asthenosphere

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A layer in the Earth's interior occurring approximately 50 mi (80 km) below the surface and extending to a depth of about 180 mi (300 km); it consists of rocks possessing less mechanical strength than the rocks above or below it. The asthenosphere is a relatively thin layer contained in a much larger region known as the mantle. The mantle is the solid portion of the Earth's interior that is located between the bottom of the Earth's crust (at about 15 mi or 25 km depth) and the top of the liquid outer core (at 1800 mi or 2900 km depth). The layers in the mantle that are above and below the asthenosphere are known as the lithosphere and the mesosphere respectively. The lithosphere is broken into 12 major tectonic plates that possess much greater mechanical strength than the underlying asthenosphere. See also Lithosphere.

The thermal structure of the asthenosphere, and indeed the very existence of this layer, is determined by the thermal convection process in the mantle. The convective flow in the mantle transports heat vertically upward from the deep interior, and it drives the observed horizontal motions of the tectonic plates. In the deep mantle, below the lithosphere, the vertical advection of heat by the convective flow is sufficiently rapid to create an adiabatic depth variation of mantle temperature. In the lithosphere the velocities of the vertical flows are much smaller than in the deep mantle; therefore the depth variation of temperature in this region is determined by a balance between the horizontal advection of heat (due to the horizontal flow associated with the tectonic plate motions) and the vertical conduction of heat to the surface. The asthenosphere is, in effect, a layer in which the depth variation of temperature changes from a steep gradient in the lithosphere to a relatively flat gradient in the deep mantle.

Since the mantle flow occurs over geological time scales, the long-term mechanical strength of the mantle rocks may be defined as the amount of stress that must be applied to produce some specified flow velocity. The flow of the solid mantle is made possible by the presence of naturally occurring microscopic defects in the crystal grains that constitute mantle rocks. The movement of these defects, due to thermally generated internal stresses, allows the mantle to creep as though it were a fluid with an extremely high viscosity. The effective viscosity of mantle rocks is a direct measure of their long-term mechanical strength, and it is strongly dependent on the ratio between the temperature (T) and the melting temperature (T_m) of the rocks. An increase of the scaled temperature T/T_m (also called the homologous temperature) produces exponentially large decreases in the effective viscosity of rocks. In the asthenosphere the average mantle temperature is closest to the melting temperature; thus the effective viscosity (that is, mechanical strength) is lower there than above or below the asthenosphere. There is a smooth transition between the zone of reduced mechanical strength in the asthenosphere and the zone of greater strength in the adjoining portions of the mantle. Therefore it is not possible, or meaningful, to specify precise locations for the upper and lower boundaries of the asthenosphere. See also Rheology.

The analysis of seismic data (for example, the travel times of seismic waves) has provided the only direct indication of the presence of the asthenosphere. Seismologists usually refer to the asthenosphere as a low-velocity zone on account of the reduction of seismic wave speeds in this layer. See also Seismology.

Seismologists have made considerable progress in the application of tomographic imaging techniques to map the three-dimensional variation of seismic wave speed in the mantle. A tomographic model of the relative perturbations of seismic shear velocity has been constructed; at a depth of 120 mi (200 km), this model indicates that the shear-velocity perturbations range from −2.5 to +4.5%. The coldest (that is, largest negative perturbation of) temperature is found below the continents. This local reduction of mantle temperature, and the corresponding increase of mechanical strength, may be sufficiently great that the concept of the asthenosphere (as a hotter and mechanically weak region) ceases to be valid below the continents. The concept of the asthenosphere is valid below the oceans, and there is an obvious concentration of hotter material below the plate boundaries, which are zones of active spreading (the so-called mid-oceanic ridges). This pattern suggests that the observed spreading at the mid-oceanic ridges is fed, and perhaps partially driven, by the upward ascent of hotter mantle material across the asthenosphere. When the ascent of this hotter material is sufficiently rapid (that is, adiabatic), the material begins to melt (and may thus produce surface eruptions of lava), because the temperature of this ascending material exceeds the local melting temperature. This partial melting can occur in the asthenosphere. See also Isostasy; Plate tectonics.

That zone of the earth's mantle which lies beneath the relatively rigid lithosphere, between 50 and 300 km below the surface.

The asthenosphere is composed of hot, semi-molten, and therefore deformable, rock, within which convection currents occur. Descending convective limbs can penetrate to depths of 700 km, and rising limbs are located under spreading centres (mid-oceanic ridges).

The asthenosphere is approximately commensurate with that zone of the mantle which transmits seismic waves at low velocity.

• Materials, Formations, and Substances - asthenosphere: layer of Earth’s crust near melting point, almost fluid

This section includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (March 2009)

Earth cutaway from core to crust, the asthenosphere lying between the upper mantle and the lithospheric mantle (detail not to scale)

The asthenosphere (from Greek asthenēs 'weak' + sphere) is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km (~ 62 and 124 miles) below the surface, but perhaps extending as deep as 700 km (430 mi).

Contents 1 Characteristics 2 Historical 3 References 4 External links

Characteristics

The asthenosphere is a portion of the upper layer just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. In spite of its high temperature, pressures keep it plastic, and it has a relatively low density. Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly the same. The lower boundary of the LVZ lies at a depth of 180–220 km,^[1] whereas the base of the asthenosphere lies at a depth of about 700 km.^[2] This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity.

Under the thin oceanic plates the asthenosphere is usually much closer to the seafloor surface, and at mid-ocean ridges it 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 plates of the Earth's crust move 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, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, creating the movement of crustal plates.

Historical

Although its presence was suspected as early as 1926, the worldwide occurrence of the asthenosphere was confirmed by analyses of earthquake waves from the Great Chilean Earthquake of May 22, 1960.

References

1. ^ Condie, K.C. (1997). Plate tectonics and crustal evolution. Butterworth-Heinemann. p. 282. ISBN 978-0-7506-3386-4. http://books.google.co.uk/books?id=HZrA6OQzsvgC&pg=PA123&dq=mantle+%22low-velocity+zone%22++anisotropy&cd=8#v=onepage&q=mantle%20%22low-velocity%20zone%22%20%20anisotropy&f=false. Retrieved 21 May 2010. 
2. ^ Kearey, P.; Vine, F.J. (1996). Global Tectonics (2 ed.). Wiley-Blackwell. pp. 41–42. ISBN 978-0-86542-924-6. http://books.google.co.uk/books?id=usiqam9p7GAC&pg=RA1-PA41&lpg=RA1-PA41&dq=asthenosphere+lvz#v=onepage&q=asthenosphere%20lvz&f=false. Retrieved 21 May 2010. 

• Donald L. Turcotte and Gerald Schubert. Geodynamics, 2nd ed., Cambridge University Press 2001
• GEMS Institute of higher education,Nepal.Environment Management Club, Kushal
• An Introduction to the Solar System; McBride and Gilmour; Cambridge University Press 2004

External links

• San Diego State University, "The Earth's internal heat energy and interior structure"

v t e Structure of the Earth Shells Crust · Lithosphere · Asthenosphere · Mesosphere · Mantle · Outer core · Inner core Discontinuities Conrad (continental crust)  · Mohorovičić (crust–mantle) · Lehmann (upper mantle) · Gutenberg (mantle–core)  · Lehmann (core) · Arguments Plate tectonics

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