Subduction
Subduction is the process in which a dense oceanic plate sinks beneath another plate into the mantle at a convergent boundary.
Definition
Subduction is when one plate slides down underneath another plate and sinks into the hot mantle, and the plate that sinks is always the heavier one, usually ocean floor. It occurs at convergent boundaries when dense oceanic lithosphere descends beneath a less dense plate; the sinking plate bends at a deep ocean trench, and as it heats up it releases water that causes the mantle above it to melt, feeding a chain of volcanoes parallel to the trench. Subduction zones produce Earth's deepest earthquakes and largest tsunamis. The process is driven by the negative buoyancy of old, cold oceanic lithosphere, which after about $30$ million years becomes denser than the asthenosphere beneath it, and slab pull from this sinking is the dominant plate-driving force. The slab descends at angles from nearly flat to almost vertical at rates of several centimeters per year, and its dehydration reactions (breakdown of serpentine and amphibole near $100\ \text{km}$) flux the mantle wedge and generate arc magma at the volcanic front. Slabs can stagnate at the $660\ \text{km}$ discontinuity or penetrate to the core-mantle boundary, and subduction is the primary means by which water, carbon, and sediment return to the mantle.
Example
Off the coast of Washington and Oregon, a piece of ocean floor is sliding under North America, which is why Mount St. Helens and Mount Rainier exist. The Mariana Trench, nearly $11\ \text{km}$ deep, marks where the Pacific Plate subducts beneath the Philippine Sea Plate, the Cascadia subduction zone feeds the Cascade volcanoes, and the Peru-Chile Trench feeds the Andes. Seismic tomography beneath the eastern United States images the Farallon slab, subducted beneath western North America over $100$ million years ago, still sinking through the lower mantle. Flat-slab subduction beneath Peru suppresses volcanism and transmits deformation far inland.
Key Insight
Subduction is why the ocean floor is never very old; it gets recycled back into the mantle within about $200$ million years. Continental crust does not subduct because it is too buoyant, like a cork that will not sink, which is why continents are billions of years old and ocean floors are not. Subduction zones are the largest hazard on the planet. All earthquakes above magnitude $9$ in recorded history occurred on subduction megathrusts, and the 2004 Sumatra and 2011 Tohoku tsunamis were both subduction events.