What are the two continent-sized slow regions at the mantle base?
In plain words
Below Africa and the Pacific, seismic waves slow down in two huge regions about 1000 km tall. They could be hot rock, a different rock composition, or even leftovers of the body that hit Earth to form the Moon.
Precise statement
The large low shear-velocity provinces (LLSVPs) show shear-velocity reductions of a few percent, sharp edges and heights near 1000 km. Identify their nature: purely thermal superplumes, thermochemical piles of primordial material, accumulated subducted oceanic crust, or remnants of the Moon-forming impactor. An answer is a composition and temperature model that fits shear, compressional and bulk-sound velocities, density and dynamical stability over Gyr.
What would settle it
Joint seismic, normal-mode and gravity inversion of density and velocity ratios matched to mineral-physics predictions for candidate compositions.
Status in the literature
Yuan and coauthors (Nature 2023, https://doi.org/10.1038/s41586-023-06589-1) proposed the LLSVPs are remnants of the Moon-forming impactor; thermal and recycled-crust explanations remain in play.
Related problems
See also
- Related Are hotspot plumes rooted at the core-mantle boundary?
- Related Where are the mantle's primordial helium and tungsten signatures stored?
- Related How much of Earth's heat flow comes from radioactive decay?
- Related Was the Moon-forming impactor isotopically Earth-like?
- Related Do mantle heat-flux changes cause superchrons and reversal-rate swings?