Large low-shear-velocity provinces

Directly above the core–mantle boundary is a thick layer of the lower mantle. This layer is known as the D″ ("D double-prime" or "D prime prime"). LLSVPs were discovered in full mantle seismic tomographic models of shear velocity as slow features at the D″ layer beneath Africa and the Pacific. The global spherical harmonics of the D″ layer are stable throughout most of the mantle but anomalies appear along the two LLSVPs. By using shear wave velocities, the locations of the LLSVPs can be verified, and a stable pattern for mantle convection emerges. This stable configuration is responsible for the geometry of plate motions at the surface. 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 at smaller scales have been discovered mainly at the edges of these LLSVPs. By using the solid Earth tide, the density of these regions has been determined. The bottom two thirds are 0.5% denser than the bulk of the mantle. 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 African LLSVP may be a potential cause for the South Atlantic Anomaly (a region of the Earth's magnetic field where it is significantly weaker than normal). ==Origins==

Several hypotheses have been proposed for the origin and persistence of LLSVPs, depending on whether the provinces represent purely thermal unconformities (i.e. are isochemical in nature, of the same chemical composition as the surrounding mantle) or represent chemical unconformities as well (i.e. are thermochemical in nature, of different chemical composition from the surrounding mantle). If LLSVPs represent purely thermal unconformities, then they may have formed as large mantle plumes of hot, upwelling mantle. However, geodynamical studies predict that isochemical upwelling of a hotter, lower viscosity material should produce long, narrow plumes, unlike the large, wide plumes seen in LLSVPs. It is important to remember, however, that the resolution of geodynamical models and seismic images of Earth's mantle are very different. The current leading hypothesis for the LLSVPs is the accumulation of subducted oceanic slabs. This corresponds to the locations of known slab graveyards surrounding the Pacific LLSVP. These graveyards are thought to be the reason for the high-velocity-zone anomalies surrounding the Pacific LLSVP and are thought to have formed by subduction zones that were around long before the dispersion—some 750 million years ago—of the supercontinent Rodinia. Aided by the phase transformation, the temperature would partially melt the slabs to form a dense melt that pools and forms the ultra-low-velocity-zone structures at the bottom of the core-mantle boundary closer to the LLSVP than the slab graveyards. The rest of the material is then carried upwards via chemically induced buoyancy and contributes to the high levels of basalt found at the mid-ocean ridge. The resulting motion forms small clusters of small plumes right above the core-mantle boundary that combine to form larger plumes and then contribute to superplumes. 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. Since it would be difficult for the Earth's core to maintain this high heat by itself, it gives support for the existence of radiogenic nuclides in the core, as well as implying that if fertile subducted lithosphere ceases to subduct in locations preferable for superplume consumption, it will mark the demise of that superplume. The hypothesis suggests that the LLSVPs may represent fragments of Theia's mantle which sank through to Earth's core-mantle boundary. ==Dynamics==

Geodynamic mantle convection models have included compositionally distinctive material. The material tends to get swept up in ridges or piles. These locations also correspond with known slab graveyard locations. These types of models, as well as the observation that the D″ structure of the LLSVPs is orthogonal to the path of true polar wander, suggest these mantle structures have been stable over large amounts of time. 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. However, the heat from the core is not enough to sustain the energy needed to fuel the superplumes located at the LLSVPs. There is a phase transition from perovskite to post-perovskite from the down welling slabs that causes an exothermic reaction. This exothermic reaction helps to heat the LLSVP, but it is not sufficient to account for the total energy needed to sustain it. 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. These concentrated radiogenic elements are thought to provide the high temperatures needed. So, the appearance and disappearance of slab graveyards predicts the birth and death of an LLSVP, potentially changing the dynamics of all plate tectonics. ==Structure and composition==

A study by researchers from Utrecht University revealed that LLSVPs were not only hotter but also ancient, potentially over a billion years old. The findings suggested that their seismic properties are influenced by factors beyond temperature, such as composition or mineral grain size. Seismic waves passing through LLSVPs decelerate but lose less energy than what would be expected if they were purely thermal in origin, indicating compositional differences and shedding light on their complex structure. ==See also==