Superplume

Large low-shear-velocity provinces, LLSVPs, also called superplumes, are characteristic structures of parts of the lowermost mantle (the region surrounding the outer core) of the Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of the deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP. Both extend laterally for thousands of kilometers and possibly up to 1000 km vertically from the core-mantle boundary. The Pacific LLSVP has specific dimensions of 3000 km across and 300 m higher than the surrounding ocean-floor, and is situated over four hotspots that suggest multiple mantle plumes underneath. These zones represent around 3% of the volume of the Earth. Other names for LLSVPs include superwells, thermo-chemical piles, or hidden reservoirs. Some of these names, however, are more interpretive of their geodynamical or geochemical effects, while many questions remain about their nature.

LLSVPs were discovered in full mantle seismic tomographic models of shear velocity as slow features in the lowermost mantle beneath Africa and the Pacific. The boundaries of these features appear fairly consistent across models when applying objective k-means clustering. The global spherical harmonic degree two structure is strong and aligns with its smallest moments of inertia along with the two LLSVPs. This means, by using shear wave velocities, the established locations of the LLSVPs are not only verified, a stable pattern for mantle convection emerges. This stable configuration is responsible for the geometry of plate motions at the surface due as well as mantle convection. Another name for the degree two structure, a roughly 200 km thick layer of the lower mantle directly above the core–mantle boundary (CMB), is the D″ ("D double-prime" or "D prime prime"). The LLSVPs lie around the equator, but mostly on the southern hemisphere. Global tomography models inherently result in smooth features; local waveform modeling of body waves, however, has shown that the LLSVPs have sharp boundaries. The sharpness of the boundaries makes it difficult to explain the features by temperature alone; the LLSVPs need to be compositionally distinct to explain the velocity jump. Ultra low velocity zones (ULVZ) at smaller scales have been discovered mainly at the edges of these LLSVPs.

...
Wikipedia