In geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another, with one sliding underneath the other and moving down into the mantle, at rates typically measured in centimeters per year. An oceanic plate ordinarily slides underneath a continental plate or another oceanic plate; this often creates an orogenic zone or volcanic arc subject to many earthquakes. In a sense, subduction zones are the opposite of divergent boundaries, areas where material rises up from the mantle and plates are moving apart.
Subduction results from the difference in density between lithosphere and underlying asthenosphere. Where, very rarely, lithosphere is denser than asthenospheric mantle, it can easily sink back into the mantle at a subduction zone; however, subduction is resisted where lithosphere is less dense than underlying asthenosphere. Whether or not lithosphere is denser than underlying asthenosphere depends on the nature of the associated crust. Crust is always less dense than asthenosphere or lithospheric mantle and continental lithosphere is always less dense than oceanic lithosphere. Exceptionally, the presence of the large areas of flood basalt that are called large igneous provinces (LIPs), which result in extreme thickening of the oceanic crust, can cause some sections of older oceanic lithosphere to be too buoyant to subduct. Where lithosphere on the downgoing plate is too buoyant to subduct, a collision occurs, hence the adage "Subduction leads to orogeny".
Subduction zones are arc-shaped, with the concave side oriented away from the direction of subduction because of the curvature of the Earth. This can easily be seen by making a cut into an orange, with the knife blade representing a subducting slab.
The magmatism associated with the volcanic arc occurs 100-300 km away from the trench. However, a relationship has been found, which relates volcanic arc location to depth of the subducted crust as defined by the Wadati-Benioff zone. Studies of many volcanic arcs around the world have revealed that volcanic arcs tend to form at a location where the subducted slab has reached a depth of about 100 km. This has interesting implications for the mechanism that causes the magmatism at these arcs. Arcs produce about 25% of the total volume of magma produced each year on Earth (~30-35 km³), much less than the volume produced at mid-ocean ridges. Nevertheless, arc volcanism has the greatest impact on humans, because many arc volcanoes lie above sealevel and erupt violently. Aerosols injected into the stratosphere during violent eruptions can cause rapid cooling of the Earth's climate.
Subduction zones are associated with the deepest earthquakes on the planet. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than 20 km. However, in subduction zones, earthquakes occur at depths as great as 700 km. These earthquakes define inclined zones of seismicity known as Wadati-Benioff zones (after the scientists who discovered them), which outline the descending lithosphere. Seismic tomography has helped outline subducted lithosphere in regions where there are no earthquakes. Some subducted slabs seem not to be able to penetrate the major discontinuity in the mantle that lies at a depth of about 670 km, whereas other subducted oceanic plates can penetrate all the way to the core-mantle boundary. The great seismic discontinuities in the mantle - at 410 and 670 km depth - are disrupted by the descent of cold slabs in deep subduction zones.
Subduction zones are important for several reasons:
Subduction zones have also being considered as possible disposal sites for nuclear waste, where the action would carry the material into the planetary mantle, safely away from any possible influence on humanity or the surface environment, but this method of disposal is currently banned by international agreement.