When two tectonic plates that are comprised of oceanic crust shares a convergent type of plate boundary, then the denser one subducts below the less dense one. Due to this convergent plate motion, a subduction zone is created, which is marked by the presence of an oceanic trench. As the subducting plate penetrates downward into the deeper zone, the process of partial melting takes place, which leads to the melting of the crust and eventually the constituent minerals are mixed with the magma. This magma again comes upward due to the force exerted by the convection current and thereby results in the formation of volcanoes in the over-riding plate.
Thus, the collision of two oceanic plates gives rise to the formation of a subduction zone and a trench.
