In a collision of tectonic plates, the Pacific Plate is being subducted beneath the Australian Plate, bringing along oceanic crust and a thin veneer of pelagic sediment. Behind the subduction zone, the crust of the Australian Plate is extended, forming backarc basins. At certain depths, usually around 200 kilometers (about 100 nautical miles), subducted materials melt, producing magmas that rise buoyantly to pond in the overlying mantle wedge and periodically erupt on Earth's surface as lavas, forming arc volcanoes. (Illutration courtesy of Institute of Geological and Nuclear Sciences Ltd.)[back]
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