Lower mantle

[5] The upper boundary is defined by the sharp increase in seismic wave velocities and density at a depth of 660 kilometers (410 mi).

The proportion of each component has been a subject of discussion historically where the bulk composition is suggested to be, Laboratory multi-anvil compression experiments of pyrolite simulated conditions of the adiabatic geotherm and measured the density using in situ X-ray diffraction.

The electronic environment of two iron-bearing minerals in the lower mantle (bridgmanite, ferropericlase) transitions from a high-spin (HS) to a low-spin (LS) state.

[5] The HS to LS transition are reported to affect the physical properties of the iron bearing minerals.

[15] The effects of the spin transition on the transport properties and rheology of the lower mantle is currently being investigated and discussed using numerical simulations.

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.

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 exist.