The process of subduction takes place when

The Subduction process takes place when denser oceanic plates are forces beneath the continental plates.

Explanation:

A geological process occurring at tectonic plate's convergent boundaries is a subduction process. In this process one plate moves beneath another plate and this makes to sink because of presence of gravity in the mantle.  

The zone in which this process occurs is called as subduction zones. A stable subduction zone is one that involves the plates the oceanic lithosphere to slide beneath the continental of another plate because of the oceanic lithosphere's high density.