Asthenosphere
The asthenosphere (from
The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock) contributes to its mechanical weakness. More extensive
Characteristics
The asthenosphere is a part of the upper mantle just below the
In the oceanic mantle, the transition from the lithosphere to the asthenosphere (the LAB) is shallower than for the continental mantle (about 60 km in some old oceanic regions) with a sharp and large velocity drop (5–10%).[11] 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
Boundaries
The asthenosphere extends from an upper boundary at approximately 80 to 200 km (50 to 120 miles) below the surface[15][7] to a lower boundary at a depth of approximately 700 kilometers (430 mi).[9]
Lithosphere-asthenosphere boundary
The lithosphere-asthenosphere boundary (LAB[15][7]) is relatively sharp and likely coincides with the onset of partial melting or a change in composition or anisotropy.[16] Various definitions of the boundary reflect various aspects of the boundary region. In addition to the mechanical boundary defined by seismic data, which reflects the transition from the rigid lithosphere to ductile asthenosphere, these include a thermal boundary layer, above which heat is transported by thermal conduction and below which heat transfer is mainly convective; a rheological boundary, where the viscosity drops below about 1021 Pa-s; and a chemical boundary layer, above which the mantle rock is depleted in volatiles and enriched in magnesium relative to the rock below.[17]
Lower boundary of asthenosphere
The lower boundary of the asthenosphere is less well-defined, but has been placed at the base of the upper mantle.
Origin
The mechanical properties of the asthenosphere are widely attributed to the partial melting of the rock.[4] It is likely that a small amount of melt is present through much of the asthenosphere, where it is stabilized by the traces of volatiles (water and carbon dioxide) present in the mantle rock.[2] However, the likely amount of melt, not more than about 0.1% of the rock, seems inadequate to fully explain the existence of the asthenosphere. This is not enough melt to fully wet grain boundaries in the rock, and the effects of melt on the mechanical properties of the rock are not expected to be significant if the grain boundaries are not fully wetted. The sharp lithosphere-asthenosphere boundary is also difficult to explain by partial melting alone.[10] It is possible that melt accumulates at the top of the asthenosphere, where it is trapped by the impermeable rock of the lithosphere.[2] Another possibility is that the asthenosphere is a zone of minimum water solubility in mantle minerals so that more water is available to form greater quantities of melt.[21] Another possible mechanism for producing mechanical weakness is grain boundary sliding, where grains slide slightly past each other under stress, lubricated by the traces of volatiles present.[10]
Numerical models of mantle 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 significant contributing mechanism.[22]
Magma generation
Decompression melting in upwelling asthenosphere likely begins at a depth as great as 100 to 150 kilometers (60 to 90 mi), where the small amounts of volatiles in the mantle rock (about 100
See also
References
- S2CID 224832862.
- ^ a b c d Hirschmann 2010.
- ^ Self, Steve; Rampino, Mike (2012). "The Crust and Lithosphere". Geological Society of London. Retrieved 27 January 2013.
- ^ a b c Kearey, Klepeis & Vine 2009, p. 49.
- .
- OCLC 655917296.
- ^ )
- ISBN 978-0-7506-3386-4. Retrieved 21 May 2010.
- ^ a b c Kearey, Klepeis & Vine 2009, p. 51.
- ^ a b c d Karato 2012.
- .
- ^ Kearey, Klepeis & Vine 2009, pp. 48–49.
- )
- OCLC 1100670264.)
{{cite book}}: CS1 maint: location missing publisher (link - ^ OCLC 745002805.
- S2CID 329976.
- ISBN 978-0-511-97541-7.
- ISSN 8755-1209.
- ISBN 978-0521893077.
- .
- S2CID 33006157.
- .
- S2CID 4352616.
- S2CID 224923541.
- S2CID 11405514.
- S2CID 29842432.
- .
- PMID 32382159.
Bibliography
- Hirschmann, Marc M. (March 2010). "Partial melt in the oceanic low velocity zone". Physics of the Earth and Planetary Interiors. 179 (1–2): 60–71. .
- Karato, Shun-ichiro (March 2012). "On the origin of the asthenosphere". Earth and Planetary Science Letters. 321–322: 95–103. .
- Kearey, P.; Klepeis, Keith A.; Vine, F. J. (2009). Global tectonics (3rd ed.). Oxford: Wiley-Blackwell. OCLC 132681514.
- McBride, Neil; Gilmour, Iain (2004). An Introduction to the Solar System. Cambridge University Press. ISBN 978-0-521-54620-1. Retrieved 24 January 2016.
- Turcotte, Donald L.; Schubert, Gerald (2002). Geodynamics (2nd ed.). ISBN 978-0-521-66624-4. Retrieved 24 January 2016.
