When two oceanic plates converge, island arcs. Japan, the Philippines, the islands of the Caribbean, and the Marianas are all examples of island arcs created by converging oceanic plates.

When two oceanic plates converge, one will subduct under the other. Although all oceanic crust is more dense than continental crust, the density of oceanic crust varies, and the more dense oceanic plate will subduct underneath the less dense oceanic plate. This is why there are both very deep trenches and very large mountain chains on the seafloor!