When melting first occurs, it happens at the peripheries of individual crystal grains (b - right), yielding minute pockets of magma. As it moves away from the ridge, it cools. The Earth’s crust triggers the questions of Earth scientists regarding its formation and evolution through time. This place is known as a subduction zone. All maps, graphics, flags, photos and original descriptions © 2020 worldatlas.com. (Modified from Lillie, 2005.) Knowing what subduction is, does not explain why it happens. Subduction zone, oceanic trench area marginal to a continent in which, according to the theory of plate tectonics, older and denser seafloor underthrusts the continental mass, dragging downward into the Earth’s upper mantle the accumulated trench sediments. Remember, oceanic plates are formed from mantle material at midocean ridges. Oceanic lithosphere is formed hot and thin at mid-ocean ridges and grows thick as more rock hardens underneath it. The exact angle of the subduction depends on the age of the subducting plate; older plates fall at a sharper angle. The crust sticks in some places, storing up energy that is released in earthquakes. These earthquakes of the Benioff Zone (or Wadati-Benioff Zone) occur near the upper surface of the descending plate (or slab). Although tectonic plates may be oceanic or continental, the act of subduction (sliding under another plate) only happens to oceanic plates. Beneath the active volcanic arc lie intrusive igneous rocks formed from magma that didn't make it all the way to the surface before crystallizing. Subduction is one of the two major processes of plate tectonics, the other being seafloor spreading. Some of the largest fault lines (by both width and length) in the world are located at subduction zones. Smaller plate boundaries typically generate smaller tremors. The process of dehydration that ultimately triggers melting beneath subduction zones must occur approximately beneath the arc such that the mechanism for fluid transfer is rapid and probably vertical. When the older plate is holding a continent however, it does not sink, which is reassuring. the introduction of water B.) Aside from triggering mantle melt, this reaction may also trigger partial melting of the subducting slab itself. These extremely hot liquids, which primarily consist of carbon dioxide and ocean water, effectively melt the plate that has remained on top. This trend is strongest in subduction zone volcanism. The hotter temperatures and increased pressure found at these depths cause the basalt rock of the sinking plate, also referred to as a slab, to morph into eclogite rock. Decompression melting occurs at mid-ocean ridges. The volatiles that enter the asthenosphere trigger flux melting in the asthenosphere. Volcanoes and volcanic activity often occur as a result of the subduction process because the subducting slab releases liquids as it moves into the extreme temperatures of the mantle layer. Upon reaching a subduction zone, the sinking plate heats up and gets squeezed, resulting in the gradual release of some or all of its water. The explanation of subduction perhaps makes the process sound relatively smooth, with one plate slowly sinking and falling off into the mantle layer below. At the surface, subduction zones result in arc volcanism, and during eruptions carbon and other volatiles are released into the atmosphere. convergent tectonic plate boundaries or where two tectonic plates come crashing together Most scientists, however, describe subduction as a rough, scraping activity that is characterized by high amounts of friction as the two plates rub past each other. As the ocean crust sinks deeper into the mantle the pressure increases (the temperature of the ocean crust rocks increases more slowly because rocks are poor conductors of heat). As the subducting plate moves under its neighboring tectonic plate, gravity pushes it further down and into the mantle layer of the earth. They occur down to depths of around 670 km at some subduction zones. An oceanic plate will sink back into the mantle. This wave activity occurs because earthquakes cause the crust of the earth to snap and rebound. Common concepts about “how subduction zones work” are reassessed—with particular emphasis on the atypical Cascades volcanic arc. Let's visualize what decompression meltin… In geological terms, subduction is the act of one tectonic plate moving under another tectonic plate at the point of their convergent boundary. The place where subduction has occurred, however, does not result in the formation of mountains. Subduction zones form where a plate with thinner (less-buoyant) oceanic crust descends beneath a plate with thicker (more-buoyant) continental crust. Mid-Oceanic Ridge (MOR) is the place where magma from below the Earth emerges an… Subduction is a geological process in which oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries.Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. There is a deep ocean trench where the oceanic plate bends downward. How does subduction trigger melting? Remember that flux melting refers to the process whereby the addition of the volatile to an already hot rock will cause a hot solid rock to start to melt. Introduction. Earthquakes are well-known geological phenomena, usually caused when two pieces of the Earth’s crust, called tectonic plates, bang or rub against each other. The reason for this is that oceanic plates, also known as the oceanic lithosphere, start out as thin and hot sections of earth. /Earth Planetary Science letters 121 (1994) 227-244 subduction shear zone exceeds the temperature of the partial melting reactions (~ 750 at 2.5 GPa). What Happens During the Process of Seafloor Spreading. enough to “sweat” fluids and initiate melting. Phlogopite + Diopside + Orthopyroxene = H 2 O + Melt [1] Lawsonite remains stable up to 1080 °C and 9.4 GPa. Why are the subduction zone volcanoes so steep? This accounts for the copious volcanism near subduction zones, where descending plates carry down water, sediment, carbonaceous matter and hydrated mineral with them. But as it spreads away from the ridge and cools and contracts (becomse denser) it is able to sink into the hotter underlying mantle. The overriding slab insulates the subducting slab, increasing temperature and causing melting. Tsunamis may occur over a period of just a few minutes or even hours, as the displaced water rushes onto nearby land. Volcanic Arcs:  The basaltic ocean crust contains hydrous minerals like amphiboles, some of which formed by hydrothermal alteration as seawater seeped through hot, fractured, young ocean crust at the midocean ridge. melting of mantle-wedge peridotite exert the main controls on melt pro-duction in volcanic arcs. Where two tectonic plates converge, if one or both of the plates is oceanic lithosphere, a subduction zone will form. By Amber Pariona on March 6 2018 in Environment. A plane of earthquake focci descend from the area around the trench underneath the overriding plate. Geologists explain that subduction occurs to oceanic plates because they are denser and cooler than continental plates. Peacock et al. The water gradually seeps upward into the overlying wedge of hot mantle. Many mountain ranges around the world were created in this manner. 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