Subduction zones mark sites of convective downwelling peculiar to the Earth. In plate tectonics, subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with and sinks below another lithospheric plate into the mantle. Subduction zones are where 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. Without subduction zones, Earth's crust would not have differentiated into continents and oceans and all of the solid Earth would lie beneath a global ocean. Mammals could not have evolved without dry land, so without continents, human beings could not exist. Ultimately, human life and civilization would not exist without subduction zones.
Subduction results from the density contrast between lithosphere and underlying asthenosphere. Where lithosphere is denser than asthenospheric mantle, it can easily sink back into the mantle at a subduction zone, but subduction is resisted where lithosphere is less dense than underlying asthenosphere. Whether or not lithosphere (the crust plus the strong portion of the upper mantle) is more or less dense than underlying asthenosphere depends on the nature of the associated crust. Because continental crust is thicker and less dense than oceanic crust, continental lithosphere is less dense than oceanic lithosphere. Oceanic lithosphere is generally heavier than asthenosphere but continental lithosphere is lighter. The presence of features called large igneous provinces (LIPs) which result in extreme thickening of the oceanic crust can cause older oceanic lithosphere to win the density contrast battle with younger oceanic lithosphere. Seismic tomography appears to show that some of the subducted oceanic plates can reach as far as the core-mantle boundary.
Subduction results in creation of oceanic trenches, such as the Mariana trench. renches mark where one plate begins its descent beneath another. Volcanoes that occur along these boundaries, such as Mt. Saint Helens and Mt. Fuji, often occur in groups that take the shape of an arc, hence the term volcanic arc or island arc. The arcuate shape is a result of the geometry of plate collision on a sphere.
The magmatism associated with volcanic arc formation does not occur a uniform distance away from the trench. However, a relationship has been found that relates volcanic arc ___location to depth of the subducted crust as defined by the Wadati-Benioff zone. Studies of many of the volcanic arcs from around the world have revealed that volcanic arcs tend to form at a ___location where the subducted slab has reached a depth of ~ 100 km. This has interesting implications for the mechanism which causes the magmatism at these arcs.
Subduction zones are also notorious for producing large scale earthquakes because of the intense geological activity. The introduction of cold oceanic crust into the mantle depresses the local geothermal gradient and causes a larger portion of the earth to deform in a brittle fashion than would in a normal geothermal gradient setting. Because earthquakes can only occur when a rock is deforming in a brittle fashion, subduction zones have the potential to create very large earthquakes. If this earthquake occurs under the ocean it has the potential to create tsunamis, such as the earthquake caused by subduction of the Indo-Australian Plate under the Eurasian Plate on December 26, 2004, that devastated the areas around the Indian Ocean. Small tremors that create tiny, unnoticeable tsunamis happen all the time because of the dynamics of the earth.
Why are Subduction Zones important?
1) Subduction Zone Physics: Sinking of mantle lithosphere provides most of the force needed to drive plate motion and are the dominant mode of mantle convection. 2) Subduction Zone Chemistry: The cold material sinking in subduction zones releases water into the overlying mantle, causing mantle melting and fractionating elements (buffering) between surface and deep mantle reservoirs, producing island arcs and continental crust. 3) Subdution Zone Biology: Because subduction zones are the coldest parts of the Earth’s interior and life cannot exist at temperatures >150°C, subduction zones are almost certainly associated with the deepest (highest pressure) biosphere
Learning more about the physics, chemistry, and biology of subduction zones requires efforts that are increasingly interdisciplinary and international. Because of the central role that subduction plays in the solid Earth system as well as its role in maintaining equilibrium between the mantle and the hydrosphere, understanding and teaching how subduction zones operate is a scientific challenge of the first importance.
Reference
- Stern, R.J., 2002, Subduction zones: Reviews of Geophysics, v. 40, 1012, doi: 10.1029/2001RG000108.