Subduction zones occur at collision boundaries where at least one of the colliding lithospheric plates contains oceanic crust . In accord with plate tectonic theory, collision boundaries are sites where lithospheric plates move together and the resulting compression causes either subduction (where one or both lithospheric plates are driven down and destroyed in the molten mantle) or crustal uplifting that results in orogeny (mountain building). Subduction zones are usually active earthquake zones. Subduction zones are the only sites of deep earthquakes. The areas of deep earthquakes, ranging to a depth of 415 mi (670 km), are termed Benioff zones. Deep earthquakes occur because of forces due to plate drag and mineral phase transitions. The release of forces due to sudden slippage of plates during subduction can be quick and violent. Subduction zones can also experience shallow and intermediate depth earthquakes.
Oceanic crust is denser than continental crust and is subductable. Moreover, as oceanic crust–bearing plates move away from their site of origin (divergent boundaries), the oceanic crust. The cooling results in an increase in general density of the oceanic crust. The concurrent loss of buoyancy makes it easier to subduct the crust. In addition, colliding plates create tremendous force. Although lithospheric plates move very slowly, the plates have tremendous mass. Accordingly, at collision each lithospheric plate carries tremendous momentum (the mathematical product of velocity and mass) that provides the energy to drive subduction. In zones of convergence, including subduction zones, compressional forces (i.e., compression of lithospheric plate material) dominates.
Earth's crust is fractured into approximately 20 lithospheric plates. Lithospheric plates move on top of the asthenosphere (the outer plastically deforming region of Earth's mantle). Because Earth's diameter remains constant, there is no net creation or destruction of lithospheric plates. Each lithospheric plate is composed of a layer of oceanic crust or continental crust superficial to an outer layer of the mantle. Oceanic crust is composed of high-density rocks, such as olivine and basalt . In contrast, continental crust is composed of lower density rocks such as granite and andesite .
Within subducting zones, oceanic crust can make material contributions of lighter crustal materials to overriding continental crust. As the oceanic crust subducts, parts may be scraped off to form an accretion prism. Rising material at sites where oceanic crust subducts may form island arcs .
When oceanic crust collides with oceanic crust, both subduct to form an oceanic trench (e.g., the Marianas trench). Dual plate subduction can result in ocean trenches with depths approximating 38,000 ft.
When oceanic crust collides with continental crust, the oceanic crust subducts under the lighter continental crust. The subducting oceanic crust pushes the continental crust upward into mountain chains (e.g., the Andes) and may contribute lighter molten materials to the overriding continental crust to form volcanic arcs (e.g., the "ring of fire"; a ring of volcanoes bordering the Pacific Rim. Because continental crust does not subduct, when continental crust collides with continental crust, there is a uplift of both crusts.
At triple points where three plates converge, the situation becomes more complex and in some cases there is a mixture of subduction and uplifting.
Convergent plate boundaries are, of course, three-dimensional. Because Earth is an oblate sphere, lithospheric plates are not flat, but are curved and fractured into curved sections akin to the peeled sections of an orange. Convergent movement of lithospheric plates can best be conceptualized by the movement together of those peeled sections over a curved surface.
See also Divergent plate boundary; Earth, interior structure; Earthquakes; Geologic time; Magma chamber; Magma; Mohorovicic discontinuity (Moho); Plate tectonics; Volcanic eruptions; Volcano