Mesosphere (mantle)
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The mesosphere refers to the lower mantle in the region between the asthenosphere and the outer core. It is the the largest layer of the earth. The upper boundary is poorly constrained in a depth of 300 km, but occurs where the more ductile asthenosphere transitions to more rigid material. 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. The expressive mineral phase transition take place from a depth of 400 km, from alpha olivine to betha olivine, and 450 km, from betha olivine to gamma olivine. At a depth of 670 km, the gamma olivine decomposes into magnesio-perovskite and manesio-ustite. This reaction remarks the boundary between upper mantle and lower mantle. This measurement is estimated from seismic data and high pressure laboratory expreiments.
The term "mesosphere" has lost favor in geologic circles. The word "mesosphere" is most commonly known as a layer in the upper atmosphere.
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