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Island ArcAn island arc is a type of archipelago formed by plate tectonics as one oceanic tectonic plate subducts under another and produces magma. It is a type of volcanic arc. Partial melting of both the subducted oceanic lithosphere and the overriding mantle generates low-density, calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. The resulting volcano chain has the shape of an arc parallel to the convergent plate boundary and convex toward the subducting plate. Weathering and erosion of these volcanic rocks produce black-green beaches composed of olivine sand eroded from the volcanic cones. On the subducting side of the island arc is a deep and narrow oceanic trench, which is the trace at the Earth’s surface of the boundary between the downgoing and overriding plates. This trench is created by the friction of the subducting plate pulling the leading edge of the overriding plate downward. Great frictional forces heat the rock on both plates in this area. Multiple earthquakes occur along this subduction boundary with the seismic epicenters located at increasing depth under the island arc. Oceans basins that are being reduced by subduction are called 'remnant oceans' as they will slowly be shrunken out of existence and crushed in the subsequent orogenic collision. This process has happened over and over in the geologic history of the Earth. Island arcs
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