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The asthenosphere (from Ancient Greek ἀσθενός (asthenós) 'without strength') is the mechanically weak [1] and ductile region of the upper mantle of Earth. It lies below the lithosphere , at a depth between c. 80 and 200 km (50 and 120 mi) below the surface, and extends as deep as 700 km (430 mi).
A diagram of the internal structure of Earth. The lithosphere consists of the crust and upper solid mantle (lithospheric mantle). The green dashed line marks the LAB. The lithosphere–asthenosphere boundary (referred to as the LAB by geophysicists) represents a mechanical difference between layers in Earth's inner structure.
The modern understanding of the Earth's upper mantle is that there are two distinct components - the lithospheric part and the asthenosphere. The lithosphere, which includes the continental plates , acts as a brittle solid whereas the asthenosphere is hotter and weaker due to mantle convection.
The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field in Earth's outer core is estimated to measure 2.5 milliteslas (25 gauss), 50 times stronger than the magnetic field at the surface.
If the viscosity of the upwelling asthenosphere is greater than that of the mantle lithosphere, delamination will stop. The upwelling asthenosphere forms two chilled, solid boundary layers on the top and bottom of the sill layer. This reduces the thickness of the portion of the lowermost crust which behaves viscously.
Below the asthenosphere, the mantle is again relatively rigid. The Earth's mantle is divided into three major layers defined by sudden changes in seismic velocity: [ 6 ] the upper mantle (starting at the Moho, or base of the crust around 7 to 35 km [4.3 to 21.7 mi] downward to 410 km [250 mi]) [ 7 ]
Based on calculations of Earth's cooling rate, which assumed constant conductivity in the Earth's interior, in 1862 William Thomson, later Lord Kelvin, estimated the age of the Earth at 98 million years, [10] which contrasts with the age of 4.5 billion years obtained in the 20th century by radiometric dating. [11]
In geology, the low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity.