Oceanic crust is typically 5 kilometers thick. The continental crust thickens from its normal 40 kilometers to 70 beneath the high Andes. When the descending slab reaches a depth of about 100 kilometers, it begins to melt, causing, directly or indirectly, many of the events associated with mountain-building. Layered rocks shorten by folding, but the stack of layers also becomes thicker. Thrust-faulting thickens the crust by stacking slices of crust atop one another.
Intrusions add volume to the crust.
A great deal of magma never invades the crust but accumulates at its base, a process called underplating . Where the crust is heated, thermal expansion causes the rocks to become lighter and more buoyant. Both plates are oceanic crust Because both plates are basaltic, most volcanism is basaltic Initially results in an island chain, but long-continued subduction and accumulation of volcanic rocks and sediments can create a substantial land mass. Ocean-Continent Descending plate is oceanic crust, overriding plate is a continent. Often continent-ocean subduction will bring a continent into contact with an island arc or small continental fragment.