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'{{short description|A level representing a mechanical difference between layers in Earth’s inner structure}}[[File:Earth's_Inner_Layers_denoting_the_LAB.png|thumb|alt=A color diagram of the internal structure of Earth|A diagram of the internal structure of Earth|upright=1.35]]The '''Lithosphere–asthenosphere boundary''' ('''LAB''') represents a [[Mechanics|mechanical]] difference between layers in [[Structure of the Earth|Earth's inner structure]]. Earth's inner structure can be described both [[Chemical substance|chemically]] ([[Crust (geology)|crust]], [[Mantle (geology)|mantle]], [[Core (geology)|core]]) and mechanically. The Lithosphere–asthenosphere boundary (referred to as the LAB by geophysicists) lies between Earth's cooler, rigid [[lithosphere]] and the warmer, ductile [[asthenosphere]]. The actual depth of the boundary is still a topic of debate and study, although it is known to vary according to the environment.<ref name="globalview2">{{cite journal|last1=Rychert|first1=Catherine A.|last2=Shearer|first2=Peter M.|date=24 April 2009|title=A Global View of the Lithosphere-Asthenosphere Boundary|journal=Science|volume=324|issue=5926|pages=495–498|bibcode=2009Sci...324..495R|doi=10.1126/science.1169754|pmid=19390041}}</ref> ==Defining the LAB== 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 melting|partial melt]]; these are factors that affect the [[Rheology|rheological]]<nowiki/>differences in the lithosphere and asthenosphere. [[File:1.4-color.jpg|link=https://en.wikipedia.org/wiki/File:1.4-color.jpg|thumb|Schematic diagram showing the relative depths of the lithosphere-asthenosphere boundary (LAB) depending on the definition used: as a mechanical boundary layer (MBL), a thermal boundary layer (TBL), a rheological boundary layer (RBL), or as a chemical boundary layer (CBL).]] === Mechanical Boundary Layer (MBL) === 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).<ref name=":02">{{Cite journal|last=Anderson|first=Don L.|date=1995|title=Lithosphere, asthenosphere, and perisphere|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/94RG02785|journal=Reviews of Geophysics|language=en|volume=33|issue=1|pages=125–149|doi=10.1029/94RG02785|issn=1944-9208}}</ref> Flexure is one observation of strength, but [[Earthquake|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.<ref name=":02" /> This criterion works particularly well in [[Lithosphere#Oceanic lithosphere|oceanic lithosphere]], where it is reasonably simple to estimated the temperature at depth based upon the age of the rocks.<ref>{{Cite web|url=https://www.cambridge.org/core/books/geodynamics/D20BD7359E157591F75CF3BCFEDF88A8|title=Geodynamics by Donald L. Turcotte|last=Schubert|first=Gerald|last2=Turcotte|first2=Donald L.|date=2002|website=Cambridge Core|language=en|doi=10.1017/cbo9780511807442|access-date=2019-03-18}}</ref> The LAB is most shallow when using this definition. The MBL is rarely equated to the lithosphere, as in some tectonically active regions (e.g. the [[Basin and Range Province]]) the MBL is thinner than the crust and the LAB would be above the [[Mohorovičić discontinuity]]. === Thermal Boundary Layer (TBL) === The definition of the LAB as a thermal boundary layer (TBL) comes not from temperature, but instead from the dominant mechanism of [[Heat transfer#Mechanisms|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 [<nowiki/>[[Heat Conduction|conduction]] vs. convection].<ref name=":12">{{Cite web|url=https://doi.org/10.1017/CBO9780511975417|title=The Lithosphere|last=Artemieva|first=Irina|date=July 28, 2011|website=Cambridge Core|language=en|orig-year=2011|archive-url=|archive-date=|dead-url=|access-date=2019-02-11}}</ref> 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. The top of the thermal boundary layer is the maximum depth at which heat is transported only by conduction. The bottom of the TBL is the shallowest depth at which heat is transported only by convection. At depths internal to the TBL, heat is transported by a combination of both conduction and convection. === Rheological Boundary Layer (RBL) === The LAB is a [[Rheology|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. In practice, the RBL is defined by the depth at which the viscosity of the mantle rocks drops below ~<math>10^{21} Pa\cdot s.</math><ref name=":12" /> === Compositional Boundary Layer (CBL) === Another definition of the LAB involves differences in composition of the mantle at depth. Lithospheric mantle is [[Ultramafic rock|ultramafic]] and has lost most of its volatile constituents, such as [[water]], [[calcium]], and [[Aluminium|aluminum]].<ref name=":12" /> Knowledge of this depletion is based upon the composition of mantle [[Xenolith|xenoliths]]. The depth to the base of the CBL can be determined from the amount of [[forsterite]] within samples of [[olivine]] extracted from the mantle. This is because partial melting of [[Primitive mantle|primitive or asthenospheric mantle]] leaves behind a composition that is enriched in [[magnesium]], with the depth at which the concentration of magnesium matches that of the primitive mantle being the base of the CBL.<ref name=":12" /> == Measuring the LAB Depth == ===Gutenberg Discontinuity=== The LAB is often observed and imaged via signal processing techniques and seismic waves. [[Seismic tomography|Seismic tomographic]] studies suggests that the LAB is not determined by a purely thermal model, but rather it is affected by the presence of partial melt material in the asthenosphere. Evidence from converted seismic phases indicates a sharp decrease in [[S-wave|shear-wave]] velocity 90–110&nbsp;km below continental crust.<ref>{{cite journal|last1=Rychert|first1=Catherine|last2=Fischer|first2=Karen|last3=Rondenay|first3=Ste´phane|title=A sharp lithosphere–asthenosphere boundary imaged beneath eastern North America|journal=Nature|date=July 2005|volume=436|issue=28|pages=542–545|doi=10.1038/nature03904|pmid=16049485|bibcode=2005Natur.436..542R}}</ref> Recent seismological studies indicate a 5 to 10 percent reduction in shear-wave velocity in the depth range of 35 to 120&nbsp;km beneath ocean basins. The seismic discontinuity often associated with this sharp contrast in wave velocity and presence of partial melt is known as the Gutenberg discontinuity or "G" to many geophysicists. The Gutenberg discontinuity coincides with the expected LAB depth in many studies and has also been found to become deeper under older crust, thus supporting the suggestion that the discontinuity is closely interrelated to the LAB.<ref>{{cite journal|last1=Schmerr|first1=Nicholas|title=The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary|journal=Science|date=2012|volume=335|issue=6075|pages=1480–1483|doi=10.1126/science.1215433|pmid=22442480|bibcode=2012Sci...335.1480S}}</ref> ===Beneath oceanic lithosphere=== Beneath oceanic crust, the LAB ranges anywhere from 50 to 140&nbsp;km in depth except at [[mid-ocean ridge]]s where the LAB is no deeper than the depth of the new crust being created.<ref name= "lithodispersion">{{cite journal|last1=Pasyanos|first1=Michael|title=Lithospheric thickness modeled from long period surface wave dispersion|date=2008|url=https://e-reports-ext.llnl.gov/pdf/361453.pdf|accessdate=26 March 2015}}</ref> Seismic evidence shows that oceanic plates do thicken with age. This would suggest that the Lithosphere-Asthenosphere boundary underneath oceanic lithosphere also deepens with plate age. Data from ocean seismometers indicate a sharp age-dependent LAB beneath the [[Pacific Plate|Pacific]] and [[Philippine Plate|Philippine]] plates and has been interpreted as evidence for a thermal control of oceanic-lithosphere thickness.<ref name="sharplab">{{cite journal|last1=Kawakatsu|first1=Hitoshi|last2=Kumar|first2=Prakash|last3=Takei|first3=Yasuko|last4=Shinohara|first4=Masanao|last5=Kanazawa|first5=Toshihiko|last6=Araki|first6=Eiichiro|last7=Suyehiro|first7=Kiyoshi|title=Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates|journal=Science|date=2009|volume=324|issue=499|doi=10.1126/science.1169499|pages=499–502|pmid=19390042|bibcode=2009Sci...324..499K}}</ref><ref>{{cite journal|last1=Fischer|first1=Karen M.|last2=Ford|first2=Heather|last3=Abt|first3=David|last4=Rychert|first4=Catherine|title=The LithosphereAsthenosphere Boundary|journal=Annual Review of Earth and Planetary Sciences|date=2010|doi=10.1146/annurev-earth-040809-152438|volume=38|issue=1|pages=551–575|bibcode=2010AREPS..38..551F}}</ref> ===Beneath continental lithosphere=== The continental lithosphere contains an ancient, stable part known as the [[craton]]. The LAB is particularly difficult to study in these regions and evidence suggests that the lithosphere within this old part of the continent is at it thickest and even appears to exhibit large variations in thickness beneath the cratons,<ref>{{cite journal|last1=Eaton|first1=David|last2=Darbyshire|first2=Fiona|last3=Evans|first3=Rob|last4=Grutter|first4=Herman|last5=Jones|first5=Alan|last6=Yuan|first6=Xiaohui|title=The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons|journal=[[Lithos (journal)|Lithos]]|date=2009|volume=109|issue=1–2|pages=1–22|doi=10.1016/j.lithos.2008.05.009|bibcode=2009Litho.109....1E}}</ref> thus supporting the theory that lithosphere thickness and LAB depth are age-dependent. Depths of the LAB beneath these regions (also known as shields and platforms) are estimated to be between 200 and 250&nbsp;km deep.<ref name="maplab">{{cite journal|last1=Plomerova|first1=Jaroslava|last2=Kouba|first2=Daniel|last3=Babusˇka|first3=Vladislav|title=Mapping the lithosphere–asthenosphere boundary through changes in surface-wave anisotropy|journal=Tectonophysics|date=2002|volume=358|issue=1–4|pages=175–185|doi=10.1016/s0040-1951(02)00423-7|bibcode=2002Tectp.358..175P}}</ref> Beneath Phanerozoic continental crust, the LAB is roughly 100&nbsp;km deep.<ref name= "maplab"/> ==References== {{Reflist}}{{Earthsinterior}} {{DEFAULTSORT:Lithosphere asthenosphere boundary}} [[Category:Structure of the Earth]] [[Category:Planetary geology]] [[Category:Plate tectonics]]'