Definition and introduction
Earth’s lower mantle extends from the base of the transition zone (commonly assigned a depth of 660 km, corresponding to the average depth of a global seismic velocity discontinuity) to the core-mantle boundary (CMB) at a depth of ∼2,900 km below the surface. This thick layer of rock comprised of silicate and oxide minerals has gradual increases with depth of P- and S-wave seismic velocities and density that are generally consistent with adiabatic self-compression of a uniform composition material over most of the depth range (see Earth’s Structure, Lower Mantle ). Large-scale lateral heterogeneities in the seismic velocities of about ±1% are apparent in the central lower mantle, and these likely involve thermal and chemical signatures of upwellings and downwellings in the slowly convecting deep mantle system (see Mantle Convection ). The deepest few hundred kilometers of the lower mantle have generally reduced seismic velocity gradients, localized seismic...
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Lay, T. (2011). Mantle D′′ Layer. In: Gupta, H.K. (eds) Encyclopedia of Solid Earth Geophysics. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-90-481-8702-7_42
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