Subduction zones occur at collision boundaries where at least one of the colliding lithospheric plates contains oceanic crust. In accord with plate tectonic theory, collision boundaries are sites where lithospheric plates move together and the resulting compression causes either subduction (where one or both lithospheric plates are driven down and destroyed in the molten mantle) or crustal uplifting that results in orogeny (mountain building). Subduction zones are usually active earthquake zones. Subduction zones are the only sites of deep earthquakes. The areas of deep earthquakes, ranging to a depth of 415 mi (670 km), are termed Benioff zones. Deep earthquakes occur because of forces due to plate drag and mineral phase transitions. The release of forces due to sudden slippage of plates during subduction can be quick and violent. Subduction zones can also experience shallow and intermediate depth earthquakes.

Oceanic crust is denser than continental crust and is...