Lithosphere–asthenosphere boundary
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Earth’s inner structure can be described both chemically (crust, mantle, core) and mechanically. The Lithosphere- Asthenosphere boundary represents a mechanical difference between layers in Earth’s inner structure. The Lithosphere-Asthenosphere boundary (referred to as the LAB by geophysicists) lies between Earth's cold, rigid Lithosphere and the hot, 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. [1]
Defining the LAB
The boundary can be studied by using seismic waves. Wave speeds differ between the rigid lithosphere and ductile asthenosphere due to the differing physical properties of each layer. The Lithosphere-Asthenosphere boundary is a mechanical boundary and because of this, seismic methods are not generally the only methods used to study the LAB. Often, thermal and rheological information are used as well.
Beneath Oceanic lithosphere
Beneath oceanic crust, the LAB ranges anywhere from 50 to 140 km in depth except at mid ocean ridges where the LAB is no deeper than the depth of the new crust being created. [2] 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 and Philippine plates and has been interpreted as evidence for a thermal control of oceanic-lithosphere thickness. [3][4]
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, [5] 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 km deep. [6] Beneath Phanerozoic continental crust, the LAB is roughly 100 km deep. [6]
References
- ^ Rychert, Catherine A.; Shearer, Peter M. (24 April 2009). "A Global View of the Lithosphere-Asthenosphere Boundary". Science. 324 (5926): 495–498. doi:10.1126/science.1169754.
{{cite journal}}:|access-date=requires|url=(help) - ^ Pasyanos, Michael (2008). "Lithospheric thickness modeled from long period surface wave dispersion" (PDF). Retrieved 26 March 2015.
{{cite journal}}: Cite journal requires|journal=(help) - ^ Kawakatsu, Hitoshi; Kumar, Prakash; Takei, Yasuko; Shinohara, Masanao; Kanazawa, Toshihiko; Araki, Eiichiro; Suyehiro, Kiyoshi (2009). "Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates". Science. 324 (499). doi:10.1126/science.1169499.
- ^ Fischer, Karen M.; Ford, Heather; Abt, David; Rychert, Catherine (2010). "The LithosphereAsthenosphere Boundary". Annual Review of Earth and Planetary Sciences. doi:10.1146/annurev-earth-040809-152438.
- ^ Eaton, David; Darbyshire, Fiona; Evans, Rob; Grutter, Herman; Jones, Alan; Yuan, Xiaohui (2009). "The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons". Lithos (109): 1-22.
- ^ a b Plomerova, Jaroslava; Kouba, Daniel; Babusˇka, Vladislav (2002). "Mapping the lithosphere–asthenosphere boundary through changes in surface-wave anisotropy". Techtonophysics. 358: 175-185.