It's not entirely accurate to say that oceanic crust *always* subducts underneath continental crust. While it's the more common scenario, there are exceptions. Here's a breakdown of the factors involved:

Why oceanic crust typically subducts under continental crust:

* Density: Oceanic crust is denser than continental crust due to its composition. Oceanic crust is primarily composed of basalt and gabbro, which are denser than the granite and other felsic rocks that make up continental crust.

* Plate Tectonics: When two plates collide, the denser plate (oceanic) will bend and slide beneath the less dense plate (continental). This is the principle of subduction.

* Subduction Zones: These zones are marked by deep ocean trenches, volcanic arcs, and earthquakes. The descending oceanic plate melts as it plunges into the mantle, creating magma that rises to the surface, forming volcanoes.

Exceptions to the rule:

* Oceanic-Oceanic Convergence: When two oceanic plates collide, the older, colder, and denser plate will subduct beneath the younger, warmer plate. This can lead to the formation of island arcs.

* Continental-Continental Convergence: When two continental plates collide, neither is dense enough to subduct fully. Instead, they buckle and fold, forming mountain ranges like the Himalayas.

In summary:

While oceanic crust typically subducts beneath continental crust due to density differences and the principles of plate tectonics, there are exceptions in cases of oceanic-oceanic and continental-continental collisions.