Answer:
When an oceanic plate strikes a continental plate, it results in the formation of a subduction zone, where the high density oceanic plate sink below the less denser continental plate. This subducting plate undergoes partial melting at greater depth. As a result of this, the magma at greater depth becomes lighter and less dense, and eventually rises up towards the surface. This magma comes out to the surface in the over-riding plate, in the form of volcanoes or a chain of volcanoes.
Thus, due to this convergent motion of two plates, namely the oceanic and the continental plate, it forms a subduction zone that is marked by a deep sea trench and volcanoes. For example, collision between South American plate and Pacific plate gave rise to the formation of St. Andes mountain.
