Subduction Zones

Where 2 tectonic key converge, if one or both the the plates is oceanic lithosphere, a subduction zone will form. An oceanic plate will sink ago into the mantle. Remember, oceanic bowl are formed from mantle material at midocean ridges. Young oceanic lithosphere is hot and also buoyant (low density) as soon as it forms at a midocean ridge. But as that spreads away from the ridge and also cools and contracts (becomse denser) the is able come sink into the hotter underlying mantle. There is a deep s trench whereby the oceanic plate bends downward.

Volcanic Arcs: The basaltic ocean crust includes hydrous minerals favor amphiboles, few of which developed by hydrothermal change as seawater seeped with hot, fractured, young s crust at the midocean ridge. Together the s crust sink deeper into the mantle the pressure rises (the temperature the the s crust rocks increases an ext slowly since rocks are poor conductors of heat). At depth of roughly 100 km beneath the surface, the press is great enough for the hydrous minerals to undergo metamorphism. The resulting minerals are denser and they don"t save the external inspection water. This metamorphic dewatering procedure liberates water indigenous the descending crust. The water gradually seeps upward right into the overlying wedge of warm mantle. The addition of water come the already hot mantle rocks lowers their melting temperature causing partial melting of ultramafic mantle rocks to productivity mafic magma. Melting aided by the addition of water or other fluid is referred to as flux melting. That is somewhat more complex than this, yet metamorphic dewatering the suducting crust and flux melting of the mantle wedge appears to account for most of the magma at subduction zones.

Magma formed over a subducting plate slowly rise right into the overriding crust and also finally come the surface developing a volcanic arc, a chain of energetic volcanoes i m sorry parallels the deep s trench. In ~ the active volcanic arc lie intrusive igneous rocks developed from magma the didn"t make it all the way to the surface before crystallizing. The volcano arcs may be volcanic island arcs (e.g., Aleutians, Mariannas), whereby one oceanic key subducts beneath another oceanic plate, or continental volcanic arcs (e.g., Andes, Cascades), wherein oceanic bowl subduct under a continent plate. The most abundant igneous rock developed at volcano arcs is andesite (or intrusive diorite), though volcanic arc rocks may selection in composition from basalt come rhyolite (mafic come felsic).

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Benioff Zones: Earthquakes in and around deep s trenches are principally produced by movements on thrust faults, describe compression (converging plates). A plane of earthquake focci descend native the area about the trench under the overriding plate. The farther native the trench, the depth the earthquakes are. These earthquakes the the Benioff Zone (or Wadati-Benioff Zone) take place near the upper surface of the descending plate (or slab).


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They take place down to depth of about 670 km at some subduction zones. Note: the volcano arc lies whereby the Benioff zone earthquakes are around 100 km beneath the surface but Benioff zone earthquakes proceed past this, landward under to 60 km; because of this the slab has not melted away!