In this case, the more dense and thinner ocean crust is forced below the thicker, less dense continental crust forming an oceanic trench. Frictional heating results in partial melting of the plate itself as well as sediments carried with it. In this case, the magma is rich in volatile elements and the eruptions are quite explosive as in the Mt. Saint Helens eruption of 1980. In this case, the subduction has gone on long enough that there's no more oceanic plate left and two continental masses collide. The low density of the continental crust and the resulting buoyancy prevents subduction and massive deformation results.
Scientists believe that their location has remained constant over earth's history.
Continental crust is too light. This results in disparity in ages between the two kinds of crust.