Subduction
Subduction is the plate-tectonic process in which one tectonic plate sinks beneath another and descends into the mantle. It recycles oceanic crust, builds volcanic arcs, forms deep-sea trenches, and creates some of the largest earthquakes and tsunamis on Earth.
What subduction is
Subduction is a convergent plate-boundary process. Two plates move toward each other, and one plate is forced downward beneath the other. The sinking plate, called the slab, descends into the mantle while the overriding plate is deformed, heated, and sometimes supplied with magma.
Why one plate sinks
Oceanic lithosphere cools as it ages, becoming thicker and denser. When dense oceanic crust reaches a convergent boundary, it can bend into the mantle more easily than buoyant continental crust. This sinking motion is helped by slab pull: the weight of the descending plate helps draw more plate into the subduction zone.
Trenches and volcanic arcs
At the surface, subduction often creates a deep ocean trench where the plate bends downward. Farther inland or behind the trench, fluids released from the sinking slab can lower the melting point of hot mantle rock. That process helps generate magma, which may feed volcanic arcs such as island chains or continental volcanic belts.
Earthquakes on the plate boundary
The contact between the sinking plate and the overriding plate can lock for decades or centuries while stress builds. When that boundary slips suddenly, it can produce a megathrust earthquake. Smaller earthquakes also occur within the bending slab, inside the overriding plate, and along related faults around the subduction system.
Tsunamis and seafloor motion
Large subduction earthquakes can lift or drop broad areas of seafloor. When that motion displaces seawater, it can generate a tsunami. Not every subduction earthquake makes a damaging tsunami, but the largest tsunami hazards often come from shallow offshore megathrust ruptures, especially near populated coasts.
Metamorphism and recycling
As the slab descends, pressure and temperature increase. Sediments, oceanic crust, and hydrated minerals are squeezed, heated, transformed, and partly recycled into the mantle. Some material may be scraped off to form an accretionary wedge, while other material travels deep enough to influence mantle chemistry and magma generation.
Subduction and mountain building
Subduction can help build mountains over long periods. Compression thickens crust, volcanic arcs add new igneous rock, and collisions may later close ocean basins. The Andes are a famous example of a continental margin shaped by oceanic plate subduction, volcanism, earthquakes, uplift, and erosion.
Why it matters
Subduction connects the surface to deep Earth. It explains why many earthquakes, volcanoes, trenches, island arcs, and mountain belts line the edges of tectonic plates. It also completes the plate-tectonic cycle by returning old seafloor to the mantle while new seafloor forms elsewhere.