A tectonic plate is a huge section of rock that makes up the land on Earth. These tectonic plates are as large as entire continents and move, although they move.
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And, transform boundaries where plates slide past each other, ideally with little or no vertical movement.
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Most transform boundaries are below sea level and so not easy to see. The San Andreas fault in California is a transform boundary. One of these is that plate tectonic processes have led to the evolution of the earth. When the earth originated it contained no continents, and consisted of only a few kinds of igneous rock - roughly the composition of the moon. But through plate tectonic processes the earth has evolved first to form volcanic island chains volcanic arcs scattered across a single world wide ocean , and then for these to enlarge to form the large continental masses we live on today.
These evolutionary processes take place because igneous rocks evolve at convergent and divergent plate boundaries. At both places magma is generated, and through processes of fractional melting partial melting whole new rocks are created including those that form continental masses. A second significant idea derived from plate tectonic theory is that as the continents have grown through time they have alternately come together to form supercontinents , only to fragment again to form smaller isolated continents. The last supercontinent was Pangaea that formed about million years ago when the isolated continents collided.
During its existence only the one large continent existed, balanced by one large ocean, Panthalassa. During the fragmentation stage beginning about million years ago and still going on the present Atlantic ocean opened up, and all the continents are now scattering across the globe. But, prior to Pangaea another supercontinent called Rodinia existed about million years ago. It also formed from the accumulation of isolated continents, only to fragment shortly after its formation.
And before that there were other supercontinents.
Principles of plate tectonics
Simplistically, we can think about it this way. The present day Atlantic ocean is getting wider because of sea floor spreading. But since the earth is spherical that means things must be coming together some place else, and that is in the Pacific ocean. The Pacific ocean is getting smaller along subduction zones convergent plate boundaries under North and South America, and Japan, as western North America and Asia get closer together.
Plate tectonics
Sometime in the future the Pacific ocean will close completely and Asia and North America will collide to form another supercontinent. The history of Gondwana and Rodinia tell us that this supercontinent will not stay together long either, and will fragment into isolated continents scattered across the earth again. The cycle begins with a supercontinent perched on one side of the earth, balanced on the other side by a superocean. The supercontinent fragments, sending small continental pieces across the ocean to collide to form another supercontinent on the opposite side of the earth.
But, shortly that supercontinent fragments also to repeat the cycle.
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And this has been going on for 4 billion years, requiring about half a billion years for each cycle. 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 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.
The subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithosphere about km 60 miles thick that carries a relatively thin cap of oceanic sediments. 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. Most, but not all, earthquakes in this planar dipping zone result from compression , and the seismic activity extends to km to 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. When the downward-moving slab reaches a depth of about 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.
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. 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.
This style of subduction predominates in the western Pacific Ocean , in which a number of back-arc basins separate several island arcs from Asia. 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.
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: 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. 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 million years. 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 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 km miles in width along the western margin of the North American continent , and the collisions have resulted in important pulses of mountain building. 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 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 mids, 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.
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See above back-arc basins. 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. 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 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, km 1, 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 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. 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. 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.
Tuzo Wilson and American geophysicist W. Jason Morgan explained such topographic features as the result of hotspots.
The number of these hotspots is uncertain estimates range from 20 to , 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, km 1, 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. 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, km 2, miles northwest to Midway and a further 2, km 1, 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. 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 point of emergence of the axis through the surface of the sphere is known as the pole of rotation. 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.
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.