There are three types of convergent plate boundaries: continental-continental, oceanic-oceanic, and oceanic-continental. At continental-continental boundaries, the two plates are the same density so the crust folds and crumples, sometimes forming mountains. For oceanic-oceanic boundaries, one oceanic plate subducts under the other due to being older, colder and denser, melting in the asthenosphere. This can create deep ocean trenches and island arcs. At oceanic-continental boundaries, the oceanic plate subducts under the continental plate, also forming trenches and coastal mountains.