Oceanic lithosphere is denser than the underlying asthenosphere and can sink back into the mantle. Subduction is the process by which one tectonic plate descends beneath another at a convergent boundary. Deep ocean trenches mark the surface expression of subduction zones.

As the slab descends it dehydrates, triggering melting in the overlying mantle wedge and producing volcanic arcs. Earthquakes occur along the slab to depths of several hundred kilometers. The sinking slab also pulls the trailing plate, contributing a major driving force for plate motion.

Subduction recycles oceanic crust and sediments into the deep Earth and is the principal means of returning water and other volatiles to the mantle. The largest earthquakes and many explosive volcanoes are associated with subduction zones. The process is central to the long-term evolution of the crust and mantle.

• Descent of one plate beneath another
• Occurs at convergent boundaries
• Generates trenches, arcs, and deep earthquakes
• Recycles oceanic lithosphere into the mantle