In this case also the two plates consist of the same material. However, one oceanic plate is generally older and therefore heavier than the other plate.
The older plate gets subducted, or pushed down, beneath the younger and more buoyant plate. The sinking plate gets heated up as it is subducted and remelts.
With the increased pressure, the remelted material comes to the surface in volcanoes, forming island arcs. Japan is an example of land being formed when two oceanic plates collide.