2.2 Subduction Zones: Deep Ocean Trenches and Volcanic Island Arcs

To unravel the events that produce mountains, researchers examine ancient mountain structures as well as sites where orogenesis is currently active. Of particular interest are active subduction zones, where lithospheric plates are converging. Here the subduction of oceanic lithosphere generates Earth’s strongest earthquakes and most explosive volcanic eruptions, as well as playing a pivotal role in generating many of Earth’s mountain belts.

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Subduction zones located along convergent boundaries are the sites of plate destruction, places where slabs of oceanic lithosphere bend and plunge back into the mantle. As oceanic lithosphere slowly sinks, higher temperatures and pressures gradually alter these rigid slabs until they are fully assimilated into the mantle.

Subduction zones can be roughly divided into four regions that include: 1) a deep ocean trench, which forms where a subducting slab of oceanic lithosphere bends and descends into the asthenosphere; 2) a volcanic arc, which is built upon the overlying plate; 3) a region located behind the trench and the volcanic arc called the “forearc” region, and 4) a region on the side of the volcanic arc opposite the trench called the “backarc” region.

Although all subduction zones exhibit these features, a great deal of variation exists, along the length of an individual subduction zone, as well as among different subduction zones.

Subduction zones can also be placed into one of two categories, those in which oceanic lithosphere is subducted beneath another oceanic slab and those in which oceanic lithosphere descends beneath a continental block. An exception is the Aleutian subduction zone, where the western part is an oceanic-oceanic subduction zone, while subduction along the eastern section occurs under the Alaskan mainland (continental block).

Volcanic arcs. Perhaps the most obvious structure generated by subduction is a volcanic arc, which is built upon the overlying plate. Where two oceanic slabs converge, one is subducted beneath the other, initiating partial melting of the mantle wedge located above the subducting plate. This eventually leads to the growth of a volcanic island arc or simply an island arc, on the ocean floor. Examples of active Island arcs include the Mariana, New Hebrides, Tonga, and Aleutian arcs.

In locations where oceanic lithosphere is subducted beneath a continental block, a continental volcanic arc results. Here the volcanic arc builds upon the higher topography of older continental rocks, resulting in volcanic peaks that may reach 6000 meters, nearly 20,000 feet, above sea level.

Deep ocean trenches. Another major feature associated with subduction is a deep ocean trench. Trench depth appears to be strongly related to the age, and hence the temperature, of the subducting oceanic slab. In the Western Pacific, where oceanic lithosphere is cold, relatively dense oceanic slabs descend into the mantle and produce deep trenches. A well-known example is the Mariana Trench, where the deepest area is more than 11,000 meters, 36,000 feet, below sea level. By contrast, the Cascadia subduction zone lacks a well-defined trench. Here the warm, buoyant Juan de Fuca plate is actively subducting at a very low angle beneath Southwestern Canada and the Northwestern United States. The Peru-Chile subduction zone, on the other hand, displays trench depth between these extremes. Much of this trench is 2 to 3 km shallower than those in the Western Pacific, averaging between 7 and 8 km. One exception occurs in central Chile, where the plate boundary has a very shallow dip, making the trench virtually non-existent.

Forearc and Backarc regions. Located between developing volcanic arcs and deep ocean trenches are the forearc regions. Here pyroclastic material from the volcanic arc, as well as sediments eroded from the adjacent landmass, accumulate. In addition, ocean floor sediments are carried to the forearc region by the subducting plate.

Another site where sediments and volcanic debris may accumulate is the backarc region, which is located on the side of the volcanic arc opposite the trench. In these regions, tensional forces often dominate, causing the crust to be stretched and thinned.