along their boundaries, where they converge, diverge, or slip past one another.

Such interactions are thought to be responsible for most of Earth’s seismic and volcanic activity, although earthquakes and volcanoes can occur in plate interiors. Plate motions cause mountains to rise where plates push together, or converge, and continents to fracture and oceans to form where plates pull apart, or diverge. The continents are embedded in the plates and drift passively with them, which over millions of years results in significant changes in Earth’s geography.

The theory of plate tectonics is based on a broad synthesis of geologic and geophysical data. It is now almost universally accepted, and its adoption represents a true scientific revolution, analogous in its consequences to quantum mechanics in physics or the discovery of the genetic code in biology.

Incorporating the much older idea of continental drift, as well as the concept of seafloor spreading, the theory of plate tectonics has provided an overarching framework in which to describe the past geography of continents and oceans, the processes controlling creation and destruction of landforms, and the evolution of Earth’s crust, atmosphere, biosphere, hydrosphere, and climates. During the late 20th and early 21st centuries, it became apparent that plate-tectonic processes profoundly influence the composition of Earth’s atmosphere and oceans, serve as a prime cause of long-term climate change, and make significant contributions to the chemical and physical environment in which life evolves.

For details on the specific effects of plate tectonics, see the articles earthquake and volcano. A detailed treatment of the various land and submarine relief features associated with plate motion is provided in the articles tectonic landform and ocean. Read More on This Topic Earth sciences: The theory of plate tectonics Plate tectonics has revolutionized virtually every discipline of the Earth sciences since the late 1960s and early 1970s.

It has served as a unifying model or paradigm for explaining geologic phenomena that were formerly considered in unrelated fashion. Plate tectonics describes seismic activity,…READ MOREPrinciples of plate tectonics In essence, plate-tectonic theory is elegantly simple.

Earth ’s surface layer, 50 to 100 km (30 to 60 miles) thick, is rigid and is composed of a set of large and small plates. Together, these plates constitute the lithosphere, from the Greek lithos, meaning “ rock .” The lithosphere rests on and slides over an underlying partially molten (and thus weaker but generally denser) layer of plastic partially molten rock known as the asthenosphere, from the Greek asthenos, meaning “weak.” Plate movement is possible because the lithosphere-asthenosphere boundary is a zone of detachment.

As the lithospheric plates move across Earth’s surface, driven by forces as yet not fully understood, they interact along their boundaries, diverging, converging, or slipping past each other. While the interiors of the plates are presumed to remain essentially undeformed, plate boundaries are the sites of many of the principal processes that shape the terrestrial surface, including earthquakes, volcanism, and orogeny (that is, formation of mountain ranges).

A cross section of Earth's outer layers, from the crust through the lower mantle. Encyclopædia Britannica, Inc.plate tectonics The roles that convection currents and other forces play in the movement of Earth's tectonic plates.© Minute Earth The process of plate tectonics may be driven by convection in Earth’s mantle, the pull of heavy old pieces of crust into the mantle, or some combination of both.

For a deeper discussion of plate-driving mechanisms, see Plate-driving mechanisms and the role of the mantle.

Earth’s layers Knowledge of Earth’s interior is derived primarily from analysis of the seismic waves that propagate through Earth as a result of earthquakes. Depending on the material they travel through, the waves may either speed up, slow down, bend, or even stop if they cannot penetrate the material they encounter.crustal generation and destruction Three-dimensional diagram showing crustal generation and destruction according to the theory of plate tectonics; included are the three kinds of plate boundaries—divergent, convergent (or collision), and strike-slip (or transform).

Encyclopædia Britannica, Inc. Collectively, these studies show that Earth can be internally divided into layers on the basis of either gradual or abrupt variations in chemical and physical properties.

Chemically, Earth can be divided into three layers. A relatively thin crust, which typically varies from a few kilometres to 40 km (about 25 miles) in thickness, sits on top of the mantle. (In some places, Earth’s crust may be up to 70 km [40 miles] thick.) The mantle is much thicker than the crust; it contains 83 percent of Earth’s volume and continues to a depth of 2,900 km (1,800 miles).

Beneath the mantle is the core, which extends to the centre of Earth, some 6,370 km (nearly 4,000 miles) below the surface.

Geologists maintain that the core is made up primarily of metallic iron accompanied by smaller amounts of nickel, cobalt, and lighter elements, such as carbon and sulfur. ( See also Earth . )There are two types of crust, continental and oceanic, which differ in their composition and thickness. The distribution of these crustal types broadly coincides with the division into continents and ocean basins, although continental shelves, which are submerged, are underlain by continental crust.

The continents have a crust that is broadly granitic in composition and, with a density of about 2.7 grams per cubic cm (0.098 pound per cubic inch), is somewhat lighter than oceanic crust, which is basaltic (i.e., richer in iron and magnesium than granite) in composition and has a density of about 2.9 to 3 grams per cubic cm (0.1 to 0.11 pound per cubic inch).

Continental crust is typically 40 km (25 miles) thick, while oceanic crust is much thinner, averaging about 6 km (4 miles) in thickness. These crustal rocks both sit on top of the mantle, which is ultramafic in composition (i.e., very rich in magnesium and iron-bearing silicate minerals ).

The boundary between the crust (continental or oceanic) and the underlying mantle is known as the Mohorovičić discontinuity (also called Moho), which is named for its discoverer, Croatian seismologist Andrija Mohorovičić. The Moho is clearly defined by seismic studies, which detect an acceleration in seismic waves as they pass from the crust into the denser mantle.

The boundary between the mantle and the core is also clearly defined by seismic studies, which suggest that the outer part of the core is a liquid.seismic wave The shifting rock in an earthquake causes vibrations called seismic waves that travel within Earth or along its surface.

The four main types of seismic waves are P waves, S waves, Love waves, and Rayleigh waves. Encyclopædia Britannica, Inc. The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle.

Its average elevation above sea level is 840 metres (2,750 feet), while the average depth of oceanic crust is 3,790 metres (12,400 feet). This density difference creates two principal levels of Earth’s surface. The lithosphere itself includes all the crust as well as the upper part of the mantle (i.e., the region directly beneath the Moho), which is also rigid.

However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity.

This process begins at about 100 km (60 miles) below the surface. This change occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere. This upper portion of the mantle, which is known as the lithospheric mantle, has an average density of about 3.3 grams per cubic cm (0.12 pound per cubic inch).

The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3.4–4.4 grams per cubic cm (0.12–0.16 pound per cubic inch). In contrast, the rocks in the asthenosphere are weaker, because they are close to their melting temperatures. As a result, seismic waves slow as they enter the asthenosphere. With increasing depth, however, the greater pressure from the weight of the rocks above causes the mantle to become gradually stronger, and seismic waves increase in velocity, a defining characteristic of the lower mantle.

The lower mantle is more or less solid, but the region is also very hot, and thus the rocks can flow very slowly (a process known as creep).

During the late 20th and early 21st centuries, scientific understanding of the deep mantle was greatly enhanced by high-resolution seismological studies combined with numerical modeling and laboratory experiments that mimicked conditions near the core-mantle boundary. Collectively, these studies revealed that the deep mantle is highly heterogeneous and that the layer may play a fundamental role in driving Earth’s plates.

At a depth of about 2,900 km (1,800 miles), the lower mantle gives way to Earth’s outer core, which is made up of a liquid rich in iron and nickel. At a depth of about 5,100 km (3,200 miles), the outer core transitions to the inner core. Although it has a higher temperature than the outer core, the inner core is solid because of the tremendous pressures that exist near Earth’s centre.

Earth’s inner core is divided into the outer-inner core (OIC) and the inner-inner core (IIC), which differ from one another with respect to the polarity of their iron crystals. The polarity of the iron crystals of the OIC is oriented in a north-south direction, whereas that of the IIC is oriented east-west.

Earth's core The internal layers of Earth's core, including its two inner cores. Encyclopædia Britannica, Inc. Plate boundaries Lithospheric plates are much thicker than oceanic or continental crust. Their boundaries do not usually coincide with those between oceans and continents, and their behaviour is only partly influenced by whether they carry oceans, continents, or both.

The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west (the North American continent) and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.

A general discussion of plate tectonics. Encyclopædia Britannica, Inc. In a simplified example of plate motion shown in the figure, movement of plate A to the left relative to plates B and C results in several types of simultaneous interactions along the plate boundaries. At the rear, plates A and B move apart, or diverge, resulting in extension and the formation of a divergent margin.

At the front, plates A and B overlap, or converge, resulting in compression and the formation of a convergent margin. Along the sides, the plates slide past one another, a process called shear. As these zones of shear link other plate boundaries to one another, they are called transform faults.

Theoretical diagram showing the effects of an advancing tectonic plate on other adjacent, but stationary, tectonic plates. At the advancing edge of plate A, the overlap with plate B creates a convergent boundary. In contrast, the gap left behind the trailing edge of plate A forms a divergent boundary with plate B.

As plate A slides past portions of both plate B and plate C, transform boundaries develop. Encyclopædia Britannica, Inc. Divergent margins As plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle.

This molten material, known as magma, is basaltic in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins. Continental rifting Upwelling of magma causes the overlying lithosphere to uplift and stretch.

(Whether magmatism [the formation of igneous rock from magma] initiates the rifting or whether rifting decompresses the mantle and initiates magmatism is a matter of significant debate.) If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart.

Settling of the continental blocks creates a rift valley, such as the present-day East African Rift Valley. As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created.

The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.rift valley; Thingvellir National Park The Thingvellir fracture zone at Thingvellir National Park in southwestern Iceland is an example of a rift valley.

The Thingvellir fracture lies in the Mid-Atlantic Ridge, which extends through the centre of Iceland.© Ihervas/Shutterstock.com Seafloor spreading As upwelling of magma continues, the plates continue to diverge, a process known as seafloor spreading.

Samples collected from the ocean floor show that the age of oceanic crust increases with distance from the spreading centre —important evidence in favour of this process. These age data also allow the rate of seafloor spreading to be determined, and they show that rates vary from about 0.1 cm (0.04 inch) per year to 17 cm (6.7 inches) per year. Seafloor-spreading rates are much more rapid in the Pacific Ocean than in the Atlantic and Indian oceans.

At spreading rates of about 15 cm (6 inches) per year, the entire crust beneath the Pacific Ocean (about 15,000 km [9,300 miles] wide) could be produced in 100 million years.age of Earth's oceanic crust Map showing the age of Earth's oceanic crust and the pattern of seafloor spreading at the global scale. Encyclopædia Britannica, Inc. Divergence and creation of oceanic crust are accompanied by much volcanic activity and by many shallow earthquakes as the crust repeatedly rifts, heals, and rifts again.

Brittle earthquake -prone rocks occur only in the shallow crust. Deep earthquakes, in contrast, occur less frequently, due to the high heat flow in the mantle rock. These regions of oceanic crust are swollen with heat and so are elevated by 2 to 3 km (1.2 to 1.9 miles) above the surrounding seafloor.

The elevated topography results in a feedback scenario in which the resulting gravitational force pushes the crust apart, allowing new magma to well up from below, which in turn sustains the elevated topography.

Its summits are typically 1 to 5 km (0.6 to 3.1 miles) below the ocean surface. On a global scale, these ridges form an interconnected system of undersea “ mountains ” that are about 65,000 km (40,000 miles) in length and are called oceanic ridges. Convergentmargins Given that Earth is constant in volume, the continuous formation of Earth’s new crust produces an excess that must be balanced by destruction of crust elsewhere.

This is accomplished at convergent plate boundaries, also known as destructive plate boundaries, where one plate descends at an angle—that is, is subducted—beneath the other. Because oceanic crust cools as it ages, it eventually becomes denser than the underlying asthenosphere, and so it has a tendency to subduct, or dive under, adjacent continental plates or younger sections of oceanic crust.

The life span of the oceanic crust is prolonged by its rigidity, but eventually this resistance is overcome. Experiments show that the subducted oceanic lithosphere is denser than the surrounding mantle to a depth of at least 600 km (about 400 miles).

The mechanisms responsible for initiating subduction zones are controversial. During the late 20th and early 21st centuries, evidence emerged supporting the notion that subduction zones preferentially initiate along preexisting fractures (such as transform faults) in the oceanic crust. Irrespective of the exact mechanism, the geologic record indicates that the resistance to subduction is overcome eventually. Where two oceanic plates meet, the older, denser plate is preferentially subducted beneath the younger, warmer one.

Where one of the plate margins is oceanic and the other is continental, the greater buoyancy of continental crust prevents it from sinking, and the oceanic plate is preferentially subducted. Continents are preferentially preserved in this manner relative to oceanic crust, which is continuously recycled into the mantle. This explains why ocean floor rocks are generally less than 200 million years old whereas the oldest continental rocks are more than 4 billion years old.

Before the middle of the 20th century, most geoscientists maintained that continental crust was too buoyant to be subducted. However, it later became clear that slivers of continental crust adjacent to the deep-sea trench, as well as sediments deposited in the trench, may be dragged down the subduction zone.

The recycling of this material is detected in the chemistry of volcanoes that erupt above the subduction zone. Two plates carrying continental crust collide when the oceanic lithosphere between them has been eliminated. Eventually, subduction ceases and towering mountain ranges, such as the Himalayas, are created. See below Mountains by continental collision. Because the plates form an integrated system, it is not necessary that new crust formed at any given divergent boundary be completely compensated at the nearest subduction zone, as long as the total amount of crust generated equals that destroyed.

Subduction zones The subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithosphere about 100 km (60 miles) thick that carries a relatively thin cap of oceanic sediments.

The path of descent is defined by numerous earthquakes along a plane that is typically inclined between 30° and 60° into the mantle and is called the Wadati-Benioff zone, for Japanese seismologist Kiyoo Wadati and American seismologist Hugo Benioff, who pioneered its study. Between 10 and 20 percent of the subduction zones that dominate the circum-Pacific ocean basin are subhorizontal (that is, they subduct at angles between 0° and 20°).

The factors that govern the dip of the subduction zone are not fully understood, but they probably include the age and thickness of the subducting oceanic lithosphere and the rate of plate convergence.subducting tectonic plate A subducting plate's path (called the Benioff-Wadati [or Wadati-Benioff] zone) is defined by numerous earthquakes along a plane that is typically inclined between 30° and 60° into the mantle. Encyclopædia Britannica, Inc. Most, but not all, earthquakes in this planar dipping zone result from compression, and the seismic activity extends 300 to 700 km (200 to 400 miles) below the surface, implying that the subducted crust retains some rigidity to this depth.

At greater depths the subducted plate is partially recycled into the mantle. The site of subduction is marked by a deep trench, between 5 and 11 km (3 and 7 miles) deep, that is produced by frictional drag between the plates as the descending plate bends before it subducts.

The overriding plate scrapes sediments and elevated portions of ocean floor off the upper crust of the lower plate, creating a zone of highly deformed rocks within the trench that becomes attached, or accreted, to the overriding plate.

This chaotic mixture is known as an accretionary wedge. The rocks in the subduction zone experience high pressures but relatively low temperatures, an effect of the descent of the cold oceanic slab. Under these conditions the rocks recrystallize, or metamorphose, to form a suite of rocks known as blueschists, named for the diagnostic blue mineral called glaucophane, which is stable only at the high pressures and low temperatures found in subduction zones.

( See also metamorphic rock .) At deeper levels in the subduction zone (that is, greater than 30–35 km [about 19–22 miles]), eclogites, which consist of high-pressure minerals such as red garnet ( pyrope) and omphacite ( pyroxene ), form. The formation of eclogite from blueschist is accompanied by a significant increase in density and has been recognized as an important additional factor that facilitates the subduction process.

Island arcs When the downward-moving slab reaches a depth of about 100 km (60 miles), it gets sufficiently warm to drive off its most volatile components, thereby stimulating partial melting of mantle in the plate above the subduction zone (known as the mantle wedge). Melting in the mantle wedge produces magma, which is predominantly basaltic in composition. This magma rises to the surface and gives birth to a line of volcanoes in the overriding plate, known as a volcanic arc, typically a few hundred kilometres behind the oceanic trench.

The distance between the trench and the arc, known as the arc-trench gap, depends on the angle of subduction. Steeper subduction zones have relatively narrow arc-trench gaps. A basin may form within this region, known as a fore-arc basin, and may be filled with sediments derived from the volcanic arc or with remains of oceanic crust.

If both plates are oceanic, as in the western Pacific Ocean, the volcanoes form a curved line of islands, known as an island arc, that is parallel to the trench, as in the case of the Mariana Islands and the adjacent Mariana Trench. If one plate is continental, the volcanoes form inland, as they do in the Andes of western South America.

Though the process of magma generation is similar, the ascending magma may change its composition as it rises through the thick lid of continental crust, or it may provide sufficient heat to melt the crust.

In either case, the composition of the volcanic mountains formed tends to be more silicon -rich and iron - and magnesium -poor relative to the volcanic rocks produced by ocean-ocean convergence. Back-arc basins Where both converging plates are oceanic, the margin of the older oceanic crust will be subducted because older oceanic crust is colder and therefore more dense.

As the dense slab collapses into the asthenosphere, however, it also may “roll back” oceanward and cause extension in the overlying plate. This results in a process known as back-arc spreading, in which a basin opens up behind the island arc.

The crust behind the arc becomes progressively thinner, and the decompression of the underlying mantle causes the crust to melt, initiating seafloor-spreading processes, such as melting and the production of basalt; these processes are similar to those that occur at ocean ridges.

The geochemistry of the basalts produced at back-arc basins superficially resembles that of basalts produced at ocean ridges, but subtle trace element analyses can detect the influence of a nearby subducted slab.

The trench “roll back” process of back-arc basin formation. Encyclopædia Britannica, Inc. This style of subduction predominates in the western Pacific Ocean, in which a number of back-arc basins separate several island arcs from Asia. Examples include the Mariana Islands, the Kuril Islands, and the main islands of Japan. However, if the rate of convergence increases or if anomalously thick oceanic crust (possibly caused by rising mantle plume activity) is conveyed into the subduction zone, the slab may flatten. Such flattening causes the back-arc basin to close, resulting in deformation, metamorphism, and even melting of the strata deposited in the basin. The slab “sea anchor” process of back-arc basin formation. Encyclopædia Britannica, Inc. Mountain building If the rate of subduction in an ocean basin exceeds the rate at which the crust is formed at oceanic ridges, a convergent margin forms as the ocean initially contracts. This process can lead to collision between the approaching continents, which eventually terminates subduction.

Mountain building can occur in a number of ways at a convergent margin: mountains may rise as a consequence of the subduction process itself, by the accretion of small crustal fragments (which, along with linear island chains and oceanic ridges, are known as terranes), or by the collision of two large continents.

Many mountain belts were developed by a combination of these processes. For example, the Cordilleran mountain belt of North America —which includes the Rocky Mountains as well as the Cascades, the Sierra Nevada, and other mountain ranges near the Pacific coast—developed by a combination of subduction and terrane accretion. As continental collisions are usually preceded by a long history of subduction and terrane accretion, many mountain belts record all three processes.

Over the past 70 million years the subduction of the Neo-Tethys Sea, a wedge-shaped body of water that was located between Gondwana and Laurasia, led to the accretion of terranes along the margins of Laurasia, followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia. These collisions culminated in the formation of the Alps and the Himalayas. Jurassic paleogeography Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period.

Included in the paleogeographic reconstruction are the locations of the interval's subduction zones. Adapted from: C. R. Scotese, The University of Texas at Arlington Mountains by subduction Mountain building by subduction is classically demonstrated in the Andes Mountains of South America. Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zone, and, therefore, the rocks in this region are warm and weak.

Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward.

For example, rapid uplift of the Andes approximately 25 million years ago is evidenced by a reversal in the flow of the Amazon River from its ancestral path toward the Pacific Ocean to its modern path, which empties into the Atlantic Ocean. In addition, models have indicated that the episodic opening and closing of back-arc basins have been the major factors in mountain-building processes, which have influenced the plate-tectonic evolution of the western Pacific for at least the past 500 million years.

Mountains by terrane accretion As the ocean contracts by subduction, elevated regions within the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin.

Since the late Devonian and early Carboniferous periods, some 360 million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes.

The piecemeal addition of these accreted terranes has added an average of 600 km (400 miles) in width along the western margin of the North American continent, and the collisions have resulted in important pulses of mountain building.continental margin The broad, gentle pitch of the continental shelf gives way to the relatively steep continental slope. The more gradual transition to the abyssal plain is a sediment-filled region called the continental rise. The continental shelf, slope, and rise are collectively called the continental margin. Encyclopædia Britannica, Inc. During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends.

Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted. Where this occurs, rare slices of ocean crust, known as ophiolites, are preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land.

A classic example is the Coast Range ophiolite of California, which is one of the most extensive ophiolite terranes in North America.

These ophiolite deposits run from the Klamath Mountains in northern California southward to the Diablo Range in central California. This oceanic crust likely formed during the middle of the Jurassic Period, roughly 170 million years ago, in an extensional regime within either a back-arc or a forearc basin.

In the late Mesozoic, it was accreted to the western North American continental margin. Because preservation of oceanic crust is rare, the recognition of ophiolite complexes is very important in tectonic analyses. Until the mid-1980s, ophiolites were thought to represent vestiges of the main oceanic tract, but geochemical analyses have clearly indicated that most ophiolites form near volcanic arcs, such as in back-arc basins characterized by subduction roll-back (the collapse of the subducting plate that causes the extension of the overlying plate).

The recognition of ophiolite complexes is very important in tectonic analysis, because they provide insights into the generation of magmatism in oceanic domains, as well as their complex relationships with subduction processes.

( See above back-arc basins . )Mountains by continental collision Continental collision involves the forced convergence of two buoyant plate margins that results in neither continent being subducted to any appreciable extent. A complex sequence of events ensues that compels one continent to override the other. These processes result in crustal thickening and intense deformation that forces the crust skyward to form huge mountains with crustal roots that extend as deep as 80 km (about 50 miles) relative to Earth’s surface, in accordance with the principles of isostasy.

The subducted slab still has a tendency to sink and may become detached and founder (submerge) into the mantle. The crustal root undergoes metamorphic reactions that result in a significant increase in density and may cause the root to also founder into the mantle.

Both processes result in a significant injection of heat from the compensatory upwelling of asthenosphere, which is an important contribution to the rise of the mountains. Continental collisions produce lofty landlocked mountain ranges such as the Himalayas. Much later, after these ranges have been largely leveled by erosion, it is possible that the original contact, or suture, may be exposed.

The balance between creation and destruction on a global scale is demonstrated by the expansion of the Atlantic Ocean by seafloor spreading over the past 200 million years, compensated by the contraction of the Pacific Ocean, and the consumption of an entire ocean between India and Asia (the Tethys Sea ). The northward migration of India led to collision with Asia some 40 million years ago. Since that time India has advanced a further 2,000 km (1,250 miles) beneath Asia, pushing up the Himalayas and forming the Plateau of Tibet.

Pinned against stable Siberia, China and Indochina were pushed sideways, resulting in strong seismic activity thousands of kilometres from the site of the continental collision. Transform faults Along the third type of plate boundary, two plates move laterally and pass each other along giant fractures in Earth’s crust. Transform faults are so named because they are linked to other types of plate boundaries. The majority of transform faults link the offset segments of oceanic ridges.

However, transform faults also occur between plate margins with continental crust—for example, the San Andreas Fault in California and the North Anatolian fault system in Turkey. These boundaries are conservative because plate interaction occurs without creating or destroying crust. Because the only motion along these faults is the sliding of plates past each other, the horizontal direction along the fault surface must parallel the direction of plate motion.

The fault surfaces are rarely smooth, and pressure may build up when the plates on either side temporarily lock.

This buildup of stress may be suddenly released in the form of an earthquake. Section of the San Andreas Fault in the Carrizo Plain, western California. U. S. Geological Survey Many transform faults in the Atlantic Ocean are the continuation of major faults in adjacent continents, which suggests that the orientation of these faults might be inherited from preexisting weaknesses in continental crust during the earliest stages of the development of oceanic crust.

On the other hand, transform faults may themselves be reactivated, and recent geodynamic models suggest that they are favourable environments for the initiation of subduction zones. Hotspots Although most of Earth’s volcanic activity is concentrated along or adjacent to plate boundaries, there are some important exceptions in which this activity occurs within plates. Linear chains of islands, thousands of kilometres in length, that occur far from plate boundaries are the most notable examples.

These island chains record a typical sequence of decreasing elevation along the chain, from volcanic island to fringing reef to atoll and finally to submerged seamount. An active volcano usually exists at one end of an island chain, with progressively older extinct volcanoes occurring along the rest of the chain.

Canadian geophysicist J. Tuzo Wilson and American geophysicist W. Jason Morgan explained such topographic features as the result of hotspots. The principal tectonic plates that make up Earth's lithosphere. Also located are several dozen hot spots where plumes of hot mantle material are upwelling beneath the plates. Encyclopædia Britannica, Inc.earthquake zones; volcanoes The world's earthquake zones occur in red bands and largely coincide with the boundaries of Earth's tectonic plates.

Black dots indicate active volcanoes, whereas open dots indicate inactive ones. Encyclopædia Britannica, Inc. The number of these hotspots is uncertain (estimates range from 20 to 120), but most occur within a plate rather than at a plate boundary. Hotspots are thought to be the surface expression of giant plumes of heat, termed mantle plumes, that ascend from deep within the mantle, possibly from the core-mantle boundary, some 2,900 km (1,800 miles) below the surface.

These plumes are thought to be stationary relative to the lithospheric plates that move over them.

A volcano builds upon the surface of a plate directly above the plume. As the plate moves on, however, the volcano is separated from its underlying magma source and becomes extinct. Extinct volcanoes are eroded as they cool and subside to form fringing reefs and atolls, and eventually they sink below the surface of the sea to form a seamount.

At the same time, a new active volcano forms directly above the mantle plume. Diagram depicting the process of atoll formation. Atolls are formed from the remnant parts of sinking volcanic islands. Encyclopædia Britannica, Inc.

The best example of this process is preserved in the Hawaiian-Emperor seamount chain. The plume is presently situated beneath Hawaii, and a linear chain of islands, atolls, and seamounts extends 3,500 km (2,200 miles) northwest to Midway and a further 2,500 km (1,500 miles) north-northwest to the Aleutian Trench. The age at which volcanism became extinct along this chain gets progressively older with increasing distance from Hawaii —critical evidence that supports this theory. Hotspot volcanism is not restricted to the ocean basins; it also occurs within continents, as in the case of Yellowstone National Park in western North America.

Measurements suggest that hotspots may move relative to one another, a situation not predicted by the classical model, which describes the movement of lithospheric plates over stationary mantle plumes. This has led to challenges to this classic model. Furthermore, the relationship between hotspots and plumes is hotly debated.

Proponents of the classical model maintain that these discrepancies are due to the effects of mantle circulation as the plumes ascend, a process called the mantle wind.

Data from alternative models suggest that many plumes are not deep-rooted. Instead, they provide evidence that many mantle plumes occur as linear chains that inject magma into fractures, result from relatively shallow processes such as the localized presence of water-rich mantle, stem from the insulating properties of continental crust (which leads to the buildup of trapped mantle heat and decompression of the crust), or are due to instabilities in the interface between continental and oceanic crust. In addition, some geologists note that many geologic processes that others attribute to the behaviour of mantle plumes may be explained by other forces.

Plate motion Euler’s contributions In the 18th century, Swiss mathematician Leonhard Euler showed that the movement of a rigid body across the surface of a sphere can be described as a rotation (or turning) around an axis that goes through the centre of the sphere, known as the axis of rotation. The location of this axis bears no relationship to Earth ’s spin axis. The point of emergence of the axis through the surface of the sphere is known as the pole of rotation.

This theorem of spherical geometry provides an elegant way to define the motion of the lithospheric plates across Earth’s surface.

Therefore, the relative motion of two rigid plates may be described as rotations around a common axis, known as the axis of spreading. Application of the theorem requires that the plates not be internally deformed—a requirement not absolutely adhered to but one that appears to be a reasonable approximation of what actually happens. Application of this theorem permits the mathematical reconstruction of past plate configurations. Theoretical depiction of the movement of tectonic plates across Earth's surface.

Movement on a sphere of two plates, A and B, can be described as a rotation around a common pole. Circles around that pole correspond to the orientation of transform faults (that is, single lines in the horizontal that connect to divergent plate boundaries, marked by double lines, in the vertical).

Encyclopædia Britannica, Inc. Because all plates form a closed system, all movements can be defined by dealing with them two at a time.

The joint pole of rotation of two plates can be determined from their transform boundaries, which are by definition parallel to the direction of motion. Thus, the plates move along transform faults, whose trace defines circles of latitude perpendicular to the axis of spreading, and so form small circles around the pole of rotation.

A geometric necessity of this theorem—that lines perpendicular to the transform faults converge on the pole of rotation—is confirmed by measurements. According to this theorem, the rate of plate motion should be slowest near the pole of rotation and increase progressively to a maximum rate along fractures with a 90° angle to it.

This relationship is also confirmed by accurate measurements of seafloor-spreading rates. Leonhard Euler, c. 1740s. Kean Collection/Hulton Archive/Getty Images Past plate movements Plate tectonics involves the movements of Earth’s lithospheric plates relative to one another over the planet’s weak asthenosphere. This activity changes the positions of all plates with respect to Earth’s spin axis and the Equator.

To determine the true geographic positions of the plates in the past, investigators have to define their motions, not only relative to each other but also relative to this independent frame of reference.

Hotspots, as classically interpreted, provide an example of such a reference frame, assuming they are the sources of plumes that originate within the deep mantle and have relatively fixed positions over time. If this assumption is valid, the motion of the lithosphere above these plumes can be deduced. The hotspot island chains serve this purpose, their trends providing the direction of motion of a plate. The speed of the plate can be inferred from the increase in age of the volcanoes along the chain relative to the distance between the islands.

Earth scientists are able to accurately reconstruct the positions and movements of plates for the past 150 million to 200 million years because they have the oceanic crust record to provide them with plate speeds and direction of movement. However, since older oceanic crust is continuously consumed to make room for new crust, this kind of evidence is not available for earlier intervals of geologic time, making it necessary for investigators to turn to other, less-precise techniques. ( See below Paleomagnetism, polar wandering, and continental drift . )Page 1 of 3Next page Development of tectonic theory

Effects of a convergent boundary between an oceanic and continental plate include: a zone of earthquake activity that is shallow along the continent margin but deepens beneath the continent; sometimes an ocean trench immediately off shore of the continent; a line of volcanic eruptions a few hundred miles inland from the shoreline; destruction of oceanic lithosphere.

Convergent Plate Boundaries

"Home » Teaching Plate Tectonics » Convergent Boundaries Convergent Plate Boundaries Teaching Plate Tectonics Teaching Plate Tectonics Earth's Internal Structure Divergent Boundary Convergent Boundary Transform Boundary Tectonic Features Map Convergent plate boundaries are locations where lithospheric plates are moving towards one another.

The plate collisions that occur in these areas can produce earthquakes, volcanic activity, and crustal deformation. Convergent Plate Boundary - Oceanic and Continental Plates When continental and oceanic plates collide, the thinner and more dense oceanic plate is overridden by the thicker and less dense continental plate. The oceanic plate is forced down into the mantle in a process known as ""subduction."" As the oceanic plate descends, it is forced into higher temperature environments.

At a depth of about 100 miles (160 km), materials in the subducting plate begin to approach their melting temperatures and a process of partial melting begins. This partial melting produces magma chambers above the subducting oceanic plate. These magma chambers are less dense than the surrounding mantle materials and are buoyant.

The buoyant magma chambers begin a slow ascent through the overlying materials, melting and fracturing their way upwards. The size and depth of these magma chambers can be determined by mapping the earthquake activity around them.

If a magma chamber rises to the surface without solidifying, the magma will break through in the form of a volcanic eruption. The Washington-Oregon coastline of the United States is an example of this type of convergent plate boundary. Here the Juan de Fuca oceanic plate is subducting beneath the westward-moving North American continental plate.

The Cascade Mountain Range is a line of volcanoes above the melting oceanic plate. The Andes Mountain Range of western South America is another example of a convergent boundary between an oceanic and continental plate. Here the Nazca Plate is subducting beneath the South American plate. Visit the Interactive Plate Boundary Map to explore satellite images of convergent boundaries between oceanic and continental plates.

Two locations are marked to show this type of plate boundary - the Cascade volcanoes along the Washington-Oregon coast of North America and the Andes mountain range on the western margin of South America.

Convergent Plate Boundary - Oceanic When a convergent boundary occurs between two oceanic plates, one of those plates will subduct beneath the other. Normally the older plate will subduct because of its higher density. The subducting plate is heated as it is forced deeper into the mantle, and at a depth of about 100 miles (150 km) the plate begins to melt.

Magma chambers are produced as a result of this melting, and the magma is lower in density than the surrounding rock material. It begins ascending by melting and fracturing its way through the overlying rock material. Magma chambers that reach the surface break through to form a volcanic eruption cone.

In the early stages of this type of boundary, the cones will be deep beneath the ocean surface but later grow to be higher than sea level. This produces an island chain. With continued development the islands grow larger, merge, and an elongate landmass is created.

Japan, the Aleutian Islands, and the Eastern Caribbean islands of Martinique, St. Lucia, and St. Vincent and the Grenadines are examples of islands formed through this type of plate boundary.

Visit the Interactive Plate Boundary Map to explore satellite images of these three areas. Effects that are found at this type of plate boundary include: a zone of progressively deeper earthquakes; an oceanic trench; a chain of volcanic islands; the destruction of oceanic lithosphere. Convergent Plate Boundary - Continental This is a difficult boundary to draw. First it is complex and second, it is poorly understood when compared to the other types of plate boundaries.

In this type of convergent boundary, a powerful collision occurs. The two thick continental plates collide, and both of them have a density that is much lower than the mantle, which prevents subduction (there may be a small amount of subduction, or the heavier lithosphere below the continental crust might break free from the crust and subduct). Fragments of crust or continent margin sediments might be caught in the collision zone between the continents, forming a highly deformed melange of rock.

The intense compression can also cause extensive folding and faulting of rocks within the two colliding plates. This deformation can extend hundreds of miles into the plate interior. The Himalaya Mountain Range is the best active example of this type of plate boundary. Visit the Interactive Plate Boundary Map to explore satellite images of the Himalaya Range where the Indian and Eurasian plates are currently in collision.

The Appalachian Mountain Range is an ancient example of this collision type and is also marked on the map.

Effects found at a convergent boundary between continental plates include: intense folding and faulting; a broad folded mountain range; shallow earthquake activity; shortening and thickening of the plates within the collision zone. Contributor: Hobart King Publisher, Geology.com Teaching Plate Tectonics Teaching Plate Tectonics Earth's Internal Structure Divergent Boundary Convergent Boundary Transform Boundary Tectonic Features Map More Plate Tectonics Explore Plate Boundaries Teaching Plate Tectonics What Is The Moho?

What is the San Andreas Fault? How Deep Does Magma Form What Causes a Tsunami? The Japan Earthquake The East Africa Rift System More From Geology.com: Dacite - a light-colored extrusive igneous rock intermediate between rhyolite and andesite.

Green Diamonds - one of rarest and most valuable diamond colors. Geology.com News contains news about geology and earth science from around the world. Volcanoes - Articles about volcanoes, volcanic hazards and eruptions past and present. Kick 'em Jenny is a submarine volcano and one of the most active on the Caribbean plate. Gemstones - Colorful images and articles about diamonds and colored stones. Minerals: Information about ore minerals, gem materials and rock-forming minerals. Mohs Hardness Scale is a set of reference minerals used for classroom hardness testing. "

al . Divergent boundaries occur where two plates slide apart from each other. Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or an orogenic belt (if the two simply collide and compress).

Plate tectonics

"Plate tectonics Plate tectonics is a theory of geology developed to explain the phenomenon of continental drift, and is currently the theory accepted by the vast majority of scientists working in this area. In the theory of plate tectonics the outermost part of the Earth 's interior is made up of two layers, the outer lithosphere and the inner asthenosphere.

The lithosphere essentially ""floats"" on the asthenosphere and is broken-up into ten major plates: African, Antarctic, Australian, Eurasian, North American, South American, Pacific, Cocos, Nazca, and the Indian plates.

These plates (and the more numerous minor plates) move in relation to one another at one of three types of plate boundaries: convergent (two plates push against one another), divergent (two plates move away from each other), and transform (two plates slide past one another).

Earthquakes, volcanic activity, mountain -building, and oceanic trench formation occur along plate boundaries (most notably around the so-called "" Pacific Ring of Fire ""). Plate tectonic theory arose out of two separate geological observations: continental drift, noticed in the early 20th century, and seafloor spreading, noticed in the 1960s.

The theory itself was developed during the late 1960s and has since almost universally been accepted by scientists and has revolutionized the Earth sciences (akin to the development of the periodic table for chemistry, the discovery of the genetic code for genetics, or evolution in biology ). Missing image Tectonic_plates.png The tectonic plates of the world were mapped in the second half of the 20th century.

[ edit]Key principles The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, the asthenosphere is hotter and mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ""float"" on the fluid-like asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions.

One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and famous.

These boundaries are discussed in further detail below. Tectonic plates are comprised of two types of lithosphere: continental and oceanic lithospheres; for example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction is based on the density of constituent materials; oceanic lithospheres are denser than continental ones due to their greater mafic mineral content.

As a result, the oceanic lithospheres generally lie below sea level (for example the entire Pacific Plate, which carries no continent), while the continental ones project above sea level (see isostasy for explanation of this principle, which is essentially a large-scale version of Archimedes ' Bath).

[ edit]Types of plate boundaries Three types of plate boundary. There are three types of plate boundaries, characterised by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: Transform boundaries occur where plates slide, or perhaps more accurately grind, past each other along transform faults. The relative motion of the two plates is therefore either sinistral or dextr

Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types. [ edit]Transform (conservative) boundaries The left- or right-lateral motion of one plate against another along transform or strike slip faults can cause highly visible surface effects.

Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults the accumulated potential energy is released as strain, or motion along the fault. The massive amounts of energy that are released are the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault complex, which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving north with respect to North America. [ edit]Divergent (constructive) boundaries At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below.

The genesis of divergent boundaries is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere.

The hot spot believed to have created the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimetres per century.

Such hot spots can be very productive of geothermal power and Iceland is actively developing this resource and is expected to be the world's first hydrogen economy within twenty years. Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley.

Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different massive transform faults occur.

These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis.

If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (due to thermal contraction and subsidence ). It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found.

Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centres. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched.

Scientists had been studying polar reversals and the link was made. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. In reality the banding furnishes a map in time and space of both spreading rate and polar reversals.

[ edit]Convergent (destructive) boundaries The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath, forming a subduction zone.

At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. As organic material from the ocean bottom is transformed and heated by friction a liquid magma with a great amount of dissolved gasses will be created.

This can erupt to the surface, forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of South America is dense with this type of volcano. In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type.

Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma.

The entire Pacific ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire . Where two continental plates collide the plates either crumple and compress or one plate burrows under or (potentially) overrides the other.

Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margins of the Indian subcontinental plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalaya .

When two oceanic plates converge they form an island arc as one oceanic plate is subducted below the other. A good example of this type of plate convergence would be Japan . Oceanic / Continental Continental / Continental Oceanic / Oceanic [ edit]Sources of plate motion As noted above, the plates are able to move because of the relative weakness of the asthenosphere.

Dissipation of heat from the mantle is acknowledged to be the source of energy driving plate tectonics.

Somehow, this energy must be converted into force in order for the plates to move. There are essentially two forces that could be driving plate motion: friction and gravity. These are further subdivided below. [ edit]Friction Mantle drag Convection currents in the mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.

Trench suction Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. [ edit]Gravity Ridge-push Plate motion is driven by the higher elevation of plates at mid-ocean ridges. Essentially stuff slides downhill. The higher elevation is caused by the relatively low density of hot material upwelling in the mantle.

The real motion producing force is the upwelling and the energy source that runs it. This is a mis-nomer as nothing is pushing and tensional features are dominant along ridges.

Also, it is difficult to explain continental break-up with this. Slab-pull Plate motion is driven by the weight of cold, dense plates sinking into the mantle at trenches. There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts.

However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by friction. Slab pull is widely believed to be the strongest force directly operating on plates. Recent models indicate that trench suction plays an important role as well.

The over-all driving force and its energy source are still debatable subjects of on-going research. [ edit]Major plates The main plates are African Plate, covering Africa Antarctic Plate, covering Antarctica Australian Plate, covering Australia Eurasian Plate covering Eurasia North American Plate covering North America and north-east Siberia South American Plate covering South America Pacific Plate, covering the Pacific Ocean Notable minor plates include the Indian Plate and the Arabian Plate .

The movement of plates has caused the formation and breakup of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1000 million years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea ; Pangea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the rest).

Related article List of tectonic plates [ edit]History and impact [ edit]Continental drift Main article: Continental drift Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th century. The theory has been superceded by and the concepts and data have been incorporated within plate tectonics.

By 1915 Alfred Wegener was making serious arguments for the idea with the first edition of The Origin of Continents and Oceans. In that book he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener wasn't the first to note this ( Francis Bacon and Snider-Pellegrini preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation.

However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically they did not see how continental rock could plow through the much denser rock that makes up oceanic crust.

In the early 1940s, Maurice Ewing seismically tested the Atlantic edge of the North American continental shelf, and found a granitic layer dropped down to the basaltic ocean floor. If the continent had been torn from Europe and was plowing through the ocean bottom, the edge of the continental shelf should have marked the end of granitic rocks. Later studies aboard the Atlantis found that ocean bottom was not smooth, which suggested it was much stronger than if continents could push it aside.

Beginning in the 1950s, scientists, using magnetic instruments ( magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt -- the iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral ( magnetite) and can locally distort compass readings.

This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time.

As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns.

When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping.

When the rock strata of the tips of separate continents are very similar it suggests that these rocks were formed in the same way implying that they were joined initially. For instance, some parts of Scotland contain rocks very similar to those found in eastern North America. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology .

[ edit]Floating continents The prevailing concept was that there were static shells of strata under the continents. It was early observed that although granite existed on continents, seafloor seemed to be composed of denser basalt.

It was apparent that a layer of basalt underlies continental rocks. However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had ""roots"" was confirmed by George B. Airy a hundred years later during study of Himalayan gravitation, and seismic studies detected corresponding density variations.

By the mid- 1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg. [ edit]Plate tectonic theory Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge.

The most notable was the 1962 publication of a paper by American geologist Harry Hess. Hess suggested that instead of continents moving through oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate.

In 1967, Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions.

[ edit]Explanation of magnetic striping The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading, operating over many millions of years has built the 50,000 km-long system of mid-ocean ridges.

This hypothesis was supported by several lines of evidence:at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;the youngest rocks at the ridge crest always have present-day (normal) polarity;stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.

), suggesting that the Earth's magnetic field has flip-flopped many times. By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory.

Furthermore, the oceanic crust now came to be appreciated as a natural ""tape recording"" of the history of the reversals in the Earth's magnetic field .

[ edit]Subduction discovered A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges.

This idea found great favor with some scientists who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called ""expanding Earth"" hypothesis was unsatisfactory because its supporters could offer no convincing geologic mechanism to produce such a huge, sudden expansion.

Most geologists believe that the Earth has changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?

This question particularly intrigued Harry H. Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U. S. Coast and Geodetic Survey who first coined the term seafloor spreading.

Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spread away from the ridges in a conveyor belt-like motion.

Many millions of years later, the oceanic crust eventually descends into the oceanic trenches -- very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust.

In effect, the ocean basins were perpetually being ""recycled,"" with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously.

Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

[ edit]Mapping with earthquakes During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments ( seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges.

By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40-60? from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States.

The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide. [ edit]Geological paradigm shift The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus ).

Within a matter of only several years geophysics and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question ""How?"". Before acceptance of plate tectonics, geology in particular was trapped in a ""pre-Copernican"" box.

However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s.

Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy. One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer ""how"" questions like ""How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites ?

"", or ""How did the convex and concave margins of the Alpine chain form? "", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.

With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift sea floor into thin atmospheres. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.

Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics.

A great rift, similar to the Great Rift Valley in northeastern Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge. We have inherited some of the old terminology, but the underlying concept is as radical and simple as ""The Earth moves"" was in astronomy.

[ edit]Plate tectonics on Mars As a result of 1999 observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft, it has been proposed that the mechanisms of plate tectonics may once have been active on the planet - see Geology of Mars .

[ edit]See also List of plate tectonics topics List of Tectonic Plates List of Tectonic Plate Interactions [ edit]References Earth System History, Steven M. Stanley, (W. H. Freeman and Company; 1999) pages 211-228 ISBN 0-7167-2882-6Geographica: The complete illustrated Atlas of the world, Editors of James Mills-Hicks (Barnes and Noble Books; New York; 2004) ISBN 0-7607-5974-XPlate Tectonics : An Insider's History of the Modern Theory of the Earth, Oreskes, Naomi ed., Westview Press, 2003, ISBN 0813341329 "