Divergent, or destructive plate margins cause volcanoes. As the more dense plate subducts, it will melt at depth and the water driven off, will lower the melting point of the overlying mantle wedge, this will create a type of magma. Volcanic Island Arcs are formed at oceanic-oceanic destructive plate margins. The hot, bouyant magma, is less dense that the surrounding rock, so will rise to the surface, and will collect beneath the plate that hasn't been subducted. It will then, under increased pressure erupt under the sea. Over thousands to millions of years, it will keep erupting, to form islands.
