Subduction: Difference between revisions

Revision as of 22:44, 17 May 2005

Revision as of 22:49, 17 May 2005

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[[Image:subduction.jpg|thumb|350px|The Juan de Fuca plate sinks below the North America plate at the [[Cascadia subduction zone]].]]

Subduction zones mark sites of convective [[downwelling]] peculiar to the [[Earth]]. In [[plate tectonics]], '''subduction zones''' exist at convergent [[plate tectonics|plate]] boundaries where one plate of [[lithosphere|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. [[Mammal]]s could not have evolved without dry land, so without [[continent]]s, [[human]] beings could not exist. Ultimately, human life and [[civilization]] would not exist without subduction zones.

[[Image:subduction.jpg|thumb|350px|The Juan de Fuca plate sinks below the North America plate at the [[Cascadia subduction zone]].]]

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, [[lithosphere|continental lithosphere]] is less dense than [[lithosphere|oceanic lithosphere]]. [[lithosphere|Oceanic lithosphere]] is generally heavier than asthenosphere but continental lithosphere is lighter. The presence of features called [[large igneous province]]s (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]].

[[Image:WorldSubZones.jpg|thumb|350px|Subduction zones of the world. Localities in green capital letters are collision zones, where continental crust has entered and disrupted a subduction zone]]

Subduction results in creation of [[oceanic trench]]es, such as the [[Mariana trench]],. and [[orogeny|mountain building]]renches mark where one plate begins its descent beneath another. [[Volcano]]es 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.

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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 [[2004 Indian Ocean Earthquake|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 theySubduction 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.