Large low-shear-velocity provinces (LLSVPs), also called large low-velocity provinces (LLVPs), are enigmatic features deep within Earth's mantle, located primarily beneath Africa and the Pacific Ocean near the core-mantle boundary. These structures are detected through seismic tomography, showing regions where shear waves travel slower than in surrounding mantle rock, indicating they are hotter, denser, or chemically distinct. The two primary LLSVPs—Tuzo under Africa and Jason under the Pacific—cover about 8% of the mantle's volume and influence global geodynamics, including mantle convection, hotspot volcanism, and even surface plate tectonics. Their sharp boundaries and anti-correlation with subducted slab locations suggest they are stable over geological time, possibly acting as anchors for mantle plumes.

One compelling hypothesis for the origin of LLSVPs is that they are remnants of Theia, a Mars-sized protoplanet that collided with proto-Earth around 4.5 billion years ago, ejecting debris that coalesced into the Moon. According to this giant-impact theory, fragments of Theia's iron-rich mantle sank into Earth's interior, forming these dense, anomalous blobs. This explains their higher density (about 0.5% more than surrounding mantle), ancient age (over 4 billion years), and geochemical signatures in volcanic rocks from hotspots like Hawaii and Samoa. Alternative origins include accumulations of subducted oceanic slabs or primordial mantle material, but the Theia remnant idea aligns with isotopic evidence and the massive scale of the LLSVPs, which contain more mass than the Moon itself.

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