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Name: Plate Tectonics Review Date Individual who proposed the Continental Drift Theory Boundary that produces rift valleys and mid-ocean ridges. The Earth's outer shell, the lithosphere, consisting of the crust and uppermost mantle, is divided into a patchwork of large tectonic plates that move slowly. Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and . Plate boundaries are commonly associated with geological events such as .. Later studies (discussed below on this page), therefore, invoked many of the relationships recognized during this pre-plate tectonics period to.
The continents have a crust that is broadly granitic in composition and, with a density of about 2. 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. 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. The four main types of seismic waves are P waves, S waves, Love waves, and Rayleigh waves. 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 metres 2, feetwhile the average depth of oceanic crust is 3, metres 12, feet. The lithosphere itself includes all the crust as well as the upper part of the mantle i. However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity.
This process begins at about 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. The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3. 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.
At a depth of about 5, km 3, milesthe outer core transitions to the inner core. 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 coreThe internal layers of Earth's core, including its two inner cores. Plate boundaries Lithospheric plates are much thicker than oceanic or continental crust. Their boundaries do not usually coincide with those between oceans and continentsand 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. 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. Divergent margins As plates move apart at a divergent plate boundarythe release of pressure produces partial melting of the underlying mantle. This molten material, known as magmais 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.
Plate Tectonics - Crystalinks
Settling of the continental blocks creates a rift valleysuch 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. The Thingvellir fracture lies in the Mid-Atlantic Ridge, which extends through the centre of Iceland. 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. 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, km [9, miles] wide could be produced in million years.
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. 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. 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 km about 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 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 trenchas 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. Aleutian islandsMariana Islandsand the Japanese island arcolder, 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. 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.
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.
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.
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 s  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 chemistrymineral from variations in mineral structuresor thermal through thermal expansion and contraction from heat energy.
The manifestation of this varying lateral density is mantle convection from buoyancy forces. 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: Plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. 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.
Name: Plate Tectonics Review Date
Plume tectonics 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 Learn how and when to remove this template message In the theory of plume tectonics developed during the s, 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 s, 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.
Surge tectonics 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 s and s. Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges.
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.
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.
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, Eurasianand Antarctic plates. Gravitational sliding away from mantle doming: 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. November Learn how and when to remove this template message Alfred Wegenerbeing a meteorologisthad 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.
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 sthe 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  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   The centrifugal forcewhich is treated as a slight modification of gravity  : 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: Later studies discussed below on this pagetherefore, 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.
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. InGeorge W.
Bostrom  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 study,  scientists reviewed and advocated these earlier proposed ideas.
It has also been suggested recently in Lovett 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,  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. 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.
Where oceanic crust meets continental crust The denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision. Where continental crust meets continental crust Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas.
Divergent boundaries — where two plates are moving apart. The space created can also fill with new crustal material sourced from molten magma that forms below. Divergent boundaries can form within continents but will eventually open up and become ocean basins. On land Divergent boundaries within continents initially produce rifts, which produce rift valleys.