deep-sea trenches Deep-sea trenches are the surface expressions of subduction zones, and mark the sites of convergent boundaries, where an oceanic plate is overriden either by another oceanic plate or by a continental plate. Most trenches occur around the rim of the Pacific Oceans, although they also occur in the north-east Indian Ocean (Java Trench), the Atlantic between the Caribbean and American plates (Puerto Rico Trench and Antilles subduction zone) and between the Scotia and Antarctic plates (South Sandwich Trench). The deepest point on the surface of the Earth occurs in Challenger Deep (11034 m) at the bottom of the Mariana Trench in the western Pacific.
As seen on a map, trenches are narrow, elongate, and usually arcuate. They may be thousands of kilometres long with widths of about 100 km. In cross-section they present an asymmetric V-shaped profile, the subducted plate dipping gently down towards the axis of the trench, while the side of the overriding plate is generally steeper (Fig. 1). Typical slopes are in the range 5–50°, but steeper slopes can occur locally. The floor of the trench is usually flat, having been covered by sediments. The relative size of the sedimented floor will depend on the balance between sediment supply from the surrounding area and removal by subduction or scraping off on to the overriding plate. Maximum depths (both absolute and relative to the surrounding ocean floor) vary considerably, although most are 2–4 km deeper than their surroundings.
Because oceanic lithosphere has a very similar density to that of the underlying asthenosphere, an oceanic plate converging with another plate is easily subducted; that is, it is deflected downwards into the mantle. The deepest point of the trench generally occurs at the boundary between the two plates. The vertical cross-sections of trenches are well described by the deformation of a thin elastic–plastic shell overlying a viscous substratum (Fig. 1). Such flexural calculations also predict the observed ‘outer rise’, a low rise a few hundred metres high lying some 120–150 km seaward of the trench axis on the subducting plate.
One of the most noticeable characteristics of most trenches is their arcuate plan, convex toward the subducting plate. An excellent example is the Aleutian Trench in the north Pacific. It is explicable in the way a spherical cap of lithosphere must deform when it is bent at a subduction zone, changing from convex upwards on the sea floor to concave upwards in the subducting slab. It is exactly comparable to the way the surface of a ping-pong or tennis ball can be indented. The trench effectively follows the boundary between the convex and concave parts, which delineates a circular arc on the surface.
Roger Searle
Bibliography
Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Science, Oxford.
