Mesosphere (mantle)

The mesosphere refers to the mantle in the region between the asthenosphere and the outer core. The upper boundary is defined as the sharp increase in seismic wave velocities and density at a depth of 660 km.^[1] As depth increases, pressure builds and forces atoms into a denser, more rigid structure; thus the difference between mesosphere and asthenosphere is likely due to density and rigidity differences, that is, physical factors, and not to any difference in chemical composition. At a depth of 660 km, ringwoodite (gamma-(Mg,Fe)₂SiO₄) decomposes into Mg-Si perovskite and magnesiowustite.^[1] 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 2700 to 2890 km. The base of the lower mantle is at about 2700 km.^[1]

“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)² inferred three spherical layers comprise the outer earth: lithosphere (including the crust), asthenosphere, and mesospheric shell. Daly’s hypothetical depths to the lithosphere–asthenosphere boundary ranged from 80 to 100 km and the top of the mesospheric shell (base of the asthenosphere) from 200 to 480 km. Thus, Daly’s asthenosphere was inferred to be 120 to 400 km 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

1. ^ ^{a b c} Condie, Kent C., 2001, Mantle Plumes and Their Record in Earth History, Cambridge University Press, pp. 3 - 10. ISBN 0-521-01472-7

• Daly, 1940, Strength and Structure of the Earth, Prentice Hall, New York.