Large low-shear-velocity provinces

[3] The Pacific LLSVP is 3,000 kilometers (1,900 miles) across and underlies four hotspots on Earth's crust that suggest multiple mantle plumes underneath.

[1] Other names for LLSVPs and their superstructures include superswells, superplumes, thermo-chemical piles, or hidden reservoirs, mostly describing their proposed geodynamical or geochemical effects.

[6] LLSVPs were discovered in full mantle seismic tomographic models of shear velocity as slow features at the D″ layer beneath Africa and the Pacific.

However, tidal tomography cannot determine how the excess mass is distributed; the higher density may be caused by primordial material or subducted ocean slabs.

The rest of the material is then carried upwards via chemical-induced buoyancy and contributes to the high levels of basalt found at the mid-ocean ridge.

The Pacific and African LLSVP, in this scenario, are originally created by a discharge of heat from the core (4000 K) to the much colder mantle (2000 K); the recycled lithosphere is fuel that helps drive the superplume convection.

[4] Another proposed origin for the LLSVPs is that their formation is related to the giant-impact hypothesis, which states that the Moon formed after the Earth collided with a planet-sized body called Theia.

This higher iron(II) oxide composition would also be consistent with the isotope geochemistry of lunar samples, as well as that of the ocean island basalts overlying the LLSVPs.

This geometrical relationship is consistent with the position of Pangaea and the formation of the current geoid pattern due to continental break-up from the superswell below.

So it is hypothesized that the material from the slab graveyard can become extremely dense and form large pools of melt concentrate enriched in uranium, thorium, and potassium.

[19] Seismic waves passing through LLSVPs decelerate but lose less energy than expected, indicating compositional differences and shedding light on their complex structure.