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Mesosphere (mantle)
The mesosphere refers to the mantle in the region under the lithosphere and the asthenosphere, but above the outer core. The upper boundary is defined as the sharp increase in seismic wave velocities and density at a depth of . At a depth of 660 km, ringwoodite (gamma-(Mg,Fe)2SiO4) decomposes into Mg-Si perovskite and magnesiowustite.〔 This reaction marks the boundary between upper mantle and lower mantle. This measurement is estimated from seismic data and high-pressure laboratory experiments.
The base of the mesosphere includes the D' zone which lies just above the mantle-core boundary at approximately . The base of the lower mantle is at about 2700 km.〔
"Mesosphere" (not to be confused with mesosphere, a layer of the atmosphere) is derived from “mesospheric shell”, coined by Reginald Aldworth Daly, a Harvard University geology professor. In the pre-plate tectonics era, Daly (1940) inferred that the outer earth consisted of three spherical layers: lithosphere (including the crust), asthenosphere, and mesospheric shell. Daly’s hypothetical depths to the lithosphere–asthenosphere boundary ranged from , and the top of the mesospheric shell (base of the asthenosphere) were from . Thus, Daly’s asthenosphere was inferred to be thick. According to Daly, the base of the solid Earth mesosphere could extend to the base of the mantle (and, thus, to the top of the core).
A derivative term, mesoplates, was introduced as a heuristic, based on a combination of "mesosphere" and "plate", for postulated reference frames in which mantle hotspots apparently exist.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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