When an oceanic crust converges with a an oceanic crust, volcanoes form when the two plates converge. Magma piles up until the volcano reaches sea level. When oceanic crust and continental crust converges, volcanoes form because the oceanic crust sub ducts under the continental crust pushing up the continental crust. When continental crust and continental crust converge, volcanos form the crust gets pushed up sideways.