This illustration shows the Pacific plate in the east colliding with the Philippine plate in the west. A consequence of this collision is subduction with the down-going slab comprised of oceanic crust (or lithosphere) and a thin veneer of pelagic (open sea) sediment. This causes extension behind the subduction zone, which is represented by backarc basins forming behind the arc front. At certain depths, usually around 200 km (about 100 nautical mi), there is melting of the subducted materials. The melting produces magmas that rise buoyantly to pond in the overlying mantle wedge and periodically erupt on Earth's surface as lavas, forming arc volcanoes. Image courtesy of Institute of Geological and Nuclear Sciences Ltd.