In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. A subduction zone is an area on Earth where two tectonic plates move towards one another and subduction occurs. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate a fingernail grows).
Subduction zones involve an oceanic plate sliding beneath either a continental plate or another oceanic plate (that is, the subducted plate is always oceanic while the subducting plate may or may not be oceanic). Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building. This is because subduction processes result in melt of the mantle that produces a volcanic arc as relatively lighter rock is forcibly submerged.
Orogenesis, or mountain-building, occurs when large pieces of material on the subducting plate (such as island arcs) are pressed into the overriding plate. These areas are subject to many earthquakes, which are caused by the interactions between the subducting slab and the mantle, the volcanoes, and (when applicable) the mountain-building related to island arc collisions.
Subduction zones are the opposite of divergent boundaries, where tectonic plates move apart.
Subduction zones mark sites of convective downwelling of the Earth's lithosphere (the crust plus the top brittle portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate. The down-going slab -- the leading edge of the subducting plate—is overridden by leading edge of the other plate. The slab sinks at an angle of approximately 25 to 45 degrees to the surface of the Earth. At a depth of approximately 80–120 km, the basalt of the oceanic slab is converted to a metamorphic rock called eclogite. At this point, the density of the oceanic lithosphere increases and it is carried into the mantle by the downwelling convective currents. It is at subduction zones that the Earth's lithosphere, oceanic crust, sedimentary layers, and some trapped water are recycled into the deep mantle. Earth is the only planet where subduction is known to occur. Without subduction, plate tectonics could not exist.
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