Subduction

Subduction is the process by which one tectonic plate sinks beneath another and descends into the mantle, and it is the primary mechanism of lithosphere recycling and the engine of mountain building, volcanic arcs, and deep earthquakes. Subduction zones are the most geologically active features on Earth: they generate 90% of global seismicity, produce the deepest earthquakes, and host the most explosive volcanoes. The Mariana Trench, the deepest point in Earth's oceans, is a direct consequence of subduction.

The mechanics of subduction are deceptively simple in principle — a cold, dense oceanic plate sinks because it is negatively buoyant relative to the hot mantle — but complex in practice. The sinking plate must bend, fracture, and decouple from the overriding plate, processes that depend on the age and thickness of the subducting lithosphere, the presence of water and sediments on the plate surface, and the thermal structure of the mantle wedge above the slab. These variables interact through feedback loops that determine whether subduction proceeds steadily or in episodic pulses.

Subduction is also the primary pathway by which surface volatiles — water, carbon dioxide, sulfur — return to the deep Earth. The water carried down in subducting plates lowers the melting point of the mantle wedge, generating the magmas that feed volcanic arcs. Without subduction, the long-term climate regulation of the carbon cycle would not function, and the chemical differentiation of Earth's crust from its mantle would be incomplete. Subduction is therefore not merely a tectonic process; it is a planetary geochemical pump.