Subduction - General Description

General Description

Subduction zones mark sites of convective downwelling of the Earth's lithosphere (the crust plus the top brittle portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate. The down-going slab—the subducting plate—is overridden by the leading edge of the other plate. The slab sinks at an angle of approximately 25 to 45 degrees to the surface of the Earth. At a depth of approximately 80–120 km, the basalt of the oceanic slab is converted to a metamorphic rock called eclogite. At this point, the density of the oceanic lithosphere increases and it is carried into the mantle by the downwelling convective currents. It is at subduction zones that the Earth's lithosphere, oceanic crust, sedimentary layers, and some trapped water are recycled into the deep mantle. Earth is the only planet where subduction is known to occur. Without subduction, plate tectonics could not exist.

Subduction zones dive down into the mantle beneath 55,000 km of convergent plate margins (Lallemand, 1999), almost equal to the cumulative 60,000 km of mid-ocean ridges. Subduction zones burrow deeply but are imperfectly camouflaged, and we can use geophysics and geochemistry to study them. Not surprisingly, the shallowest portions of subduction zones are known best. Subduction zones are strongly asymmetric for the first several hundred kilometers of their descent. They start to go down at oceanic trenches. Their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the 660 km discontinuity. Subduction zones are defined by the inclined array of earthquakes known as the “Wadati-Benioff Zone” after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at enormously greater depths than elsewhere on Earth, where seismicity is limited to the outermost 20 km of the solid Earth.

The subducting basalt and sediment are normally rich in hydrous minerals and clays. During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a supercritical fluid. The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle where it lowers the pressure in (and thus the melting temperature of) the mantle rock to the point of actual melting, generating magma. These magmas, in turn, rise, because they are less dense than the rocks of the mantle. These mantle-derived magmas (which are basaltic in composition) can continue to rise, ultimately to the Earth's surface, resulting in a volcanic eruption. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt (a) interacts with (melts) the Earth's crust and/or (b) undergoes fractional crystallization.

Above subduction zones, volcanoes exist in long chains called volcanic arcs. Volcanoes that exist along arcs tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive. Krakatoa, Nevado del Ruiz, and Mount Vesuvius are all examples of arc volcanoes. Arcs are also known to be associated with precious metals such as gold, silver and copper - again believed to be carried by water and concentrated in and around their host volcanoes in rock termed "ore".

Subduction results from convection in the asthenosphere. The heat from the core of the earth that is imparted to the mantle causes the mantle to convect much the way boiling water convects in a pan on the stove. Hot mantle at the core-mantle boundary rises while cool mantle sinks, causing convection cells to form. At points where two downward moving convecting cells meet (cold mantle sinking), can occur, forcing the oceanic crust below either continents or other oceanic crust. Continental crust tends to override oceanic crust because it consists of less dense granite compared to the basalt of the oceanic crust.