 As the oceanic slab sinks deep into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape.The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt.This newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Volcanic Arc) above the subduction zone.