What is an area in which an oceanic plate descends into the mantle called?

The area where an oceanic plate descends into the mantle is called a subduction zone. Oceanic plates are denser than continental plates, so when they meet and collide, the oceanic plate slides under the continental plate into the mantle.

The Earth's crust consists of a number of tectonic plates that slide over the mantle. Subduction zones are associated with many geologic phenomena, such as volcanoes, earthquakes and tsunamis. The Pacific Ring of Fire is an example of the subduction of oceanic plates. Many of the world's largest volcanoes and most powerful earthquakes are found in the waters surrounding Japan, Russia and Canada.