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Concept
Asthenosphere
Summary
The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not well defined.
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 decompression melting of the asthenosphere takes place where it wells upwards, and this is the most important source of magma on Earth. It is the source of mid-ocean ridge basalt (MORB) and of some magmas that erupted above subduction zones or in regions of continental rifting.
The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. It is composed of peridotite, a rock containing mostly the minerals olivine and pyroxene. The lithosphere-asthenosphere boundary is conventionally taken at the isotherm. Below this temperature (closer to the surface) the mantle behaves rigidly; above this temperature (deeper below the surface) it acts in a ductile fashion. 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.
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 strictly the same; the lower boundary of the LVZ lies at a depth of , whereas the base of the asthenosphere lies at a depth of about . 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). The discovery of the LVZ alerted seismologists to the existence of the asthenosphere and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity.
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