Subduction is the converging of two tectonic plates in which one is forced beneath the other. Typically, this occurs when an oceanic plate meets a continental plate. The more dense oceanic plate is forced under the less dense continental plate and into the Earth's mantle.
The difference in density is a function of the composition of the tectonic plates. The plate being subducts usually sinks at an angle between 25 and 45 degrees. Oceanic plates are composed primarily of basalts (density = 3.0 g/cm3) while continental plates are composed of granites (density = 2.7 g/cm3). As a tectonic plate is subducted it is metamorphosed due to the intense pressure. There are a variety of changes in mineral phases as the plate is metamorphosed.
Subduction zones are typically characterized by high rates of volcanism, earthquakes and mountain building. Volcanic arcs along subduction zones are due to magma from the melting plate rising and transforming surrounding continental or oceanic rock until it erupts from the ground to form a volcano.