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Where Is The Asthenosphere Located

Written By Monday, 31 October 2022 Add Comment Edit

Highly viscous, mechanically weak and ductile region of Globe's drape

The asthenosphere (from Ancient Greek ἀσθενός ( asthenós ) 'without force') is the mechanically weak[1] and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km (50 and 120 mi) below the surface, and extends equally deep as 700 km (430 mi). However, the lower purlieus of the asthenosphere is not well defined.

The asthenosphere is about solid, only a slight amount of melting (less than 0.ane% of the stone) contributes to its mechanical weakness. More than extensive decompression melting of the asthenosphere takes place where information technology wells up, and this is the most important source of magma on Earth. Information technology is the source of mid-ocean ridge basalt (MORB) and of some magmas that erupted above subduction zones or in regions of continental rifting.

Characteristics [edit]

The asthenosphere in relation to the other layers of Globe'due south structure

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic motility and isostatic adjustments. Information technology is composed of peridotite, a rock containing mostly the minerals olivine and pyroxene.[2] The lithosphere-asthenosphere purlieus is conventionally taken at the 1,300 °C (2,370 °F) isotherm. Beneath this temperature (closer to the surface) the drape behaves rigidly; above this temperature (deeper below the surface) it acts in a ductile manner.[3] The asthenosphere is where the drape rock most closely approaches its melting indicate, and a minor amount of melt is probable present in this layer.[4]

Seismic waves pass relatively slowly through the asthenosphere[five] compared to the overlying lithospheric curtain. Thus, it has been called the low-velocity zone (LVZ), although the ii are non strictly the same;[6] [seven] the lower boundary of the LVZ lies at a depth of 180 to 220 kilometers (110 to 140 mi),[8] whereas the base of the asthenosphere lies at a depth of about 700 kilometers (430 mi).[9] The LVZ as well has a high seismic attenuation (seismic waves moving through the asthenosphere lose energy) and significant anisotropy (shear waves polarized vertically have a lower velocity than shear waves polarized horizontally).[10] The discovery of the LVZ alerted seismologists to the existence of the asthenosphere and gave some information well-nigh its concrete properties, as the speed of seismic waves decreases with decreasing rigidity. This subtract in seismic wave velocity from the lithosphere to the asthenosphere could exist caused by the presence of a very small-scale percentage of melt in the asthenosphere, though since the asthenosphere transmits S waves, information technology cannot exist fully melted.[xi]

In the oceanic mantle, the transition from the lithosphere to the asthenosphere (the LAB) is shallower than for the continental mantle (about threescore km in some old oceanic regions) with a sharp and large velocity drop (v–x%).[12] At the mid-ocean ridges, the LAB rises to within a few kilometers of the ocean flooring.

The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth'south crust motion about. 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, tin can break, causing faults. The rigid lithosphere is thought to "float" or motility about on the slowly flowing asthenosphere, enabling isostatic equilibrium[13] and allowing the move of tectonic plates.[xiv] [xv]

Boundaries [edit]

The asthenosphere extends from an upper purlieus at approximately 80 to 200 km (50 to 120 miles) below the surface[sixteen] [17] to a lower boundary at a depth of approximately 700 kilometers (430 mi).[9]

Lithosphere-asthenosphere boundary [edit]

The lithosphere-asthenosphere boundary (LAB[18] [nineteen]) is relatively sharp and likely coincides with the onset of fractional melting or a change in composition or anisotropy.[20] Diverse definitions of the boundary reverberate various aspects of the purlieus region. In improver to the mechanical purlieus defined by seismic data, which reflects the transition from rigid lithosphere to ductile asthenosphere, these include a thermal boundary layer, above which rut is transported past thermal conduction and below which heat is conduction largely past convection; a rheological boundary, where the viscosity drops below near 1021 Pa-due south; and a chemic boundary layer, above which the mantle rock is depleted in volatiles and enriched in magnesium relative to the rock beneath.[21]

Lower boundary of asthenosphere [edit]

The lower boundary of the asthenosphere is less well divers, but has been placed at the base of the upper mantle. [22] This boundary is neither seismically sharp nor well understood[9] just is approximately coincident with the circuitous 670 km discontinuity.[23] This discontinuity is mostly linked to the transition from mantle stone containing ringwoodite to mantle rock containing bridgmanite and periclase.[24]

Origin [edit]

The mechanical backdrop of the asthenosphere are widely attributed to partial melting of the rock.[11] It is likely that a small amount of melt is present through much of the asthenosphere, where information technology is stabilized by the traces of volatiles (water and carbon dioxide) present in the mantle stone.[25] Nonetheless, the likely amount of melt, not more than near 0.ane% of the rock, seems inadequate to fully explain the existence of the asthenosphere. This is not enough cook to fully moisture grain boundaries in the rock, and the effects of melt on the mechanical properties of the stone are not expected to be significant if the grain boundaries are not fully wetted. The sharp lithosphere-asthenosphere boundary is too hard to explain by partial melting alone.[26] It is possible that melt accumulates at the top of the asthenosphere, where information technology is trapped past the impermeable rock of the lithosphere.[25] Some other possibility is that the asthenosphere is a zone of minimum water solubility in drapery minerals, and so that more than water is available to grade greater quantities of cook.[27] Another possible mechanism for producing mechanical weakness is grain boundary sliding, where grains slide slightly by each other nether stress, lubricated by the traces of volatiles nowadays.[10]

Numerical models of pall convection in which the viscosity is dependent both on temperature and strain rate reliably produce an oceanic asthenosphere, suggesting that strain-rate weakening is a meaning contributing mechanism.[28]

Magma generation [edit]

Decompression melting of asthenospheric rock creeping towards the surface is the well-nigh important source of magma on Earth. Well-nigh of this erupts at mid-ocean ridges to class the distinctive mid-bounding main ridge basalt (MORB) of the ocean crust.[29] [30] [31] Magmas are also generated by decompressional melting of the asthenosphere above subduction zones[32] and in areas of continental rifting.[33] [34]

Decompression melting in upwelling asthenosphere likely begins at a depth as great as 100 to 150 kilometers (60 to 90 mi), where the pocket-sized amounts of volatiles in the mantle rock (about 100 ppm of water and 60 ppm of carbon dioxide) assistance in melting not more than about 0.one% of the stone. At a depth of well-nigh 70 kilometers (40 mi), dry melting conditions are reached and melting increases substantially. This dehydrates the remaining solid rock and is probable the origin of the chemically depleted lithosphere.[25] [26]

Come across likewise [edit]

  • Seismology § History

References [edit]

  1. ^ Barrel, J. (1914). "The force of the crust, Part VI. Relations of isostatic movements to a sphere of weakness – the asthenosphere". The Periodical of Geology. 22 (7): 655–83. Bibcode:1914JG.....22..655B. doi:x.1086/622181. JSTOR 30060774. S2CID 224832862.
  2. ^ Hirschmann, Marc One thousand. (March 2010). "Partial melt in the oceanic low velocity zone". Physics of the Globe and Planetary Interiors. 179 (1–2): 60–71. doi:x.1016/j.pepi.2009.12.003.
  3. ^ Self, Steve; Rampino, Mike (2012). "The Crust and Lithosphere". Geological Society of London. Retrieved 27 January 2013.
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  11. ^ a b Kearey, Klepeis & Vine 2009, p. 49.
  12. ^ Rychert, Catherine A.; Shearer, Peter G. (2011). "Imaging the lithosphere-asthenosphere boundary beneath the Pacific using SS waveform modeling". Journal of Geophysical Enquiry: Solid Earth. 116 (B7): B07307. Bibcode:2011JGRB..116.7307R. doi:10.1029/2010JB008070.
  13. ^ Kearey, Klepeis & Vine 2009, pp. 48–49.
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  20. ^ Rychert, Catherine A.; Shearer, Peter One thousand. (24 Apr 2009). "A Global View of the Lithosphere-Asthenosphere Purlieus". Science. 324 (5926): 495–498. Bibcode:2009Sci...324..495R. doi:10.1126/science.1169754. PMID 19390041. S2CID 329976.
  21. ^ Artemieva, Irina (2011). The Lithosphere. pp. half dozen, 12. doi:10.1017/CBO9780511975417. ISBN978-0-511-97541-7.
  22. ^ Anderson, Don Fifty. (1995). "Lithosphere, asthenosphere, and perisphere". Reviews of Geophysics. 33 (1): 125. doi:10.1029/94RG02785. ISSN 8755-1209.
  23. ^ Fowler, C. M. R.; Fowler, Connie May (2005). The Solid Earth: An Introduction to Global Geophysics. ISBN978-0521893077.
  24. ^ Ito, East; Takahashi, E (1989). "Postspinel transformations in the system Mg2SiO4-Fe2SiO4 and some geophysical implications". Journal of Geophysical Enquiry: Solid Earth. 94 (B8): 10637–10646. Bibcode:1989JGR....9410637I. doi:10.1029/jb094ib08p10637.
  25. ^ a b c Hirschmann 2010.
  26. ^ a b Karato 2012.
  27. ^ Mierdel, Katrin; Keppler, Hans; Smyth, Joseph R.; Langenhorst, Falko (19 January 2007). "Water Solubility in Aluminous Orthopyroxene and the Origin of Earth'due south Asthenosphere". Science. 315 (5810): 364–368. doi:10.1126/scientific discipline.1135422.
  28. ^ Becker, Thorsten West. (November 2006). "On the effect of temperature and strain-rate dependent viscosity on global mantle menstruation, net rotation, and plate-driving forces". Geophysical Journal International. 167 (2): 943–957. doi:x.1111/j.1365-246X.2006.03172.x.
  29. ^ Connolly, James A. D.; Schmidt, Max Westward.; Solferino, Giulio; Bagdassarov, Nikolai (November 2009). "Permeability of asthenospheric pall and melt extraction rates at mid-ocean ridges". Nature. 462 (7270): 209–212. doi:x.1038/nature08517.
  30. ^ Olive, Jean-Arthur; Dublanchet, Pierre (November 2020). "Controls on the magmatic fraction of extension at mid-ocean ridges". Earth and Planetary Scientific discipline Letters. 549: 116541. doi:10.1016/j.epsl.2020.116541.
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  32. ^ Conder, James A.; Wiens, Douglas A.; Morris, Julie (August 2002). "On the decompression melting structure at volcanic arcs and back-arc spreading centers: ARC AND Dorsum-ARC MELTING". Geophysical Inquiry Messages. 29 (15): 17–1–17-4. doi:ten.1029/2002GL015390.
  33. ^ Not bad, C.E.; Courtney, R.C.; Dehler, S.A.; Williamson, M.-C. (February 1994). "Decompression melting at rifted margins: comparison of model predictions with the distribution of igneous rocks on the eastern Canadian margin". World and Planetary Scientific discipline Letters. 121 (three–4): 403–416. doi:10.1016/0012-821X(94)90080-9.
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Bibliography [edit]

  • Turcotte, Donald L.; Schubert, Gerald (2002). Geodynamics (2nd ed.). Cambridge University Press. ISBN978-0-521-66624-iv . Retrieved 24 January 2016.
  • McBride, Neil; Gilmour, Iain (2004). An Introduction to the Solar Organisation. Cambridge University Printing. ISBN978-0-521-54620-1 . Retrieved 24 January 2016.

External links [edit]

  • San Diego Country University, "The World'due south internal heat energy and interior structure" Archived 2011-03-03 at the Wayback Motorcar

Where Is The Asthenosphere Located,

Source: https://en.wikipedia.org/wiki/Asthenosphere

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