Cutaway animation of the internal structure of a volcanic arc. The arc is a chain of volcanoes running parallel to a subduction zone, where a dense slab of oceanic crust (left) is pushed below the lighter continental crust (right). The slab of crust is pushed into the Earth's mantle (red). The rock of the oceanic crust is saturated with seawater, and the loss of this volatile water into the hot mantle lowers the mantle's melting point, leading to it melting (orange) above the slab, generating liquid magma. The magma is less dense than the surrounding mantle, and rises through it to the base of the continental plate. Here it rises through the rock as an intrusion, sometimes displacing surrounding rock, and sometimes filling cracks and gaps. When the magma reaches the surface, it forms a volcano that erupts the magma as lava. The pressure and temperature at which the mantle melting occurs are always found at the same depth, so the distance from the subduction zone to the volcanoes depends only on the angle of the subducting plate. The volcanoes are generally found some 100 kilometres from the plate boundary.