Here's how it works:

* Oceanic plates are constantly moving and pushing against each other.

* When an oceanic plate collides with another oceanic plate or a continental plate, the denser plate (usually the oceanic plate) is forced beneath the less dense plate. This process is called subduction.

* As the subducted plate descends into the Earth's mantle, it is melted and recycled into the mantle.

This process is a key part of the plate tectonic cycle, constantly renewing the Earth's crust.