A. Oceanic-Continental Convergence When a plate capped with continental crust converges with a slab of oceanic lithosphere, the buoyant continental block remains ?floating?; denser oceanic crust sinks into the mantle. When oceanic slab goes down about 100km, melting is triggered in the asthenosphere above it. This is because the ?wet? oceanic rock in a high-pressure place melts at a much lower temperature than ?dry? rock of the same material does. Sediments and oceanic crusts have lots of H2O, which is carried down much depth by a subducting plate. As the plate moves down, heat + pressure drive water from voids in the rock. At a depth of 100km, the wedge of rock is hot enough so that H2O from slab, when exposed, causes some melting.