Subduction zones are sites of convective
downwelling of Earth's
crust plus the top non-convecting portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of
oceanic lithosphere converges with another plate. The descending
slab, the subducting plate, is over-ridden by the leading edge of the other plate. The slab sinks at an angle of approximately twenty-five to forty-five degrees to Earth's surface. This sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle
asthenosphere, as the colder oceanic lithosphere is, on average, denser. At a depth of approximately 80–120 kilometers, the
basalt of the oceanic crust is converted to a metamorphic rock called
eclogite. At that point, the density of the oceanic crust increases and provides additional negative
buoyancy (downwards force). It is at subduction zones that Earth's lithosphere,
sedimentary layers and some trapped water are
recycled into the deep mantle.
Earth is so far the only planet where subduction is known to occur. Subduction is the driving force behind
plate tectonics, and without it, plate tectonics could not occur.
Subduction zones dive down into the mantle beneath 55,000 kilometers of convergent plate margins (Lallemand, 1999), almost equal to the cumulative 60,000 kilometers of mid-ocean ridges. Subduction zones burrow deeply but are imperfectly camouflaged, and
geochemistry can be used to study them. Not surprisingly, the shallowest portions of subduction zones are known best. Subduction zones are strongly asymmetric for the first several hundred kilometers of their descent. They start to go down at
oceanic trenches. Their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the
660-kilometer discontinuity. Subduction zones are defined by the inclined array of earthquakes known as the
Wadati–Benioff zone after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at greater depths (up to 600 km) than elsewhere on Earth (typically <20 km depth); such deep earthquakes may be driven by deep phase transformations, thermal runaway, or dehydration embrittlement.
The subducting basalt and sediment are normally rich in
hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the subducting slab bends downward.
 During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a
supercritical fluid. The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle where it lowers the pressure in (and thus the melting temperature of) the mantle rock to the point of actual melting, generating
magma. The magmas, in turn, rise (and become labeled
diapirs) because they are less dense than the rocks of the mantle. The mantle-derived magmas (which are basaltic in composition) can continue to rise, ultimately to Earth's surface, resulting in a volcanic eruption. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with (melts) Earth's crust and/or undergoes
Above subduction zones, volcanoes exist in long chains called
volcanic arcs. Volcanoes that exist along arcs tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive.
Nevado del Ruiz, and
Mount Vesuvius are all examples of arc volcanoes. Arcs are also known to be associated with precious metals such as gold, silver and copper believed to be carried by water and concentrated in and around their host volcanoes in rock called "ore".