Subduction
The sinking of one tectonic plate beneath another at convergent boundaries, recycling oceanic lithosphere into the mantle and driving plate motion.
Slab pull: The gravitational force exerted by a dense sinking slab on the rest of the plate. The dominant driving force of plate motion, generated by subduction itself.
Harry Hammond Hess: The geologist who proposed in 1962 that oceanic crust is created at ridges and consumed at trenches. His seafloor-spreading model first required sinking ocean floor to balance creation.
Volcanic arc: A chain of volcanoes formed above a subduction zone, typically 100–200 km from the trench. The surface expression of slab-generated magmas, from the Andes to Japan.
Mariana subduction zone: The western Pacific subduction zone where old, dense oceanic lithosphere descends steeply, producing Earth's deepest trench. The archetype of a mature, fast-retreating subduction system.
Slab stagnation: The observation that some slabs flatten and pond at the mantle transition zone instead of sinking through it. Seismic imaging shows slabs stalling near 660 km depth in some regions, sinking through in others.
Benioff zone: The inclined plane of earthquakes marking a descending slab, from the surface down to about 700 km. Earthquake foci trace the slab's path into the mantle.
Seafloor spreading: The creation of oceanic crust at mid-ocean ridges as plates move apart. Crust made at ridges must be destroyed somewhere; subduction is that somewhere.
Deep-sea trench: The elongated topographic depression where an oceanic plate bends and descends, reaching depths near 11 km. The Mariana Trench, Earth's deepest point, marks a subduction zone.
Andean subduction: The subduction of the Nazca plate beneath South America, building the Andes and a long volcanic zone. The classic example of subduction building a mountain belt on a continental margin.
660-km discontinuity: The mantle boundary at 660 km depth where the spinel-to-perovskite phase transition occurs. Whether this boundary blocks or passes slabs controls whole-mantle versus layered convection.