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The process by which the leading edge of a denser lithospheric plate is forced downward into the underlying asthenosphere is called subduction. The downgoing plate is called the subducted plate or downgoing slab; the less dense plate is called the overriding plate or slab. The area where this process occurs is a subduction zone. The subducted plate, whose thickness averages 100 km, is generally composed of dense oceanic lithosphere. Subduction is the major process by which oceanic lithosphere is destroyed and recycled into the asthenosphere and deeper Earth at rates similar to its creation along the oceanic ridge system. For this reason, subduction zone plate boundaries are also called destructive plate boundaries.

Figure 1.11 Convergent plate boundary, showing trench‐arc system, inclined seismic zone and subduction of oceanic lithosphere.

The surface expressions of subduction zones are large trench‐arc systems of the kind that encircle most of the shrinking Pacific Ocean (Isacks et al. 1968). Trenches are deep, elongate troughs in the ocean floors marked by water depths that can exceed 11 km. They are formed as the downgoing slab forces the overriding slab to bend downward forming a long trough along the boundary between them.

Because the asthenosphere is mostly solid, it resists the downward movement of the subducted plate to varying degrees. This produces stresses in the cool interior of the subducted lithosphere that generate earthquakes (Figure 1.11) along an inclined seismic (Wadati‐Benioff) zone that marks the path of the subducted plate as it descends into the asthenosphere. The four largest magnitude earthquakes in the past 120 years occurred along inclined seismic zones beneath Chile (1909), Alaska (1964), Sumatra (2004), and Japan (2011). The latter two events produced the devastating 2004 Banda Aceh tsunami which killed some 300 000 people around the Indian Ocean region and the Fukushima tsunami which killed tens of thousands in eastern Japan and destroyed an atomic power plant.

What is the ultimate fate of subducted slabs? Earthquakes occur in subducted slabs to a depth of 660 km, so we know they reach the base of the asthenosphere transition zone. Earthquake records suggest that some slabs flatten out as they reach this boundary indicating that they may not penetrate into the lower mantle. Seismic tomography, which images three‐dimensional variations in seismic wave velocity within the mantle, has shed some light on this question, while raising many others. A consensus has emerged (Grand 2002; Hutko et al. 2006) that some subducted slabs become dense enough to sink all the way to the core–mantle boundary where they contribute material to the D″ layer. These slab remnants may ultimately be involved in the formation of mantle plumes, as proposed by Jeanloz (1993).

Subduction zones produce a wide range of distinctive Earth materials. The increase in temperature and pressure within the subducted plate causes it to undergo significant metamorphism. The upper part of the subducted slab, in contact with the hot asthenosphere, releases volatile fluids as it undergoes metamorphism which lowers melting temperatures and triggers partial melting. A complex set of melts rise from this region to produce volcanic‐magmatic arcs. These melts range in composition from basaltic–gabbroic through dioritic–andesitic and may differentiate or be contaminated to produce melts of granitic–rhyolitic composition. Granitic–rhyolitic melts may also be generated by partial melting of older continental crust heated by rising magma and volatiles. Melts that reach the surface produce volcanic arcs such as those that characterize the “ring of fire” of the Pacific Ocean basin. Mt. St. Helens in Washington, Mt. Pinatubo in the Philippines, Mt. Fuji in Japan and the many volcanoes in Central America and the Andes of South America are all examples of volcanic arc composite volcanoes that form over Pacific Ocean subduction zones. These volcanic arcs add to the volume of continental crust.

When magmas intrude the crust and solidify below the surface, they produce plutonic igneous rocks that add new continental crust to Earth. Most of the world's major batholith belts represent plutonic magmatic arcs, subsequently exposed by erosion of the overlying volcanic arc. In addition, many of Earth's most important ore deposits (Chapter 19) are produced in association with volcanic‐magmatic arcs over subduction zones.

Many of the magmas generated over the subducted slab cool and crystallize at the base of the lithosphere, thickening it by underplating. Underplating and intrusion are two of the major sets of processes by which new continental crust is generated. Once produced, the low density of continental crust prevents most of it from being subducted. This helps to explain its preservation potential and the great age that continental crust can achieve (>4.0 Ga).

Figure 1.12 Subduction zone depicting details of sediment distribution, sedimentary basins and volcanism in trench‐arc system forearc and backarc regions.

Areas of significant relief, such as trench‐arc systems are ideal sites for the production and accumulation of detrital (epiclastic) sedimentary rocks (Chapter 13). Huge volumes of detrital sedimentary rocks produced by the erosion of volcanic and magmatic arcs are deposited in forearc and backarc basins (Figure 1.12). They also occur with deformed abyssal sediments in the forearc subduction complex. As these sedimentary rocks are buried and deformed, they are commonly metamorphosed (Chapters 15 and 18).