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Accepted · first revision · record

Title: What are the two continent-sized slow regions at the mantle base?
Plain: 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: 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.
Settled by: Joint seismic, normal-mode and gravity inversion of density and velocity ratios matched to mineral-physics predictions for candidate compositions.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: contested
Status note: 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.
Posed since: (empty)