When a plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it can cause upwelling of magma that can cause volcanism along the subduction front on the overriding plate. This produces an oceanic chain of volcanoes, like Japan. This volcanism causes metamorphism of rocks, intrusions of magma that produce rocks such as granite, and thickens the crust by depositing additional layers of rock from volcanoes.