Subduction Zones

Earth's lithosphere is divided into plates that move relative to one another. At a convergent boundary, one plate may bend and descend beneath another, entering the mantle; this process is called subduction. Subduction commonly involves oceanic lithosphere. The descending plate bends the seafloor downward, often forming a deep ocean trench. The geometry and behavior of a subduction zone depend on the plates and surrounding geology, so not every convergent boundary produces the same features.

The contact between the descending and overriding plates can become locked by friction while the plates continue to converge. Stress then accumulates and may be released when a fault ruptures, producing a megathrust earthquake. Earthquakes also occur within the descending plate and in the overriding crust. A tsunami can be generated when an undersea earthquake vertically displaces the seafloor; submarine landslides can also displace water and generate tsunamis. Not every subduction-zone earthquake produces a tsunami.

As a slab descends, water-bearing minerals and sediments can carry fluids into the mantle. At depth, released fluids help promote partial melting in the mantle above the slab. Magma may rise through the overriding plate and feed a chain of volcanoes, forming a volcanic arc. Where an oceanic plate descends beneath a continent, the arc may lie along a continental margin; where one oceanic plate descends beneath another, an island arc can form. The composition and behavior of magma vary, so eruptions differ from place to place.

Subduction zones can shape trenches, mountain belts, volcanic arcs, and earthquake-prone faults. Their hazards include ground shaking, volcanic eruptions, landslides, and tsunamis. The U.S. Geological Survey notes that the largest earthquakes occur on major subduction-zone megathrust faults. These hazards are not uniform: local plate geometry, sediment, fluids, and the history of fault movement influence what occurs in a particular region.