Subduction is the process in which one tectonic plate sinks beneath another into the mantle.
It usually occurs at a convergent plate boundary, where plates move toward each other. Dense oceanic lithosphere commonly descends beneath another oceanic plate or beneath a continental plate. The sinking slab forms a deep ocean trench and can generate earthquakes along the plate interface and within the slab.
Water and other materials carried downward by the slab can promote melting in the mantle above it. Magma may rise to form volcanic arcs, such as the Andes or Japan. Subduction therefore links earthquakes, trenches, volcanoes, and mountain building.
Some subduction earthquakes are megathrust earthquakes, which occur on the shallow interface between the descending and overriding plates. If the seafloor moves vertically during a large undersea event, the displacement can generate a tsunami. Not every subduction earthquake produces a tsunami; the amount and direction of seafloor movement matter.