Subduction is the process in which one lithospheric plate collides with and is overridden by, or descends under, an adjacent plate. The subduction zone is the place where two lithospheric plates come together, one riding over the other. Often the upper plate is a continental plate and the lower one is an oceanic plate, and subduction leads to the formation of an ocean trench. Most volcanoes on land occur parallel to and inland from the boundary between the two plates.