<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> <Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (September 2015) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (September 2015) </Td> </Tr>

What happens when a piece of earth's crust is subducted