Subduction zones

Abstract

Subduction zones represent the primary return of material from the earth’s surface to its interior. In the process of doing so, great earthquakes occur, violent volcanoes erupt, and the largest lateral temperature gradients in the Earth’s interior are generated. At shallow depths, elastic and brittle processes control deformation including most of the planet’s large earthquakes, while at greater depths the subduction system deforms by large-scale ductile flow. Along with oceanic crust, sediment and volatiles enter the trench, lubricating the plate boundary at shallow depths, and then are metamorphosed where slabs descend below hot mantle, releasing H2O and other volatiles into the overlying mantle wedge. While some of this material may mechanically ascend back to the surface, in the form of high-pressure metamorphic rocks, much feeds explosive volcanism in the volcanic arc or descends into the deep mantle. Melting at sub-arc depths is controlled by a combination of advection via flow of hot mantle from the back-arc, and fluxing by slab-derived fluids, a combination of which produces a sharp volcanic front. Magmas ascend to form arc crust and differentiate, some fraction of which persists to form the basis of continental crust. This transfer of energy and material leads to a rich set of phenomena that play a major role in creating the geologic record.