The arcs form when an oceanic plate, often originating from a spreading oceanic trench, is subducted under a continental plate. The speed of the plates' collision determines how deep and steeply the oceanic plate is forced down under the continental plate. As the plate dives, the water that saturates the oceanic plate is subjected to great pressure, releasing the water into the mantle, which becomes softer, with a lower melting point. The melted mantle material, magma, rises through the crust and surfaces as an arc of volcanoes behind the continental plate's leading edge.