A subduction zone forms where two tectonic plates converge and one plate, usually the denser oceanic lithosphere, sinks beneath the other plate into the Earth's mantle. This occurs because oceanic plates, formed at mid-ocean ridges, cool and become denser as they move away from the ridge, eventually becoming heavy enough to sink beneath a lighter plate such as continental lithosphere or younger oceanic crust. Subduction may start spontaneously due to density contrasts or be forced by existing plate motions, creating a zone where recycled crust sinks into the mantle and new crust forms on the surface. The sinking plate releases water which lowers the melting point of mantle rocks, causing magma formation and volcanic activity often seen in arcs above subduction zones.