At depths of around km beneath the surface, the pressure is great enough for the hydrous minerals to undergo metamorphism.
The resulting minerals are denser and they don't contain the bonded water. This metamorphic dewatering process liberates water from the descending crust. The water gradually seeps upward into the overlying wedge of hot mantle.
The addition of water to the already hot mantle rocks lowers their melting temperature resulting in partial melting of ultramafic mantle rocks to yield mafic magma. Melting aided by the addition of water or other fluid is called flux melting. It is somewhat more complicated than this, but metamorphic dewatering of suducting crust and flux melting of the mantle wedge appears to account for most of the magma at subduction zones.
Magma formed above a subducting plate slowly rise into the overriding crust and finally to the surface forming a volcanic arc , a chain of active volcanoes which parallels the deep ocean trench. A study by Froitzheim et al. Classic plate tectonics concepts suggested that continents do not subduct. Instead, when two continents collide at a convergent boundary following the consumption of an ocean by subduction, they accommodate the shortening within the lithosphere, which is thickened up to twice the normal values.
The subducted oceanic slab that brought the continents together stalls and eventually breaks off and sinks into the mantle due to its negative buoyancy. In contrast to that view, modern petrologic, tectonic, and geophysical observations have completely changed this picture still prevalent in many textbooks: continental lithosphere does, in fact, subduct to great depths at major long-lived collisional boundaries, and the two colliding plates can be separated by a section of convective upper mantle mantle wedge similar to the case of oceanic subduction.
There are three important types of observations supporting those assertions. After their initial discovery in the Alps, tens of localities of UHP or near-UHP metamorphic rocks have been described globally in a variety of Phanerozoic and older orogenic belts Gilotti, Most of these are unambiguous continental crustal rocks.
Some even contain microdiamonds, indicating that they were buried to as much as km McClelland and Lappen, , and references therein. Fundamentally, all UHP rocks are eclogite facies rocks and the better-preserved ones have only limited products of retrogression overprinted along their exhumation path. Because it is unlikely that continental crust is ever — km thick anywhere on the planet, the implication is that such rocks were subducted to mantle depths before being returned to the surface Hacker et al.
Second, refined plate-tectonic reconstructions and plate kinematics models for the Indo-Asian collision van Hinsbergen et al. More than km of Indian lithosphere are missing and must have been subducted under the Asian continent. Third, seismic images of the ongoing Pamir—Hindu Kush collision system show that Indian lithosphere is being subducted to as much as km beneath the surface Sippl et al.
One can further assume that only a small fraction of subducted continental crust makes it back to the surface of Earth, making UHP rocks extraordinarily important. Six distinctive geodynamic mechanisms have been proposed in the literature for continental subduction and development of UHP metamorphic rocks, and also six mechanism exist that explain the subsequent unroofing of these rocks Hacker and Gerya, ; Gerya, The proposed mechanisms for exhumation are mechanically and geologically plausible but, at this point, also very speculative, since they are so difficult to test.
One of the proposed mechanisms for the first type of unroofing is the eduction mechanism Andersen et al. Understanding exhumation of UHP rocks remains very much a work in progress and an area in which much is left to be discovered.
Froitzheim et al. Variably retrogressed kyanite eclogites that have a mafic bulk composition containing omphacite, garnet, and minor kyanite, phengite, and quartz make up an assemblage that is mafic in bulk chemistry, but also appears continental and crustal. The assemblage was at a peak metamorphic pressure of as much as 2. Together, these results indicate that the analyzed rock was a lower crustal piece of Baltica, an old lithospheric fragment that was subducted beneath Laurentia during the broadly defined Caledonian orogeny as a consequence of the collision and continental subduction that followed after the consumption of the Iapetus Ocean.
In fact, the Lofoten rocks exposed on an island off the northern coast of Norway are spatially and tectonically correlated to the Western Gneiss Complex Root et al. The Caledonian Western Gneiss Complex has similar ages of peak metamorphism although, on average, they are older than the age determined in this new study of Froitzheim et al.
However, Lofoten rocks are much less deformed than most of the Western Gneiss, suggesting that they may represent legitimate autochthonous Baltica from the hinterland of the collision, and not some potentially allochthonous terranes positioned in between Baltica and Laurentia. The main reason is size. The size of an earthquake is related to the size of the fault that causes it, and subduction zone faults are the longest and widest in the world.
The Cascadia subduction zone offshore of Washington is about miles 1, kilometers long and about 62 miles km wide. Smaller earthquakes also strike all along the descending plate, also called a slab.
Seismic waves from these temblors and tremors help scientists "see" inside the Earth , similar to a medical CT scan. The quakes reveal that the sinking slab tends to bend at an angle between 25 to 45 degrees from Earth's surface, though some are flatter or steeper than this.
Sometimes, the slabs may tear, like a gash in wrinkled paper. Pieces of the sinking plate can also break off and fall into the mantle, or get stuck and founder. Subduction zones are usually along coastlines, so tsunamis will always be generated close to where people live, Titov said. But the bad news is sometime a tsunami is generated. When subduction zone earthquakes hit, Earth's crust flexes and snaps like a freed spring. For earthquakes larger than a magnitude 7.
However, not all subduction zone earthquakes will cause tsunamis. Also, some earthquakes trigger tsunamis by sparking underwater landslides.
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