What is subduction?

Subduction is defined as the process where one tectonic plate moves under another and sinks into the mantle. This phenomenon typically occurs at convergent plate boundaries, where two plates collide, and one is forced down into the asthenosphere due to differences in density. Oceanic plates, being denser than continental plates, are often the ones that subduct, leading to various geological activities such as the formation of deep ocean trenches, volcanic arcs, and earthquakes.

The other choices describe different geological processes. The option about plates moving apart refers to divergent boundaries, where tectonic plates separate and new crust is formed. The option concerning the gradual forming of mountains relates to continental collision but does not specifically address the sinking motion characteristic of subduction. Lastly, the choice about the splitting of tectonic plates describes another dynamic within plate tectonics but does not encompass the critical aspect of one plate descending beneath another.