Zones of subduction are integral parts of plate tectonics in that they are the location where old oceanic lithosphere is consumed to offset the new oceanic lithosphere being created along the oceanic ridges. The zones are initiated where the oceanic lithosphere offshore from a continental margin becomes subjected to compressive forces generated by an underlying convection cell within the asthenosphere. Over time, the offshore portion of the oceanic lithosphere begins to downwarp, forming an oceanic trench. Eventually, the oceanic lithosphere breaks along the base of the trench. Because the density of the oceanic lithosphere (3.1) is greater than that of the continental lithosphere (2.9), the edge of the oceanic lithosphere is driven down and under the edge of the continental lithosphere crust where it is consumed within the asthenosphere. Simultaneously, masses of granitic and andesitic magmas are created along with lesser amounts of basaltic magma. The andesitic magma erupts to the surface to create a chain of volcanic mountains, either continental arc mountains along the margin of the continent such as the Andes or offshore as a chain of volcanic islands rising from the ocean floor referred to as island arc mountains.
zone of subduction, plate tectonics