Subduction zones

(geology) Regions where portions of the earth's tectonic plates are diving beneath other plates, into the earth's interior. They are defined by deep oceanic trenches, lines of volcanoes parallel to the trenches, and zones of large earthquakes that extend from the trenches landward.

Regions where portions of the Earth's tectonic plates are diving beneath other plates, into the Earth's interior. Subduction zones are defined by deep oceanic trenches, lines of volcanoes parallel to the trenches, and zones of large earthquakes that extend from the trenches landward.

Plate tectonic theory recognizes that the Earth's surface is composed of a mosaic of interacting lithospheric plates, with the lithosphere consisting of the crust (continental or oceanic) and associated underlying mantle, for a total thickness of about 100 km (60 mi). Oceanic lithosphere is created by sea-floor spreading at mid-ocean ridges (divergent, or accretionary, plate boundaries) and destroyed at subduction zones (at convergent, or destructive, plate boundaries). At subduction zones, the oceanic lithosphere dives beneath another plate, which may be either oceanic or continental. Part of the material on the subducted plate is recycled back to the surface (by being scraped off the subducting plate and accreted to the overriding plate, or by melting and rising as magma), and the remainder is mixed back into the Earth's deeper mantle. This process balances the creation of lithosphere that occurs at the mid-ocean ridge system. The convergence of two plates occurs at rates of 1–10 cm/yr (0.4–4 in./yr) or 10–100 km (6–60 mi) per million years.

During subduction, stress and phase changes in the upper part of the cold descending plate produce large earthquakes in the upper portion of the plate, in a narrow band called the Wadati-Benioff zone that can extend as deep as 700 km (420 mi). The plate is heated as it descends, and the resulting release of water leads to melting of the overlying mantle. This melt rises to produce the linear volcanic chains that are one of the most striking features of subduction zones. See also Lithosphere.

Subduction zones can be divided in two ways, based either on the nature of the crust in the overriding plate or on the age of the subducting plate. The first classification yields two broad categories: those beneath an oceanic plate, as in the Mariana or Tonga trenches, and those beneath a continental plate, as along the west coast of South America. The first type is known as an intraoceanic convergent margin, and the second is known as an Andean-type convergent margin. See also Mid-Oceanic Ridge; Plate tectonics.

Active volcanoes are highly visible features of subduction zones. The volcanoes that have developed above subduction zones in East Asia, Australasia, and the western Americas surround the Pacific Ocean in a so-called Ring of Fire. At intraoceanic convergent margins, volcanoes may be the only component above sea level, leading to the name “island arc.” The more general term “volcanic arc” refers to volcanoes built on either oceanic or continental crust. See also Volcano.

An eventual consequence of subduction is orogeny, or mountain building. Subduction zones are constantly building new crust by the production of volcanic material or the accretion of oceanic sediments. However, the development of the greatest mountain ranges—the Alps or the Himalayas—occurs not during “normal” subduction but during the death of a subduction zone, when it becomes clogged with a large continent or volcanic arc. See also Orogeny.