<P> 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 . </P> <P> 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 geophysics 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 . </P> <P> 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 fractional crystallization . </P> <P> 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 . Krakatoa, 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". </P>

What type of rock would most likely be at a subduction zone