Message #8131

[Adam Hawthorne]

General description

Subduction zones mark sites of convective downwelling of the Earth's lithosphere (the crust plus the strong portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate and sinks below it to depth of approximately 100 km. At that depth the peridotite of the oceanic slab is converted to eclogite, the density of the edge of the oceanic lithosphere increases and it sinks into the mantle. It is at subduction zones that the Earth's lithosphere, oceanic crust, sedimentary layers, and trapped water are recycled into the deep mantle. Earth is the only planet where subduction is known to occur; neither Venus nor Mars have subduction zones. Without subduction, plate tectonics could not exist and Earth would be a very different planet: Earth's crust would not have differentiated into continents and oceans and all of the solid Earth would lie beneath a global ocean.

Subduction results from the difference in density between lithosphere and underlying asthenosphere. Where, very rarely, lithosphere is denser than asthenospheric mantle, it can easily sink back into the mantle at a subduction zone; however, subduction is resisted where lithosphere is less dense than underlying asthenosphere. Whether or not lithosphere is denser than underlying asthenosphere depends on the nature of the associated crust. Crust is always less dense than asthenosphere or lithospheric mantle, but because continental crust is always thicker and less dense than oceanic crust, continental lithosphere is always less dense than oceanic lithosphere. Oceanic lithosphere is generally not denser than asthenosphere but continental lithosphere is lighter. Exceptionally, the presence of the large areas of flood basalt that are called large igneous provinces (LIPs), which result in extreme thickening of the oceanic crust, can cause some sections of older oceanic lithosphere to be too buoyant to subduct. Where lithosphere on the downgoing plate is too buoyant to subduct, a collision occurs, hence the adage "Subduction leads to orogeny". Most subduction zones are arcuate, where the concave side is directed towards the continent. This is especially so where a back-arc basin develops between the subduction zone and the continent. The arcuate configuration probably results from differential friction between the tectonic plates, and the likely agent that would reduct the interplate friction is serpentinite, but a large batch of unconsolidated sediment could cause similar effects as well.

Subduction zones are associated with the deepest earthquakes on the planet. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than 20 km. However, in subduction zones, earthquakes occur at depths as great as 700 km. These earthquakes define inclined zones of seismicity known as Wadati-Benioff zones (after the scientists who discovered them), which outline the descending lithosphere. Seismic tomography has helped outline subducted lithosphere in regions where there are no earthquakes. Some subducted slabs seem not to be able to penetrate the major discontinuity in the mantle that lies at a depth of about 670 km, whereas other subducted oceanic plates can penetrate all the way to the core-mantle boundary. The great seismic discontinuities in the mantle - at 410 and 670 km depth - are disrupted by the descent of cold slabs in deep subduction zones.