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In plate tectonics, subduction zones exist at convergent plate boundaries where Oceanic lithosphere collides with either continental lithosphere or oceanic lithosphere and sinks below the latter plate into the mantle. Subduction is driven by the density contrast between the plates involved. While there is no significant density contrast between oceanic crust and continental crust (both about 2.8 g/cc), because continental crust is much thicker than oceanic crust it displaces more of the dense mantle and as a consequence the continental lithosphere (the crust plus the strong portion of the upper mantle) is less dense than oceanic lithosphere. Oceanic lithosphere is then preferentially subducted at ocean/continent convergent margins due to its high density relative to continental lithosphere. Where ocean/ocean convergence occurs, the older, colder, thicker, denser, oceanic lithosphere will typically subduct beneath the younger, hotter, thinner oceanic lithosphere. However the presence of features called large igneous provinces (LIPS) which result in extreme thickening of the oceanic crust can cause older oceanic lithosphere to win the density contrast battle with younger oceanic lithosphere. This is because a LIP located on a piece of old oceanic lithosphere will displace high density mantle allowing the old oceanic lithosphere to have a lower density than the younger oceanic lithosphere. Seismic tomography appears to show that some of the subducted oceanic plates can reach as far as the core-mantle boundary.
Subduction results in creation of oceanic trenches, such as the Mariana trench, and mountain building. Volcanoes that occur along these boundaries, such as Mt. Saint Helens and Mt. Fuji, often occur in groups that take the shape of an arc, hence the term volcanic arc. The arcuate shape is a result of the geometry of plate collision on a sphere.
The magmatism associated with volcanic arc formation does not occur a uniform distance away from the trench. However, a relationship has been found that relates volcanic arc location to depth of the subducted crust as defined by the Wadati-Benioff zone. Studies of many of the volcanic arcs from around the world have revealed that volcanic arcs tend to form at a location where the subducted slab has reached a depth of ~ 100 km. This has interesting implications for the mechanism which causes the magmatism at these arcs.
Subduction zones are also notorious for producing large scale earthquakes because of the intense geological activity. The introduction of cold oceanic crust into the mantle depresses the local geothermal gradient and causes a larger portion of the earth to deform in a brittle fashion than would in a normal geothermal gradient setting. Because earthquakes can only occur when a rock is deforming in a brittle fashion, subduction zones have the potential to create very large earthquakes. If this earthquake occurs under the ocean it has the potential to create tsunamis, such as the earthquake caused by subduction of the Indo-Australian Plate under the Eurasian Plate on December 26, 2004, that devastated the areas around the Indian Ocean. Small tremors that create tiny, unnoticeable tsunamis happen all the time because of the dynamics of the earth.
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