These plates are not static, but move relative to each other at varying speeds, "gliding" over a viscous asthenosphere.
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Volcanism running alongside is a further surface manifestation of the ongoing process of continental break up and the proximity of the hot molten asthenosphere to the surface.
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Exactly what mechanism or mechanisms are behind their movement is still debated, but are likely to include convection currents within the asthenosphere and the forces generated at the boundaries between plates.
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Mesosphere (not to be confused with mesosphere, a layer of the atmosphere) is derived from "mesospheric shell", coined by Reginald Aldworth Daly, a Harvard University geology professor. In the pre-plate tectonics era, Daly (1940) inferred that the outer Earth consisted of three spherical layers: lithosphere (including the crust), asthenosphere, and mesospheric shell. Daly's hypothetical depths to the lithosphere–asthenosphere boundary ranged from , and the top of the mesospheric shell (base of the asthenosphere) were from . Thus, Daly's asthenosphere was inferred to be thick.
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The LAB is determined from the differences in the lithosphere and asthenosphere including, but not limited to, differences in grain size, chemical composition, thermal properties, and extent of partial melt; these are factors that affect the rheological differences in the lithosphere and asthenosphere 12\. Fjeldskaar, W., 1994. Viscosity and thickness of the asthenosphere detected from the Fennoscandian uplift. Earth and Planetary Science Letters, 126, 4 399-410..
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Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. This is because the chemically differentiated oceanic crust is lighter than asthenosphere, but thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones.
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The outer layers of the Earth are divided into the lithosphere and asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient.
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The Lithosphere-Asthenosphere boundary is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation.
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Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, allowing the movement of tectonic plates.
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Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches. Oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at ocean trenches. Two processes, ridge- push and slab pull, are thought to be responsible for spreading at mid-ocean ridges. Ridge push refers to the gravitation sliding of the ocean plate that is raised above the hotter asthenosphere, thus creating a body force causing sliding of the plate downslope.
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These metamorphic mineral reactions cause the dehydration of the upper part of the slab as the hydrated slab sinks. Heat is also transferred to it from the surrounding asthenosphere. As heat is transferred to the slab, temperature gradients are established such that the asthenosphere in the vicinity of the slab becomes cooler and more viscous than surrounding areas, particularly near the upper part of the slab. This more viscous asthenosphere is then dragged down with the slab causing less viscous mantle to flow in behind it.
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These raised features produce ridge push; gravity pulling down on the lithosphere at the mid-ocean ridge is mostly opposed by the normal force from the underlying rock, but the remainder acts to push the lithosphere down the sloping asthenosphere and away from the ridge. Because the asthenosphere is weak, ridge push and other driving forces are enough to deform it and allow the lithosphere to slide over it, opposed by drag at the Lithosphere-Asthenosphere boundary and resistance to subduction at convergent plate boundaries. Ridge push is mostly active in lithosphere younger than 90 Ma, after which it has cooled enough to reach thermal equilibrium with older material and the slope of the Lithosphere-Asthenosphere boundary becomes effectively zero.
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The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. The lithosphere-asthenosphere boundary is conventionally taken at the 1300 °C isotherm. Below this temperature (closer to the surface) the mantle behaves in a rigid way; above this temperature (deeper below the surface) it acts in a ductile fashion. Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly the same.
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Evidence supports that the force of gravity will increase plate velocity. As the relatively cool subducting slab sinks deeper into the mantle, it is heated causing dehydration of hydrous minerals. This releases water into the hotter asthenosphere, which leads to partial melting of asthenosphere and volcanism. Both dehydration and partial melting occurs along the isotherm, generally at depths of .
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Applied Geothermics. Springer Science & Business. pp. 318–. . This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity. This decrease in seismic wave velocity from lithosphere to asthenosphere could be caused by the presence of a very small percentage of melt in the asthenosphere.
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The lower boundary of the LVZ lies at a depth of 180–220 km, whereas the base of the asthenosphere lies at a depth of about 700 km. In the oceanic mantle, the transition from the lithosphere to the asthenosphere (the LAB) is shallower than for continental mantle (about 60 km in some old oceanic regions) with a sharp and large velocity drop (5–10%). At the mid-ocean ridges the LAB rises to within a few kilometers of the ocean floor. The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about.
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In collisions between two oceanic plates, the cooler, denser oceanic lithosphere sinks beneath the warmer, less dense oceanic lithosphere. As the slab sinks deeper into the mantle, it releases water from dehydration of hydrous minerals in the oceanic crust. This water reduces the melting temperature of rocks in the asthenosphere and causes partial melting. Partial melt will travel up through the asthenosphere, eventually, reach the surface, and form volcanic island arcs.
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The oceanic crust contains hydrated minerals such as the amphibole group. During subduction, oceanic lithosphere is heated and metamorphosed causing dehydration of these hydrous minerals contained within basalts, releasing water into the asthenosphere. The release of water into the asthenosphere leads to partial melting. Partial melting allows the rise of more buoyant, hot material and can lead to volcanism at the surface and emplacement of plutons in the subsurface.
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The total amount of uplift produced by this mechanism could be as much as 500 m. Other geoscientists have implied diapirism in the asthenosphere as being the cause of uplift. One hypothesis states that the early uplift of the Scandinavian Mountains could be indebted to changes in the density of the lithosphere and asthenosphere caused by the Iceland plume when Greenland and Scandinavia rifted apart about 53 million years ago.
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However, the plate hypothesis is inconsistent with both the geochemistry of shallow asthenosphere melts (i.e., Mid-ocean ridge basalts) and with the isotopic compositions of ocean island basalts.
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This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/year (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year (Nazca Plate; about as fast as hair grows)., .
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The current view, though still a matter of some debate, asserts that as a consequence, a powerful source of plate motion is generated due to the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.
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A lithosphere creeping over the asthenosphere is a logical consequence of an Earth with internal heat by radioactivity decay, the Airy-Heiskanen isostasy, thrust faults and Niskanen's mantle viscosity determinations.
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The older, and denser plate moves below the lighter plate. The further down it moves, the hotter it becomes, until finally melting altogether at the asthenosphere and inner mantle and the crust is actually destroyed. The location where the two oceanic plates actually meet become deeper and deeper creating trenches with each successive action. There is an interplay of various densities of lithosphere rock, asthenosphere magma, cooling ocean water and plate movement for example the Pacific Ring of Fire.
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Schematic diagram of the formation of a continental arc. When two tectonic plates collide, relatively denser oceanic crust will be subducted under relatively lighter continental crust. Because of the subduction process, the relatively cooler oceanic crust, along with water, is subducted to the asthenosphere, where pressures and temperatures are much higher than the surface of Earth. Under such conditions, the downgoing plate releases volatiles such as H2O and CO2, which cause partial melting of the above asthenosphere.
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Weight causes crustal flexure and subsidence Lithospheric stretching/thinning during rifting results in regional necking of the lithosphere (the elevation of the upper surface decreases while the lower boundary rises). The underlying asthenosphere passively rises to replace the thinned mantle lithosphere. Subsequently, after the rifting/stretching period ends, this shallow asthenosphere gradually cools back into mantle lithosphere over a period of many tens of millions of years. Because mantle lithosphere is denser than asthenospheric mantle, this cooling causes subsidence.
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New York: Prentice-Hall. They have been broadly accepted by geologists and geophysicists. These concepts of a strong lithosphere resting on a weak asthenosphere are essential to the theory of plate tectonics.
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Although its presence was suspected as early as 1926, the global presence of the asthenosphere was confirmed by analyses of seismic waves from the 9.5 Mw Great Chilean earthquake of May 22, 1960.
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The geothermal structure in a subduction zone determines the melting rate of subduction slab and asthenosphere. The change in isotherm structure may have significant impact on the intensity of magmatism. Some factors may contribute to the change in geothermal structure: a) the change in convergence velocity of two plates in subduction zone; b) the dipping angle of subduction slab; c) the amounts of subducted low temperature materials (water and oceanic sediments); d) the mantle/asthenosphere upwelling event (slab window/slab breakoff).
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Two plates collide and create an island arc between them in the process. Understanding the source of heat that causes the melting of the mantle was a contentious problem. Researchers believed that the heat was produced through friction at the top of the slab. However, this is unlikely because the viscosity of the asthenosphere decreases with increasing temperature, and at the temperatures required for partial fusion, the asthenosphere would have such a low viscosity that shear melting could not occur.
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These volcanic centers can be dormant for tens of millions of years between eruptions. This implies that the mechanism of formation is connected to the lithosphere unlike some other intraplate volcanoes such as the Hawaii island chain, which are rooted in the asthenosphere. One mechanism for the creation of these volcanoes is the flaking off of the base of Zealandia's lower lithosphere into the asthenosphere. Zealandia has a thin lithosphere as it has been extended while rafting away from Australia.
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Incompatible elements and rare-earth elements are enriched in these lavas. The volcanic rocks are derived from decompression melting of the asthenosphere, with garnet and lherzolite as precursors. Dunite xenoliths are found within the erupted basalts.
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The rocks have shoshonitic, mafic and calc-alkaline composition. The magma feeding these volcanic centres appears to come from the asthenosphere and the ascent of mafic magmas is facilitated by the extensional tectonic regime and by faulting.
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The main rift or "syn-rift" stage describes the phase of active stretching and fault block rotation. Syn-rift subsidence results from the elastic/isostatic adjustment of the crust due to mechanical stretching of the lithosphere. The subsidence is counteracted by upwelling of the asthenosphere into the space created by the mechanical stretching and thermal upward displacement of the asthenosphere-lithosphere boundary, causing uplift of the rift zone. The fundamental architectural element in many extensional basins is the half-graben, formed within the hanging walls of major rift-bounding or intra-rift basin faults.
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The reason for this was discovered upon analyzing data from the USARRAY project. It was found that the asthenosphere had invaded the overlying lithosphere, as a result of an area of mantle upwelling stemming from either the disintegration of the descending Farallon Plate, or the survival of the subducted spreading center connected to the East Pacific Rise and Gorda Ridge beneath western North America, or possibly both. The asthenosphere erodes the lower levels of the Plateau. At the same time, as it cools, it expands and lifts the upper layers of the Plateau.
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Earth cutaway from core to crust, the lithosphere comprising the crust and lithospheric mantle (detail not to scale) The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere. The modern understanding of the Earth's upper mantle is that there are two distinct components - the lithospheric part and the asthenosphere. The lithosphere, which includes the continental plates, acts as a brittle solid whereas the asthenosphere is hotter and weaker due to mantle convection. The boundary between these two layers is rheologically based and is not necessarily a strict function of depth.
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One example of the effects of lithosphere delamination is seen in the Sierra Nevada (US)², Basin and Range Province and Colorado Plateau in the western USA. During crustal extension in the Basin and Range Province 10 million years ago, the upwelling of asthenosphere thinned the lithosphere. Heating caused by the rise of the warmer asthenosphere created a crustal lower-viscosity zone and delamination occurred on the flanks of the Basin and Range. Uplift of the Sierra Nevada mountain range in California and the Colorado Plateau has occurred on the flanks as a result of the loss of high density lower lithosphere.
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This is analogous to the "Andean" style of orogenesis where subduction of an oceanic plate to approximately 110 km beneath the surface of Earth results in melting of the down-going slab and convecting asthenosphere. This melting may be assisted by the presence of water in what is known as Flux melting. The melt from the slab then rises up through the asthenosphere and through the crust to create large batholiths and volcanism. Although deformation in the western and central regions of the Sierra Nevada is widespread, deformation from the Nevadan Orogeny in the Eastern Belt is somewhat limited.
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The volcanism might have been mostly generated by asthenospheric upwelling possibly by displacement along the transform fault. If the transform fault had a section of vertical tearing to contain potentially different dip angles between the Explorer and Juan de Fuca Plates, the subducted plate asthenosphere may possibly flow upward into the mantle wedge. Similarly, if the displacement had a section of extension, a horizontal slab window-like gap would have developed, again allowing a pathway for upwelling magma. In either case, the unsettled asthenosphere might have experienced low degrees of decompressional melting and interacted with North American lithosphere to yield within plate compositions.
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Granite plutons have formed in many parts of the LFB where there has been significant heating. They were formed at the time of extension, when hot asthenosphere rose towards the surface. Granites cover 61000 km2. There are 875 lithological units of granite.
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Many continental rift zones are associated with magmatism due to upwelling of the asthenosphere as the lithosphere is thinned, which leads to decompression melting. The magmatism is often bimodal in character as the mantle-derived basaltic magmas cause partial melting of the continental crust.
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Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice the typical thickness of mature oceanic or non-cratonic, continental lithosphere.Petit (2010) p. 24 At that depth, craton roots extend into the asthenosphere.
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If large sections of this already thin lithosphere sank into the asthenosphere, it would be replaced with hotter rock leading to decompression melting. This theoretically could cause volcanic activity that is locked to the moving lithosphere over many millions of years.Hoernle 2006 et al.
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The subduction of oceanic crust, whether beneath oceanic or continental crust, is associated in almost all cases with partial melting of the overlying asthenosphere due to the addition of volatiles (especially water) expelled from the downgoing slab. Only when the slab fails to reach sufficient depth as in the earliest stages of subduction or where there are periods of flat-slab subduction that completely pinch out the asthenosphere, is magmatism absent. The magmatism is mostly calc-alkaline in type along a well-defined curvilinear magmatic arc. Only the volcanic parts of modern arcs are exposed at the surface and the understanding of the underlying magma chambers relies on geophysical methods.
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During the climax of lithospheric rifting, as the crust is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30 °C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges.
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The trend of a seamount chain traces the direction of motion of the lithospheric plate over a more or less fixed heat source in the underlying asthenosphere, the part of the Earth's mantle beneath the lithosphere.Seamounts are made by extrusion of lavas piped upward in stages from sources within the Earth's mantle to vents on the seafloor. Seamounts provide data on movements of tectonic plates on which they ride, and on the rheology of the underlying lithosphere. The trend of a seamount chain traces the direction of motion of the lithospheric plate over a more or less fixed heat source in the underlying asthenosphere part of the Earth's mantle.
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The passive rift model infers that slab pull stretches the lithosphere and thins it. To compensate for lithospheric thinning, asthenosphere upwells, melts due to adiabatic decompression, and derivative melts rise to the surface to erupt. Melts push up through faults towards the surface, forming dikes and sills.
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Only with the arrival of plate tectonic theory in the 1950s an explanation was found. In plate tectonics, the horizontal movement of tectonic plates over the Earth's soft asthenosphere causes horizontal forces within the crust. Presently, geologists believe most mountain chains are formed by convergent movements between tectonic plates.
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Such changes include the thickening of the lithosphere by overthrusting, changes in rock density of the lithosphere caused by metamorphism or thermal expansion and contraction, increases in the volume of the asthenosphere (part of the upper mantle supporting the lithosphere) caused by hydration of olivine, and orogenic, or mountain- building, movements.
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Back-arc extension often leads to the formation of oceanic crust and relatively short- lived spreading centres. As the asthenosphere behind the arc has been partly affected by volatiles from the downgoing slab, the typical back-arc basin basalts are intermediate in character between MORB type basalts and IAB type basalts.
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Mid-ocean ridge spreading centres are the sites of almost continuous magmatism. The basalts erupted at mid-ocean ridges are tholeiitic in character and result from the partial melting of upwelling asthenosphere. The composition of Mid-Ocean Ridge Basalts (MORB) shows little variation globally as they come from a mostly homogeneous source.
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Tectonophysics, a branch of geophysics, is the study of the physical processes that underlie tectonic deformation. The field encompasses the spatial patterns of stress, strain, and differing rheologies in the lithosphere and asthenosphere of the Earth; and the relationships between these patterns and the observed patterns of deformation due to plate tectonics.
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One way to determine if the LVZ is generated by partial melt is to measure the electrical conductivity of the Earth as a function of depth using magnetotelluric (MT) methods. Partial melt tends to increase conductivity, in which case the LAB can be defined as a boundary between the resistive lithosphere and conductive asthenosphere. Because mantle flow induces the alignment of minerals (such as olivine) to generate observable anisotropy in seismic waves, another definition of the seismic LAB is the boundary between the anisotropic asthenosphere and the isotropic (or a different pattern of anisotropy) lithosphere. The seismic LVZ was first recognized by Beno Gutenberg, whose name is sometimes used to refer to the base of the seismic LAB beneath oceanic lithosphere.
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The lithosphere-asthenosphere boundary is roughly deep in the TVF and deep in the surrounding regions.Fan, Q., Ma, M., King, D.T., Li, S., Zhao, Y., and Zou, H (2017), Genesis and open-system evolution of Quaternary magmas beneath southeastern margin of Tibet: Constraints from Sr- Nd-Pb-Hf isotope systematics, Lithos, 272-273, 279-290.
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A small portion of the continental crust may be subducted until the slab breaks, allowing the oceanic lithosphere to continue subducting, hot asthenosphere to rise and fill the void, and rebound of the continental lithosphere. Evidence of this continental rebound include ultrahigh pressure metamorphic rocks which form at depths of that are exposed at the surface.
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A close-up of the youngest lava flow The Lunar Crater vents have erupted alkali basalts; trachyte occurs at two lava domes and basalts, basanite, tephrite and trachybasalt have been reported as well. In general, the volcanic rocks define an ocean island basalt suite that originated in the asthenosphere. The rocks contain phenocrysts. inclusions and nodules.
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Volcanism in this field appears to be related to volcanism which is widespread in this part of Asia. Localized asthenosphere upwellings may be responsible for these volcanic events at Hangai and other volcanic centres around Lake Baikal. Other theories for the Hangai postulate a mantle plume or the removal of part of the lithosphere by asthenospheric currents.
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The debris from the continental arc would deposit in the subduction zone as turbidite. The undergoing subduction forces sediments to accretively add to the accretionary wedge or to subduct into the asthenosphere. Then part of sediments would be recycled through volcanic activities, and thus return to the continental crust, while another part would form new mantle material.
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In the Late Stephanian the zone was bent around a vertical axis to make the current crescent shape. This kind of bending is called an orocline. Two theories explain the Permian basin formation due to crustal extension, lithosphere delamination as solid mantle sinks from the bottom of the lithosphere, being replaced by hot asthenosphere; or a continental rift.
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Columnar basalt at Szent György Hill, Hungary Vesicular basalt at Sunset Crater, Arizona. US quarter for scale. In the Hadean, Archean, and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica (SiO2) contents below 45% are usually classified as komatiites.
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The LAB is a rheological boundary layer (RBL). Colder temperatures at Earth's shallower depths affect the viscosity and strength of the lithosphere. Colder material in the lithosphere resists flow while the "warmer" material in the asthenosphere contributes to its lower viscosity. The increase in temperature with increasing depth is known as the geothermal gradient and is gradual within the rheological boundary layer.
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Likewise, partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts. Basalt is generally created as a partial melt of peridotite at 20–120 km depth. Eclogite is denser than the surrounding asthenosphere. Unless the eclogite is created in very young oceanic crust, it is cool at the time of initial subduction and so is carried down into the mantle.
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Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches. Marine geology or geological oceanography is the study of the history and structure of the ocean floor. It involves geophysical, geochemical, sedimentological and paleontological investigations of the ocean floor and coastal zone. Marine geology has strong ties to geophysics and to physical oceanography.
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The isostatic response to the detachment of the downgoing slab is rapid uplift. Slab detachment is also followed by the upwelling of relatively hot asthenosphere to fill the gap created, leading in many cases to magmatism. The uncritical use of the slab-detachment model to explain disparate observations of magmatism, uplift and exhumation in continental collision zones has been criticised.
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The temperature of the core is approximately 1,000 degrees Celsius higher than that of the overlying mantle. Plumes are postulated to rise as the base of the mantle becomes hotter and more buoyant. Plumes are postulated to rise through the mantle and begin to partially melt on reaching shallow depths in the asthenosphere by decompression melting. This would create large volumes of magma.
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A residual forms composed from garnet pyroxenite at a depth of . This residual is denser than the mantle peridotite and can cause delamination of the lower crust containing the residual. Between 18 and 12 mya the Puna-Altiplano region was subject to an episode of flat subduction of the Nazca Plate. A steepening of the subduction after 12 mya resulted in the influx of hot asthenosphere.
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Ridge push is primarily opposed by plate drag, which is the drag force of the rigid lithosphere moving over the weaker, ductile asthenosphere. Models estimate that ridge push is probably just sufficient to overcome plate drag and maintain the motion of the plate in most areas. Slab pull is similarly opposed by resistance to the subduction of the lithosphere into the mantle at convergent plate boundaries.
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Greenland is isostatically depressed by the Greenland ice sheet such that parts of the bedrock surface in the interior are below sea level. Isostatic depression is the sinking of large parts of the Earth's crust into the asthenosphere. The sinking is caused by a heavy weight placed on the Earth's surface. Often this is caused by the heavy weight of glacial ice due to continental glaciation.
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This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe. Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle.
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A rift is the result of pulling apart or extension of both the lithosphere and crust (note that the crust is a part of the lithosphere). This is a product of what is referred to as mantle upwelling where hotter asthenosphere rises up into colder lithosphere. This rise is associated with thinning and stretching of the lithosphere. The internal dynamics of a rift system.
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The extension of the crust behind volcanic arcs is believed to be caused by processes in association with subduction. As the subducting plate descends into the asthenosphere it is heated up causing the volcanism at the island arcs. Another result of this heating is a convection cell is formed (See figure 1). The rising magma and heat in the convection cell cause a rift to form.
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Large iron blobs cannot be dragged by convective forces in the primordial mantle, therefore they do not have enough time to hydrodynamically equilibrate and reach the stabilized size. Hence, they deposit at a rheological boundary (such as present day lithosphere-asthenosphere boundary), forming iron ponds. Eventually, ponded iron would sink into the comparatively less dense silicates underneath them. The mechanism is thought to resemble salt diapirs.
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Imbabura is a volcano in the southern Ring of Fire. As the Nazca Plate is subducted beneath the South American Plate, the former melts with exposure to the hotter asthenosphere. This melted rock, which is less dense than the crust above it, rises to the surface. The result is an arc of volcanoes, which includes Imbabura, 100–300 km away from the subduction zone.
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Earth's lithosphere includes the crust and the uppermost mantle, which constitutes the hard and rigid outer layer of the Earth. The lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere, and biosphere through the soil-forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, hotter, and deeper part of the upper mantle.
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Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is stretched and thinned, so that the hot asthenosphere rises and heats the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting of volcanic deposits. When the basin grows due to continued stretching of the lithosphere, the rift grows and the sea can enter, forming marine deposits.
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S-waves (seismic shear waves) cannot propagate in liquids, leading to negligible velocity in the liquid outer core. The seismic velocities very near the surface () are markedly lower than at greater depths, demarking the LVZ. The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals.
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They contain phenocrysts of clinopyroxene, olivine and plagioclase. The chemical composition is unlike that of other regional basaltic volcanoes, and reflects the influence of oceanic asthenosphere. The basement in the region is formed by various Paleozoic to Mesozoic sediments and volcanic rocks. The plutons of the Northern Patagonian Batholith were intruded into this basement and may have an origin in the subduction of the Nazca Plate-Farallon Plate.
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This event was followed by low-angle extensional faulting along the East Antarctic plate. No uplift occurred during this initial phase of faulting due to counteracting erosional events. Shortly after, during the Mid Cretaceous (100 Ma), the West Antarctic Rift System began to form. Certain models show that the rapid rifting and intense thermal forces are due to a shallow (50 km depth) lithosphere-asthenosphere boundary under the Transantarctic Mountains.
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By identifying a reversal with a known age and measuring the distance of that reversal from the spreading center, the spreading half-rate could be computed. magnetic stripes formed during seafloor spreading In some locations spreading rates have been found to be asymmetric; the half rates differ on each side of the ridge crest by about five percent. This is thought due to temperature gradients in the asthenosphere from mantle plumes near the spreading center.
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The total thickness of the sedimentary infill in a sag basins can thus exceed 10 km. A third type of basin exists along convergent plate boundaries – places where one tectonic plate moves under another into the asthenosphere. The subducting plate bends and forms a fore-arc basin in front of the overriding plate – the an elongated, deep asymmetric basin. Fore-arc basins are filled with deep marine deposits and thick sequences of turbidites.
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Because of the different physical and chemical properties between the asthenosphere and lithosphere, viscous materials and a heater (for mantle convection) are also used. The simple analogue modelling of the extension tectonics which showing the formation of normal fault and salt dome (diapirism). This model is built in a glass box. The darker greyish layer is silicone which represents salt, and brownish layers are dry quartz sands which represent the brittle sedimentary rocks.
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First from Valanginian to Hauterivian (142–130 Ma) expansion happened at around 7 mm per year. Secondly from Hauterivian to Albian (130–113 Ma) the mantle was exhumed at around 13 mm per year. After this, the asthenosphere penetrated to the surface, a mid-oceanic ridge formed and normal oceanic crust was formed. The shallower 2–3 km of peridotite has been converted to green serpentine by alteration by seawater at depth.
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These small alkalic volcanoes are small percent melts of asthenosphere that exploit bending-related lithospheric faults to reach the seafloor. Hirano et al., (2006) proposed that these small volcanoes erupted along lithospheric fractures in response to plate flexure during subduction. If bending-related faulting and serpentinization is an important process beneath outer trench swells, there are probably also abundant low-temperature hydrothermal vents on the swells, similar to those of the Lost City (hydrothermal field).
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The same holds for the African, Eurasian, and Antarctic plates. Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them (see the paragraph on Mantle Mechanisms). This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of van Bemmelen.
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Earth's crust and mantle, Moho discontinuity between bottom of crust and solid uppermost mantle The Mohorovičić discontinuity , , (), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth's crust and the mantle. It is defined by the distinct change in velocity of seismological waves as they pass through changing densities of rock. The Moho lies almost entirely within the lithosphere. Only beneath mid-ocean ridges does it define the lithosphere–asthenosphere boundary.
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Furthermore, the Galápagos Hotspot is at the northern boundary of the Pacific Large Low Shear Velocity Province while the Easter Hotspot is on the southern boundary. The Galápagos Archipelago is characterized by numerous contemporaneous volcanoes, some with plume magma sources, others from the asthenosphere, possibly due to the young and thin oceanic crust. The GSC caused structural weaknesses in this thin lithosphere leading to eruptions forming the Galápagos Platform. Fernandina and Isabela in particular are aligned along these weaknesses.
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Relative depth may be controlled by the age of the lithosphere at the trench, the convergence rate, and the dip of the subducted slab at intermediate depths. Finally, narrow slabs can sink and roll back more rapidly than broad plates, because it is easier for underlying asthenosphere to flow around the edges of the sinking plate. Such slabs may have steep dips at relatively shallow depths and so may be associated with unusually deep trenches, such as the Challenger Deep.
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There is no evidence of rifting until the formation of Rodinia, 1.25 Gya in North Laurentia, and 1 Gya in East Baltica and South Siberia. However, breakup did not occur until 0.75 Gya, marking the end of the Boring Billion. This tectonic stasis may have been related in ocean and atmospheric chemistry. It is possible the asthenosphere—the molten layer of Earth's mantle that tectonic plates essentially float and move around upon—was too hot to sustain modern plate tectonics at this time.
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Calcium carbonate contributors, including plankton (such as coccoliths and planktic foraminifera), coralline algae, sponges, brachiopods, echinoderms, bryozoa and mollusks, are typically found in shallow water environments where sunlight and filterable food are more abundant. Cold-water carbonates do exist at higher latitudes but have a very slow growth rate. The calcification processes are changed by ocean acidification. Where the oceanic crust is subducted under a continental plate sediments will be carried down to warmer zones in the asthenosphere and lithosphere.
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In contrast to pure shear, simple shear describes constant volume strain with rotations. If a cube undergoes simple shearing, the result will be a parallelogram with sides that increase in length and are no longer parallel to the sides of the original cube. The top and bottom of the cube will neither stretch nor shorten. In a simple shear model, a basin is stretched asymmetrically by a large scale detachment fault extending from the upper crust to the lower lithosphere and even asthenosphere.
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The lithosphere, which rides atop the asthenosphere, is divided into a number of tectonic plates that are continuously being created and consumed at their opposite plate boundaries. Oceanic crust and tectonic plates are formed and move apart at mid-ocean ridges. Abyssal hills are formed by stretching of the oceanic lithosphere. Consumption or destruction of the oceanic lithosphere occurs at oceanic trenches (a type of convergent boundary, also known as a destructive plate boundary) by a process known as subduction.
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The LAB separates the mechanically strong lithosphere from the weak asthenosphere. The depth to the LAB can be estimated from the amount of flexure the lithosphere has undergone due to an applied load at the surface (such as the flexure from a volcano). Flexure is one observation of strength, but earthquakes can also be used to define the boundary between "strong" and "weak" rocks. Earthquakes are primarily constrained to occur within the old, cold, lithosphere to temperatures of up to ~650°C.
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Specifically, oceanic lithosphere (lithosphere underneath the oceanic plates) and subcontinental lithosphere, is defined as a mechanical boundary layer that heats via conduction and the asthenosphere is a convecting adiabatic layer. In contrast to oceanic lithosphere, which experiences quicker rates of recycling, subcontinental lithosphere is chemically distinct, cold, and older. This translated into the differences between the SCLM and the oceanic lithospheric mantle. There are two different types of subcontinental lithosphere that formed at different times in Earth's history: Archean and Phanerozoic subcontinental mantle.
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1930, Radioactivity and Earth movements. Geological Society of Glasgow Transactions, 18, pp.559-606. However, some studies have shown that the upper mantle (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along. Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from seismic tomography and from observations of the seismic discontinuity in the upper mantle at about 400 km (250 mi).
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Flat slab subduction is characterized by a low subduction angle (<30 degrees to horizontal) beyond the seismogenic layer and a resumption of normal subduction far from the trench. A slab refers to the subducting lower plate. Although, some would characterize flat slab subduction as any shallowly dipping lower plate as in western Mexico. Flat slab subduction is associated with the pinching out of the asthenosphere, an inland migration of arc magmatism (magmatic sweep), and an eventual cessation of arc magmatism.
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This required that plumes were sourced from beneath the shallow asthenosphere that is thought to be flowing rapidly in response to motion of the overlying tectonic plates. There is no other known major thermal boundary layer in the deep Earth, and so the core-mantle boundary was the only candidate. The base of the mantle is known as the D″ layer, a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle, and may contain partial melt.
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Heat flow at passive margins changes significantly over its lifespan, high at the beginning and decreasing with age. In the initial stage, the continental crust and lithosphere is stretched and thinned due to plate movement (plate tectonics) and associated igneous activity. The very thin lithosphere beneath the rift allows the upwelling mantle to melt by decompression. Lithospheric thinning also allows the asthenosphere to rise closer to the surface, heating the overlying lithosphere by conduction and advection of heat by intrusive dykes.
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304-308 One hypothesis is that uplift was a result of delamination, where the lowest layer of the North American tectonic plate below the Colorado Plateau detached and sank into the underlying mantle. This would have allowed hotter rock from the asthenosphere, the part of the earth's mantle that underlies its tectonic plates, to rise and lift the overlying crust.Ranney 2012, p.113 Another possibility is that the uplift was the result of heating at the base of the crust.
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Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact, oceanic lithosphere is a thermal boundary layer for the convectionDonald L. Turcotte, Gerald Schubert, Geodynamics.
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Such old continental crust and the underlying mantle asthenosphere are less dense than elsewhere in Earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense orogeny; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle.
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Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. Each red dot is a measuring point and vectors show direction and magnitude of motion. It has generally been accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming.
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Cross-section of subduction zone and associated stratovolcanoes Stratovolcanoes are common at subduction zones, forming chains and clusters along plate tectonic boundaries where oceanic crust is drawn under continental crust (continental arc volcanism, e.g. Cascade Range, Andes, Campania) or another oceanic plate (island arc volcanism, e.g. Japan, Philippines, Aleutian Islands). The magma forming stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust is released into mantle rock of the asthenosphere above the sinking oceanic slab.
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The upper mantle causes the tectonic plates to move. Crust and mantle are distinguished by composition while the lithosphere and asthenosphere are defined by a change in mechanical properties. The top of the mantle is defined by a sudden increase in the speed of seismic waves, which was first noted by Andrija Mohorovičić in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho". The Moho defines the base of the crust and varies from to below the surface of the Earth.
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The separation from Antarctica changed the tectonics of the Fuegian Andes into a transpressive regime with transform faults. About 15 million years ago in the Miocene the Chile Ridge begun to subduct beneath the southern tip of Patagonia (55° S). The point of subduction, the triple junction has gradually moved to the north and lies at present at 47° S. The subduction of the ridge has created a northward moving "window" or gap in the asthenosphere beneath South America.Charrier et al. 2006, p. 112.
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Whole-mantle convection Mantle convection is the very slow creeping motion of Earth's solid silicate mantle caused by convection currents carrying heat from the interior to the planet's surface. Physics Department, University of Winnipeg The Earth's surface lithosphere rides atop the asthenosphere and the two form the components of the upper mantle. The lithosphere is divided into a number of tectonic plates that are continuously being created or consumed at plate boundaries. Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading.
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This sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere has, on average, a greater density. At a depth of greater than 60 kilometers, the basalt of the oceanic crust is converted to a metamorphic rock called eclogite. At that point, the density of the oceanic crust increases and provides additional negative buoyancy (downwards force). It is at subduction zones that Earth's lithosphere, oceanic crust and continental crust, sedimentary layers and some trapped water are recycled into the deep mantle.
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Back arc basins form behind a volcanic arc and are associated with extensional tectonics and high heat flow, often being home to seafloor spreading centers. These spreading centers are like mid ocean ridges, though the magma composition of back arc basins is generally more varied and contains a higher water content than mid ocean ridge magmas. Back arc basins are often characterized by thin, hot lithosphere. Opening of back arc basins are still being studied but it is possible that movement of hot asthenosphere into lithosphere causes extension.
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Globally, forearcs have the lowest heatflow from the interior Earth because there is no asthenosphere (convecting mantle) between the forearc lithosphere and the cold subducting plate. The inner trench wall marks the edge of the overriding plate and the outermost forearc. The forearc consists of igneous and metamorphic crust, and this crust may act as buttress to a growing accretionary wedge (formed from sediments scraped off the top of the downgoing plate). If the flux of sediments is high, material transfers from the subducting plate to the overriding plate.
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Ascension is a geologically young formation, the tip of an undersea volcano which rose above the waves only a million years ago. Although volcanic activity is mainly associated with the Mid-Atlantic Ridge plate boundary 80 km to the west, Ascension also displays some features which are commonly attributed to "hotspot" volcanism. Such volcanism is typically assumed to arise from a deep mantle thermal plume from the core-mantle boundary. Alternatively it may result from minor deformations of the oceanic crust that cause extension and permit magma to rise passively up from the asthenosphere.
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The Cocos Plate was created by sea floor spreading along the East Pacific Rise and the Cocos Ridge, specifically in a complicated area geologists call the Cocos-Nazca spreading system. From the rise the plate is pushed eastward and pushed or dragged (perhaps both) under the less dense Caribbean Plate, in the process called subduction. The subducted leading edge heats up and adds its water to the mantle above it. In the mantle layer called the asthenosphere, mantle rock melts to make magma, trapping superheated water under great pressure.
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The Earth's surface or lithosphere comprises tectonic plates which average approximately 50 miles in thickness, and are continuously moving very slowly upon a bed of magma in the asthenosphere and inner mantle. The plates converge upon one another, and one subducts below the other, or, where there is only shear stress, move horizontally past each other (see transform plate boundary below). Little movements called fault creep are minor and not measurable. The plates meet with each other, and if rough spots cause the movement to stop at the edges, the motion of the plates continue.
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Abyssal plains cover more than 33% of the ocean floor (about 23% of Earth's surface), but they are poorly preserved in the sedimentary record because they tend to be consumed by the subduction process. The abyssal plain is formed when the lower oceanic crust is melted and forced upwards by the asthenosphere layer of the upper mantle. As this basaltic material reaches the surface at mid-ocean ridges, it forms new oceanic crust. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt.
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Hotspots are sites of upwelling of relatively hot mantle, possibly associated with mantle plumes, that cause partial melting of the asthenosphere. This type of magmatism forms volcanic seamounts or oceanic islands when they become emergent. Over short geological timescales the hotspots appear to be fixed relative to one another, forming a reference frame against which plate motions can be measured. As tectonic plates move relative to a hotspot, the location of magmatic activity on the plate shifts, causing the development of time-progressive chains of volcanoes such as the Hawaiian–Emperor seamount chain.
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The definition of the LAB as a thermal boundary layer (TBL) comes not from temperature, but instead from the dominant mechanism of heat transport. The lithosphere is unable to support convection cells because it is strong, but the convecting mantle beneath is much weaker. In this framework, the LAB separates the two heat transport regimes [conduction vs. convection]. However, the transition from a domain that transports heat primarily through convection in the asthenosphere to the conducting lithosphere is not necessarily abrupt and instead encompasses a broad zone of mixed or temporally variable heat transport.
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The thickness of the crust beneath the rift is disputed, as the structures of the rift deep beneath the surface are unknown. The difference in thickness of the crust, between the crust under the rift and that under the surrounding areas, has been bounded to be less than . Although some seismic data is evidence for a rise in the Lithosphere-Asthenosphere boundary, other researchers have claimed that there are deep structures which influence seismic activity, and that the lower crusts is intruded by mafic sills. They interpret the extension as a pure shear process.
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The diorites found in the area are high in magnesium and REE (rare-earth elements), which suggest further that the rocks were formed from a mixture of the mantle and remelted basaltic crust. Overall, the presence of tonalite- trondhjemite-granodiorites, as well as the iron present for the banded iron formations (BIFs), are determined to have been caused by hydrothermal alteration at high temperatures. BIFs are typically found in intra-oceanic, arc or forearc settings, which are associated with convergent boundaries. The intrusions were formed when upwelling asthenosphere partially melted thick overlying oceanic crust.
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Off the western coast of South America, the oceanic Nazca Plate subducts beneath the South America Plate in the Peru-Chile Trench. Volcanism associated with subduction in the region has been ongoing since the Jurassic. Dehydration of the downgoing slab causes melts to form in the abovelying asthenosphere which drive the activity in the volcanic arc. East of the main volcanic arc of the Central Volcanic Zone the back-arc region has been volcanically active since the Oligocene, generating volcanic edifices ranging from small monogenetic volcanoes to large calderas with their ignimbrites.
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The cones were constructed by basaltic andesite, which contains clinopyroxene, olivine and plagioclase. The petrologically primitive composition suggests that they were constructed from primitive asthenosphere derived magmas that reached the surface directly, through the Liquiñe-Ofqui fault system. The cones may be extremely young, one eruption with a volume of about may have occurred about 9,000 years ago based on stratigraphy, although the date is fairly uncertain. The glacial isostasy phenomena at the end of the last ice age may have triggered the magma ascent and thus eruptions.
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Unlike those on Earth, the deformations on Venus are directly related to regional dynamic forces within the planet's mantle. Gravitational studies suggest that Venus differs from Earth in lacking an asthenosphere--a layer of lower viscosity and mechanical weakness that allows Earth's crustal tectonic plates to move. The apparent absence of this layer on Venus suggests that the deformation of the Venusian surface must be explained by convective movements within the planet's mantle. The tectonic deformations on Venus occur on a variety of scales, the smallest of which are related to linear fractures or faults.
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Magmatism associated with subduction occurred not near the plate edges (as in the volcanic arc of the Andes, for example), but far to the east, called the Coast Range Arc. Geologists call such a lack of volcanic activity near a subduction zone a magmatic gap. This particular gap may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotter asthenosphere. One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which were the progenitors of the Rocky Mountains.
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West of South America, the Nazca Plate and the Antarctic Plate subduct beneath the South America Plate at a rate of , giving rise to the Andean volcanic belt. The volcanic belt is not continuous and is interrupted by gaps where the subduction is shallower and the asthenosphere between the two plates missing. North of the Payún Matrú, flat slab subduction takes place; in the past flat slab subduction occurred farther south as well and had noticeable influence on magma chemistry. In general, the mode of subduction in the region over time has been variable.
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Diagram of a mid-ocean ridge showing ridge push near the mid-ocean ridge and the lack of ridge push after 90 Ma Ridge push is the result of gravitational forces acting on the young, raised oceanic lithosphere around mid-ocean ridges, causing it to slide down the similarly raised but weaker asthenosphere and push on lithospheric material farther from the ridges. Mid-ocean ridges are long underwater mountain chains that occur at divergent plate boundaries in the ocean, where new oceanic crust is formed by upwelling mantle material as a result of tectonic plate spreading and relatively shallow (above ~60 km) decompression melting. The upwelling mantle and fresh crust are hotter and less dense than the surrounding crust and mantle, but cool and contract with age until reaching equilibrium with older crust at around 90 Ma. This produces an isostatic response that causes the young regions nearest the plate boundary to rise above older regions and gradually sink with age, producing the mid-ocean ridge morphology. The greater heat at the ridge also weakens rock closer to the surface, raising the boundary between the brittle lithosphere and the weaker, ductile asthenosphere to create a similar elevated and sloped feature underneath the ridge.
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Lost City Expedition. :For a general explanation of mid-oceanic ridges, see mid- oceanic ridge and seafloor spreading The ridge sits atop a geologic feature known as the Mid-Atlantic Rise, which is a progressive bulge that runs the length of the Atlantic Ocean, with the ridge resting on the highest point of this linear bulge. This bulge is thought to be caused by upward convective forces in the asthenosphere pushing the oceanic crust and lithosphere. This divergent boundary first formed in the Triassic period, when a series of three-armed grabens coalesced on the supercontinent Pangaea to form the ridge.
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Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches Oceanic trenches are topographic depressions of the seafloor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab.
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The angle of dip of the subducting slab, and therefore the Benioff seismic zone, is dominantly controlled by the negative buoyancy of the slab and forces from the flowing of the asthenosphere. Younger lithosphere is hotter and more buoyant, resulting in shallow-dipping Benioff zones, whereas older lithosphere is denser and colder, causing steeper dips. The Benioff zone spans from near-surface to depths of up to 670 km. The upper bound is just beneath the weak sediments in the toe of the wedge of the subduction zone, and the lower bound is where the brittle-ductile transition occurs.
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Approximate location of Mesoproterozoic (older than 1.3 Ga) cratons in South America and Africa (the Saharan Metacraton is not shown). The Earth formed about 4.54 billion years ago. As it cooled, the lithosphere, consisting of the crust and the rigid uppermost part of the mantle, solidified. The lithosphere rides on the asthenosphere, which is also solid but can flow like a liquid on geological time scales. The lithosphere is broken up into tectonic plates, which slowly move in relation to one another at speeds of 50–100 mm annually, colliding, combining into continents, splitting and drifting apart to form new continental configurations.
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Temperatures of have been estimated for the lava erupted by the Black Tank cone. The magma erupted in the field ultimately appears to originate from the lithospheric or asthenospheric mantle with little contribution of crustal components, unlike earlier felsic volcanism. Upwelling of asthenosphere material appears to be responsible for the volcanism at the end, possibly associated with the change in the tectonics of the region from subduction-dominated to tectonics of a transform boundary. Fractional crystallization, magma ponding in the crust, differences in the mantle sources and partial melting processes have been invoked to explain certain compositional differences in the erupted rocks.
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The proto-rift stage is sometimes characterized by deposition in a wide, slowly subsiding flexural basin with only minor fault activity. During this stage, sedimentation is controlled primarily by climatic and, in marine settings, by relative sea- level fluctuations. In other rifts, progressive, thermally induced, upward displacement of the asthenosphere - lithosphere boundary by mantle plumes cause the gradual upward motion of broad rift domes that reach their maximum dimensions before or at the onset of active stretching. Proto-rift basins are typically saucer-shaped, slightly deepening towards the future graben axis, which can lead to large axial sediment transport systems.
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That spreading ridges could be subducted was recognized early in the development of plate tectonic theory, but there was little consideration of the ensuing effects. In the 1980s came realization that the magma welling up from the asthenosphere through the subducted ridge would not reach seawater, and thus not be quenched to form rock and close the gap. Continued spreading would lead to a widening gap or "window" in the subducting plate through which there could be increased flow of magma.. The implications of this for Siletzia were first shown by and (following the pioneering work by ).; .
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The subduction of the Nazca plate and the Antarctic plate beneath the western side of South America has generated a belt of volcanic activity named the Andean Volcanic Belt. The belt is separated in a number of volcanic zones by segments lacking recent volcanic activity; in these segments, shallow subduction of the plates presumably displaces the asthenosphere away from these segments. The segments with active volcanism are the Northern Volcanic Zone (NVZ), the Central Volcanic Zone (CVZ), the Southern Volcanic Zone (SVZ) and the Austral Volcanic Zone (AVZ). The "Volcanoes of the World" catalogue counts about 575 eruptions in the entire volcanic belt.
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Another explanation for slab flattening is the lateral movement of the overriding plate in a direction opposite to that of the downgoing slab. The overriding plate is often equipped with a cratonic keel of thick continental lithosphere which, if close enough to the trench, can impinge upon the flow in the mantle wedge. Trench suction is included in this causal mechanism. Trench suction is induced by the flow of the asthenosphere in the mantle wedge area; trench suction increases with subduction velocity, a decrease of the mantle wedge thickness, or an increase in the mantle wedge viscosity.
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With hindsight, we can discount Dana's conjecture that this contraction was due to the cooling of the Earth (aka the cooling Earth theory). The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, fiercely contested by proponents of vertical movements in the crust (similar to tephrotectonics), or convection within the asthenosphere or mantle. Gustav Steinmann (1906) recognised different classes of orogenic belts, including the Alpine type orogenic belt, typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure.
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Cross-sectional diagram of magmatic processes in a continental arc The dehydration of the downgoing slab and the partial melting of asthenosphere together generate the primary magma of continental arcs. Primary magma is composed of olivine tholeiitic basalt because of mixture of peridotites from the mantle wedge and large ion lithophile enriched (LIL- enriched) fluids from the dehydrating subducting plate. Because the larger thickness and lower density, the continental crust is likely to prevent the upwards rising of primary magma. Ascending primary magma is likely to pond at the bottom of continental crust, forming a magma chamber.
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"Andean" style of orogenesis involving the subduction of an oceanic plate beneath a continental plate causing partial melt of the asthenosphere wedge atop the down going plate. The partial melt then rises and causes volcanism to start to build up mountains through lava flows and deposits from eruptions. The eastern belt of the Sierra Nevada consists of the Northern Sierra Terrane. The Northern Sierra Terrane was formed from volcanism at the western edge of North America due to the subduction of an oceanic plate, which eventually resulted in the accretion of the Tuolumne River and Slate Creek terranes to North America.
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Mineral Resource Reviews. Springer-Verlag Heidelberg, 398 pp Rock (1991) considered lamphrophyres to be possible source rocks for the gold, but this view is not generally supported. The more reasonable explanation for the correlation is that lamprophyres, representing "wet" melts of the asthenosphere and mantle, correlate with a period of high fluid flow from the mantle through the crust, during subduction-related metamorphism, which drives gold mineralisation. Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands and Southern Uplands of Scotland;Thorpe R.S., Gaskarth J.W. & Henney P.J., 1993.
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Volcanic activity in the Andes and the region between the Andes and the Atlantic Ocean is caused by the subduction of the Nazca plate beneath the South America plate. While the main volcanic arc is formed by the dehydration of the descending slab of oceanic lithosphere, the origin of volcanism beneath the main volcanic arc is unclear. One of these volcanic structures is the Payenia volcanic field between 34°30′–38°S southern latitude; it was probably formed by magmas generated by asthenosphere overriding a steepening subducting Nazca plate. This province, with a surface area of , includes Payún Matrú and Llancanelo volcanic field.
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Most earthquakes occur within the 1000 °C isotherm, in the interior of the slab that has not yet heated up to match the temperature of the surrounding mantle into which it is being subducted. At depths below the thickness of the lithosphere, earthquakes are no longer generated by thrusting at the interface of the two plates, because the asthenosphere is weak and cannot support the stresses necessary for faulting. In this region, internal deformation of the still-cool down-going slab is the source of the earthquakes. Up to depths of 300 km, dehydration reactions and the formation of eclogite are the main causes of seismicity.
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Ridge push (also known as gravitational sliding) or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid- ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.
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Structure of Earth The internal structure of Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through Earth, measurements of the gravitational and magnetic fields of Earth, and experiments with crystalline solids at pressures and temperatures characteristic of Earth's deep interior.
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The creation of the abyssal plain is the result of the spreading of the seafloor (plate tectonics) and the melting of the lower oceanic crust. Magma rises from above the asthenosphere (a layer of the upper mantle), and as this basaltic material reaches the surface at mid-ocean ridges, it forms new oceanic crust, which is constantly pulled sideways by spreading of the seafloor. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine- grained sediments, mainly clay and silt. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons into deeper water.
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Break-off of the subducting slab following the end of subduction would lead to the upwelling of hot asthenosphere, causing melting of the overlying lithosphere producing lamprophyric magmas, underplating and injecting into the crust. The granitic magmas may be a result of either partially melting the lamprophyric underplate or by differentiation from the lamprophyric magmas. Further upwelling would lead to an increase degree of melting within the crust, contributing to a decrease in the amount of mantle component in the granitic melts. This is consistent with an overall change from more basic to more acidic with time observed in the plutons and a reduction of Barium and Strontium.
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Schematic depiction of the process of slab detachment. OC=oceanic crust, OLM=oceanic lithospheric mantle, CC=continental crust, CLM=continental lithospheric mantle In plate tectonics, slab detachment or slab break-off may occur during continent-continent or arc-continent collisions. When the continental margin of the subducting plate reaches the oceanic trench of the subduction zone, the more buoyant continental crust will in normal circumstances experience only a limited amount of subduction into the asthenosphere. The slab pull forces will, however, still be present and this normally leads to the breaking off or detachment of the descending slab from the rest of the plate.
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The concept of the lithosphere as Earth's strong outer layer was described by A.E.H. Love in his 1911 monograph "Some problems of Geodynamics" and further developed by Joseph Barrell, who wrote a series of papers about the concept and introduced the term "lithosphere". The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong, solid upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere). These ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work "Strength and Structure of the Earth."Daly, R. (1940) Strength and structure of the Earth.
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Rising convection currents occur where two plates are moving away from each other. In the gap, thus produced hot magma rises up, meets the cooler sea water, cools, and solidifies, attaching to either or both tectonic plate edges creating an oceanic spreading ridge. When the fissure again appears, again magma will rise up, and form new lithosphere crust. If the weakness between the two plates allows the heat and pressure of the asthenosphere to build over a large amount of time, a large quantity of magma will be released pushing up on the plate edges and the magma will solidify under the newly raised plate edges, see formation of a submarine volcano.
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The most recent estimate of the subduction angle for the Nazca Plate is 20° to a depth of at inland. At depth, approximately inland, the plate shifts to a horizontal orientation, and continues to travel horizontally for up to inland, before resuming subduction into the asthenosphere. Image showing the lack of continental volcanism adjacent to subducting ridges Large magnitude earthquakes occur in association with the area around the Nazca Ridge subduction zone, known at the Peru megathrust. These include, but are not limited to, a magnitude 8.1 earthquake in 1942, a magnitude 8.0 earthquake in 1970, a magnitude 7.7 earthquake in 1996, a magnitude 8.4 earthquake in 2001, and a magnitude 8.0 earthquake in 2007.
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John Tuzo Wilson (October 24, 1908 – April 15, 1993) was a Canadian geophysicist and geologist who achieved worldwide acclaim for his contributions to the theory of plate tectonics. Plate tectonics is the idea that the rigid outer layers of the Earth (crust and part of the upper mantle), the lithosphere, is broken up into around 13 pieces or "plates" that move independently over the weaker asthenosphere. Wilson maintained that the Hawaiian Islands were created as a tectonic plate (extending across much of the Pacific Ocean) shifted to the northwest over a fixed hotspot, spawning a long series of volcanoes. He also conceived of the transform fault, a major plate boundary where two plates move past each other horizontally (e.g.
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The basaltic rocks in the oceanic and continental sectors of the Cameroon line are similar in composition, although the more evolved rocks are quite distinct. The similarity in basaltic rocks may indicate they have the same source. Since the lithosphere mantle below Africa must be different in chemical and isotopic composition from the younger lithosphere below the Atlantic, one explanation is that the source is in the asthenosphere rather than in metasomatized lithosphere. A different view is that the similarities are caused by shallow contamination of the oceanic section, which could be caused by sediments from the continent or by rafted crustal blocks that were trapped in the oceanic lithosphere during the separation between South America and Africa.
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The tectonic plates of the world were mapped in the second half of the 20th century. Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the , from the ) is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of Earth's lithosphere, since tectonic processes began on Earth between 3.3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.
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Geologists think one of the reasons that volcanoes on Mars are able to grow so large is because Mars lacks plate tectonics. The Martian lithosphere does not slide over the upper mantle (asthenosphere) as on Earth, so lava from a stationary hot spot is able to accumulate at one location on the surface for a billion years or longer. On 17 October 2012, the Curiosity rover on the planet Mars at "Rocknest" performed the first X-ray diffraction analysis of Martian soil. The results from the rover's CheMin analyzer revealed the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the "weathered basaltic soils" of Hawaiian volcanoes.
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The Altiplano plateau was formed during the Tertiary, with several mechanisms proposed; all attempt to explain why the topography of the Andes incorporates a large area of low relief at high altitude (high plateau): # Existence of weaknesses in the Earth's crust prior to tectonic shortening. Such weaknesses would cause the partition of tectonic deformation and uplift into eastern and western cordillera, leaving the necessary space for the formation of the Altiplano basin. # Magmatic processes rooted in the asthenosphere might have contributed to uplift the plateau. # Climate controlled the spatial distribution of erosion and sediment deposition, creating the lubrication along the Nazca Plate subduction and hence influencing the transmission of tectonic forces into South America.
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Current research indicates that complex convection within the Earth's mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary. This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere (or upper mantle) beneath the rift area, forming large flood basalt or lava flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other. Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary.
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The magma that forms them arises when water, which is trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering", and occurs at specific pressure/temperature conditions for specific minerals as the plate subducts to lower depths. The water freed from the subducting slab lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica rich crustal rock, leading to a final intermediate composition.
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Magmatism before the Laramide orogeny migrated all the way to western South Dakota. Eventually, the magmatic activity above the flat slab may completely cease as the subducting plate and upper plate pinch out the mantle wedge. Upon the failure of the flat slab, the mantle wedge can again start circulating hot asthenosphere (1300 degrees C) in an area that has been heavily hydrated, but that had not produced any melt; this leads to widespread ignimbritic volcanism, which is seen in both the Andean flat slab effected regions and the western United States. Adakites are dacitic and andesitic magmas that are highly depleted in heavy rare-earth elements and high strontium/yttrium ratios and may be derived of melting of the oceanic crust.
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Initially, there will be decreased alkalic magmatism, horizontal shortening, hydration of the lithosphere above the flat-slab, and low heat flow. Upon a return to normal subduction, the hot asthenosphere will once again interact with the hydrated mantle, causing wet melting, crustal melting will ensue as mantle melts pass through, and lithospheric thinning and weakening due to the increased heat flow. The subducting slab can be lifted by aseismic ridges, seamount chains, or oceanic plateaus – which can provide a favourable environment for the development of a porphyry deposit. This interaction between subduction zones and the aforementioned oceanic features can explain the development of multiple metallogenic belts in a given region; as each time the subduction zone interacts with one of these features it can lead to ore genesis.
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Oceanic crust is formed at a mid-ocean ridge, while the lithosphere is subducted back into the asthenosphere at oceanic trenches Age of oceanic crust (red is youngest, and blue is oldest) Oceanic crust, which forms the bedrock of abyssal plains, is continuously being created at mid- ocean ridges (a type of divergent boundary) by a process known as decompression melting. Plume-related decompression melting of solid mantle is responsible for creating ocean islands like the Hawaiian islands, as well as the ocean crust at mid-ocean ridges. This phenomenon is also the most common explanation for flood basalts and oceanic plateaus (two types of large igneous provinces). Decompression melting occurs when the upper mantle is partially melted into magma as it moves upwards under mid-ocean ridges.
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Through subduction, oceanic crust and lithosphere returns to the convecting mantle. Areas of the crust where new crust is created are called divergent boundaries, those where it is brought back into the Earth are convergent boundaries and those where plates slide past each other, but no new lithospheric material is created or destroyed, are referred to as transform (or conservative) boundaries Earthquakes result from the movement of the lithospheric plates, and they often occur near convergent boundaries where parts of the crust are forced into the earth as part of subduction. Volcanoes result primarily from the melting of subducted crust material. Crust material that is forced into the asthenosphere melts, and some portion of the melted material becomes light enough to rise to the surface—giving birth to volcanoes.
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This is a misnomer as nothing is "pushing" horizontally and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate which produces a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges, and mantle plumes and hot spots, which are postulated to impinge on the underside of tectonic plates. Slab-pull: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere.
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In a rift setting, the alkaline magma is produced by very small degrees of partial melting (<1%) of garnet peridotite in the upper mantle (200 to 600 km depth). This melt becomes enriched in incompatible elements, like the rare-earth elements, by leaching them out of the crystalline residue. The resultant magma rises as a diapir, or diatreme, along pre-existing fractures, and can be emplaced deep in the crust, or erupted at the surface. Typical REE enriched deposits types forming in rift settings are carbonatites, and A- and M-Type granitoids. Near subduction zones, partial melting of the subducting plate within the asthenosphere (80 to 200 km depth) produces a volatile-rich magma (high concentrations of CO and water), with high concentrations of alkaline elements, and high element mobility that the rare-earths are strongly partitioned into.
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After dehydration, solute-rich fluids are released from the slab and metasomatise the overlying mantle wedge of MORB-like asthenosphere, enriching it with volatiles and large ion lithophile elements (LILE). The current belief is that the generation of andesitic magmas is multistage, and involves crustal melting and assimilation of primary basaltic magmas, magma storage at the base of the crust (underplating by dense, mafic magma as it ascends), and magma homogenization. The underplated magma will add a lot of heat to the base of the crust, thereby inducing crustal melting and assimilation of lower-crustal rocks, creating an area with intense interaction of the mantle magma and crustal magma. This progressively evolving magma will become enriched in volatiles, sulfur, and incompatible elements – an ideal combination for the generation of a magma capable of generating an ore deposit.
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Upon further opening of the ocean and plate drift, the plume and the mid-Atlantic Ridge are postulated to have approached one another, and finally met. The excess magmatism that accompanied the transition from flood volcanism on Greenland, Ireland and Norway to present- day Icelandic activity was the result of ascent of the hot mantle source beneath progressively thinning lithosphere, according to the plume model, or a postulated unusually productive part of the mid-ocean ridge system. Some geologists have suggested that the Iceland plume could have been responsible for the Paleogene uplift of the Scandinavian Mountains by producing changes in the density of the lithosphere and asthenosphere during the opening of the North Atlantic. To the south the Paleogene uplift of the English chalklands that resulted in the formation of the Sub-Paleogene surface has also been attributed to the Iceland plume.
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The Donegal batholith was formed during the Early Devonian, towards the end of the Caledonian orogeny between about 418 and 397 Ma. It is interpreted to have been intruded along a major SW-NE trending sinistral shear zone. The space to allow the intrusion of such large volumes of granitic magma into the crust is thought to be a result of movement along the shear zone combined with the sinistral strike-slip reactivation of a major SSW-NNE trending fault, running approximately through the centres of the Ardara, Trawenagh Bay, Rosses and Thorr plutons. The chemistry and timing of the granites does not suggest that they are subduction-related. Subduction is thought to have ceased by the end of the Silurian (~419 Ma) and there is no evidence of significant involvement of mid-ocean ridge basalt or asthenosphere sources for the granitic melts.
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Cartoon showing the isostatic vertical motions of the lithosphere (grey) in response to a vertical load (in green) The lithospheric flexure (also called regional isostasy) is the process by which the lithosphere (rigid outer layer of the Earth) bends under the action of forces such as the weight of a growing orogen or changes in ice thickness related to (de)glaciations. The lithosphere is the thin, outer, rigid layer of the Earth resting on the asthenosphere, a viscous layer that in geological time scales behaves as a fluid. Thus, when loaded, the lithosphere progressively reaches an isostatic equilibrium, which is the name of the Archimedes principle applied to these geological settings. This phenomenon was first described in the late 19th century to explain the shorelines uplifted in Scandinavia due to the removal of large ice massed during the last glaciation.
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Although the process of subduction as it occurs today is fairly well understood, its origin remains a matter of discussion and continuing study. Subduction initiation can occur spontaneously if denser oceanic lithosphere is able to founder and sink beneath adjacent oceanic or continental lithosphere; alternatively, existing plate motions can induce new subduction zones by forcing oceanic lithosphere to rupture and sink into the asthenosphere. Both models can eventually yield self-sustaining subduction zones, as oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. Results from numerical models generally favor induced subduction initiation for most modern subduction zones, which is supported by geologic studies, but other analogue modeling shows the possibility of spontaneous subduction from inherent density differences between two plates at passive margins, and observations from the Izu-Bonin-Mariana subduction system are compatible with spontaneous subduction nucleation.
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Intraplate volcanoes occur in many places of the Western United States, including along the Sierra Nevada, on the Colorado Plateau, the Basin and Range province and the Rio Grande Rift. Lunar Crater volcanic field lies within the Basin and Range province along with other volcanic fields, but in an unusually central position. Upwelling of asthenospheric mantle in response to the tectonic regime of the Basin and Range may be responsible for the eruptive activity there, although other processes have also been proposed such as mantle downwelling and compensating flow in the asthenosphere; older volcanism in the region is related to the subduction of the Farallon Plate. The Basin and Range province has had a complicated geological history and in the last 20 million years features extensional tectonics (tectonic processes involving a dilatation of the crust) represented by normal faults (faults where the downmoving blocks move in a way consistent with gravity).
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The Nazca Plate and Antarctic Plate subduct beneath the South America Plate in the Peru-Chile Trench at a pace of and , respectively, resulting in volcanic activity and geothermal manifestations in the Andes. Present-day volcanism occurs within four discrete belts: the NVZ (between 2°N–5°S), the CVZ (16°S–28°S), the SVZ (33°S–46°S) and the Austral Volcanic Zone (AVZ) (49°S-55°S). Between them they contain about 60 active volcanoes and 118 volcanoes which appear to have been active during the Holocene, not including potentially active very large silicic volcanic systems or very small monogenetic ones. These belts of active volcanism occur where the Nazca Plate subducts beneath the South America Plate at a steep angle, while in the volcanically inactive gaps between them the subduction is much shallower; thus there is no asthenosphere between the slab of the subducting plate and the overriding plate in the gaps.
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Early models of plate tectonics, such as Harry Hess's seafloor spreading model, assumed that the motions of plates and the activity of mid-ocean ridges and subduction zones were primarily the result of convection currents in the mantle dragging on the crust and supplying fresh, hot magma at mid-ocean ridges. Further developments of the theory suggested that some form of ridge push helped supplement convection in order to keep the plates moving, but in the 1990s, calculations indicated that slab pull, the force that a subducted section of plate exerts on the attached crust on the surface, was an order of magnitude stronger than ridge push. As of 1996, slab pull was generally considered the dominant mechanism driving plate tectonics. Modern research, however, indicates that the effects of slab pull are mostly negated by resisting forces in the mantle, limiting it to only 2-3 times the effective strength of ridge push forces in most plates, and that mantle convection is probably much too slow for drag between the lithosphere and the asthenosphere to account for the observed motion of the plates.
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But while the track of the Yellowstone hotspot across the Snake River Plain conforms to what is expected from the motion of the North American Plate across some sort of "hotspot" fixed in the underlying mantle, the Newberry "hotspot" track is oblique to the motion of the North American Plate; this is inconsistent with the hotspot model. Alternative models include: 1) flow of material from the top layer of the mantle (asthenosphere) around the edge of the Juan de Fuca Plate (a.k.a. "Vancouver slab"), 2) flows reflecting lithospheric topography (such as the edge of the craton), 3) faulting in the lithosphere, or 4) extension of the Basin and Range province (which in turn may be due to interactions between the North American, Pacific, and Farallon Plates, and possibly with the subduction of the triple point where the three plates came together), but none is yet fully accepted.E.g., concluded that the Newberry track is the product of a lithosphere-controlled process (such as lithospheric faulting or Basin and Range extension); disagree, arguing for mantle flow around the sinking Gorda-- Juan de Fuca slab.
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In Grenoble, he set up in 1959 at the master level a new syllabus in general geophysics which will flourish in the 1960s when the Earth's sciences will be refounded by the plate tectonics "theory". Two articles published in 1969 and 1970 on the modelling of convection within the Earth's mantle showed him, with Claude Allègre, Xavier Le Pichon and Dan McKenzie, in the very closed circle of European scientists at the leading edge of the new theory. He was the first to notice that the viscosity of the asthenosphere, due to partial melting (of the order of one percent), is analogous to what happens in so-called "temperate glaciers" where ice is also partially melted in the same order of magnitude, with the coexistence of a liquid phase and a solid phase. He also modelled the postglacial rebound of the lithosphere as observed in Fennoscandia or Canada following the disappearance of Quaternary ice caps, which allowed him to infer the mechanical properties of the Earth's mantle, its rheology and its viscosity.
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