subduction zone

subduction zone large-scaled narrow region in the earth's crust where, according to plate tectonics , masses of the spreading oceanic lithosphere bend downward into the earth along the leading edges of converging lithospheric plates where it slowly melts at about 400 mi (640 km) deep and becomes reabsorbed. Subduction zones are usually marked by deep ocean trenches that often exceed 6 mi (10 km) compared to the ocean's overall depth of 2 to 4 mi (3 to 5 km). A pattern of earthquakes of shallow, intermediate, and deep focus occurs along the same angle as the descending plate, which is steeply inclined (30°-60°) toward the continent behind the trench in a zone called the Benioff Zone, discovered by the U.S. seismologist Hugo Benioff. This earthquake pattern enables geophysicists to trace the descending plate to depths of 600 to 700 km (370-440 mi), where temperatures are thought to be between 1,000°C and 2,000°C (1,800°-3,600°F). As the oceanic plate descends, friction between the two plates probably causes partial melting of the descending plate forming a magma of andesitic composition that rises along fractures. If the overlying crustal plate is oceanic, the magma may erupt to form volcanic island arcs, such as Japan or the Aleutians. If the overlying plate is continental, a line of batholiths and volcanoes may be created as in the Coast Ranges of Canada and the W United States. See continent ; continental drift ; seafloor spreading .

subduction zone The zone, at an angle to the surface of the Earth, down which a lithospheric plate descends. Most present day subduction zones extend from trenches on the ocean floor, from where a zone of earthquake hypocentres (called a Benioff zone) extends, at an angle ranging from near-horizontal to near-vertical, to a depth of up to 700 km. Andesitic volcanoes form approximately 100 km above the subducting slab, and the presence of andesitic volcanoes in the geologic record is regarded as evidence of an ancient subduction zone and thus of a destructive plate margin.