Editing Asthenosphere (section)

== Characteristics ==
[[File:Earth cutaway schematic-en.svg|thumb|upright=1.8|The asthenosphere in relation to the other layers of Earth's structure]]  
The asthenosphere is a part of the upper mantle just below the [[lithosphere]] that is involved in [[Plate tectonics|plate tectonic movement]] and [[Isostasy|isostatic]] adjustments. It is composed of [[peridotite]], a rock containing mostly the minerals [[olivine]] and [[pyroxene]].{{sfn|Hirschmann|2010}} The lithosphere-asthenosphere boundary is conventionally taken at the {{convert|1300|C||sp=us}} [[Isotherm (contour line)|isotherm]]. Closer to the surface at lower temperatures, the mantle behaves rigidly; deeper below the surface at higher temperatures, the mantle moves in a [[ductility (Earth science)|ductile]] fashion.<ref>{{cite web |last1=Self |first1=Steve |last2=Rampino |first2=Mike |year=2012 |title=The crust and lithosphere |url=http://www.geolsoc.org.uk/Education-and-Careers/Resources/Papers-and-Reports/Flood-basalts-mantle-plumes-and-mass-extinctions/The-Crust-and-Lithosphere |access-date=27 January 2013 |publisher=[[Geological Society of London]]}}</ref> The asthenosphere is where the mantle rock most closely approaches its melting point, and a small amount of melt is likely to present in this layer.{{sfn|Kearey|Klepeis|Vine|2009|p=49}}

[[Seismic wave]]s pass relatively slowly through the asthenosphere<ref>{{cite journal |last=Forsyth |first=Donald W. |year=1975 |title=The early structural evolution and anisotropy of the oceanic upper mantle |journal=Geophysical Journal International |volume=43 |issue=1 |pages=103–162 |bibcode=1975GeoJ...43..103F |doi=10.1111/j.1365-246X.1975.tb00630.x |doi-access=free}}</ref> compared to the overlying lithospheric mantle. Thus, it has been called the ''[[low-velocity zone]]'' (LVZ), although the two are not strictly the same;<ref>{{Cite book |url=https://www.worldcat.org/oclc/655917296 |title=The Encyclopedia of the Solid Earth Sciences |year=1993 |publisher=Blackwell Science |isbn=978-1-4443-1388-8 |editor-last=Kearey |editor-first=P. |location=Oxford |oclc=655917296}}</ref><ref name=Epplbm-Kutsv-Pilchn-2013>{{cite book |last1=Eppelbaum |first1=Lev V. |last2=Kutasov |first2=I.M. |last3=Pilchin |first3=Arkady |year=2013 |title=Applied Geothermics |location=Berlin, Germany |isbn=978-3-642-34023-9 |oclc=879327163 |url=https://www.worldcat.org/oclc/879327163 }}</ref> the lower boundary of the LVZ lies at a depth of {{convert|180 to 220|km||sp=us}},<ref>{{cite book |last=Condie |first=Kent C. |year=1997 |title=Plate Tectonics and Crustal Evolution |publisher=[[Butterworth-Heinemann]] |isbn=978-0-7506-3386-4 |page=123 |url=https://books.google.com/books?id=HZrA6OQzsvgC&pg=PA123 |access-date=21 May 2010}}</ref> whereas the base of the asthenosphere lies at a depth of about {{convert|700|km||sp=us}}.{{sfn|Kearey|Klepeis|Vine|2009|p=51}} The LVZ also 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).{{sfn|Karato|2012}} The discovery of the LVZ alerted [[seismology|seismologists]] to the existence of the asthenosphere and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing [[Stiffness|rigidity]].
This decrease in seismic wave velocity from the lithosphere to the asthenosphere could be caused by the presence of a very small percentage of melt in the asthenosphere, though since the asthenosphere transmits [[S waves]], it cannot be fully melted.{{sfn|Kearey|Klepeis|Vine|2009|p=49}}

In the oceanic [[mantle (geology)|mantle]], the transition from the lithosphere to the asthenosphere (the LAB) is shallower than for the continental mantle (about 60&nbsp;km in some old oceanic regions) with a sharp and large velocity drop (5–10%).<ref>{{cite journal |last1=Rychert |first1=Catherine A. |last2=Shearer |first2=Peter M. |year=2011 |title=Imaging the lithosphere-asthenosphere boundary beneath the Pacific using SS waveform modeling |journal=Journal of Geophysical Research: Solid Earth |volume=116 |issue=B7 |pages=B07307 |bibcode=2011JGRB..116.7307R |doi=10.1029/2010JB008070 |doi-access=}}</ref> At the [[mid-ocean ridge]]s, 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 [[tectonic plate|plates of the Earth's crust]] move about. Due to the temperature and [[pressure]] conditions in the asthenosphere, [[Rock (geology)|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 [[Fault (geology)|faults]]. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, enabling [[isostatic equilibrium]]{{sfn|Kearey|Klepeis|Vine|2009|pp=48–49}} and allowing the movement of [[tectonic plates]].<ref>{{cite book |last1=Hendrix |first1=Mark |last2=Thompson |first2=Graham R. |last3=Turk |first3=Jonathan |year=2015 |title=Earth |edition=2nd |isbn=978-1-285-44226-6 |location=Stamford, CT |oclc=864788835 |url=https://www.worldcat.org/oclc/864788835}}</ref><ref>{{cite book |last1=Garrison |first1=Tom |last2=Ellis |first2=Robert |year=2017 |title=Essentials of Oceanography |edition=8th |isbn=978-1-337-51538-2 |location=Pacific Grove, CA |oclc=1100670264 |url=https://www.worldcat.org/oclc/1100670264 }}</ref>