Mantle Plume - The Lower Mantle and The Core

The Lower Mantle and The Core

The most prominent thermal contrast known to exist in the deep (≳ 1000 km) mantle is at the core-mantle boundary. Mantle plumes were originally postulated to rise from this layer because the "hot spots" that are assumed to be their surface expression were thought to be fixed relative to one another. This required that plumes were sourced from beneath the shallow asthenosphere that is thought to be flowing rapidly in response to motion of the overlying tectonic plates. There is no other known major thermal boundary layer in the deep Earth, and so the core-mantle boundary was the only candidate.

The base of the mantle is known as the D″ layer, a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle, and may contain partial melt.

Two very large, broad, low-seismic-velocity bodies exist in the lower mantle, nicknamed the "superplumes". They are generally assumed to be hot because of their low seismic velocities, and scientists have postulated that small plumes rise from their surface or their edges. However, it has recently been shown that they are not hot and that they owe their low seismic velocities to their distinct composition.