When a continental crustal plate collides with an oceanic crustal plate, the denser oceanic crust is subducted beneath the less dense continental crust. This means it gets forced down into the mantle.

Here's why:

* Density: Oceanic crust is primarily composed of basalt, which is denser than the granite that makes up continental crust. This density difference is the key factor in subduction.

* Plate Tectonics: The Earth's lithosphere (crust and upper mantle) is broken into large plates that are constantly moving. These plates interact at their boundaries, and when a denser plate collides with a less dense plate, the denser plate is forced under the less dense plate.

The primary reason the continental crust stays on top is its lower density. This density difference drives the subduction process, leading to the formation of features like volcanic arcs and deep ocean trenches.