Subduction - Wikipedia
The rate of spreading along the Mid-Atlantic Ridge averages about The location where sinking of a plate occurs is called a subduction zone. PDF | The relationship between the angle of the subducting plate and the dynamics Age, spreading rates, and spreading asymmetry of the World's ocean crust. spread, with the result that plate tectonics and subduc- tion zones are .  The relationship between subduction zones and convergent plate .. Importance of convergence rate and age of subducted lithosphere. (a) Plot of.
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 compressionand 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 glaucophanewhich 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]eclogiteswhich consist of high-pressure minerals such as red garnet pyrope and omphacite pyroxeneform.
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 km 60 milesit 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 magmawhich 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 arctypically 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 islandsknown as an island arcthat 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. 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 processessuch 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 ridgesbut subtle trace element analyses can detect the influence of a nearby subducted slab. This style of subduction predominates in the western Pacific Oceanin 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 deformationmetamorphismand even melting of the strata deposited in the basin.
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 continentswhich 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 Cascadesthe Sierra Nevadaand 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 Seaa wedge-shaped body of water that was located between Gondwana and Laurasialed 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 paleogeographyDistribution 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. Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zoneand, 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.
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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.
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 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 continentand the collisions have resulted in important pulses of mountain building.
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. 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 ophiolitesare 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 Californiawhich 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 Periodroughly million years ago, in an extensional regime within either a back-arc or a forearc basin. In the late Mesozoicit 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. 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. 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 erosionit 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 Oceanand 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 SiberiaChina 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.
Two main tectonic structures are spreading centers and subduction zones. Spreading centers occur at the boundary between two plates that are moving apart, called divergent plate boundaries.
Here the plate motion opens a gap between the plates and magma from the mantle rises up through it. When the magma reaches water at the ocean floor most spreading centers are in the oceanit cools and hardens, and becomes new oceanic crust.
As the plates continue to move apart, more and more new basaltic crust is created.
The most famous spreading center is the Mid-Atlantic Ridgebut there are many others. California has a small spreading center located in the Brawley Seismic Zone of Imperial County, the northern-most spreading center of the East Pacific Rise. The largest earthquakes are associated with subduction zones. Because of subduction, the sea floor is relatively young, no where more than about million years old. The continents, on the other hand, are much older. The Science Teacher, v. Return to top Videos: Looks at the relationship between plate tectonics and marine mineral deposits; shows how the ocean floor is being mapped and looks at recovery systems for marine resources including underwater scoops and shovels and giant "vacuum cleaners".
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Plate tectonics, continental drift, spreading centers, subduction zones
Describes methods of exploring the oceans; interaction of oceans with the biosphere, lithosphere, and atmosphere to create a unique environment; and the three main characteristics of oceans: Order from Scott Resources, P. Collins, CO, ; phone This set illustrates submarine research using deep water submersibles and remotely operated vehicles to study ocean floor rift systems. Includes photos of black smokers, tube worms, and equipment used by oceanographers. Order from the American Geophysical Union, Attn.: Orders, Florida Avenue, N.
In this module students examine data from sediments on the ocean floor, determine whether the data support the theory of sea-floor spreading, and calculate the rate of spreading of the East Pacific Rise.