Yet no plates have subducted over the blob regions for the past few hundred million years, says Saskia Goes of Imperial College London. “It’s the absence of cold material that makes these relatively hot.”
The opposing camp, meanwhile, doesn’t doubt that plumes rise from the hot-blob regions. They just argue that the blobs are special in and of themselves.
Since the mid-2000s, several teams of seismologists have looked at earthquake signals that merely graze the edges of these regions. Those signals show complicated patterns, indicating that the waves were skimming across a relatively crisp boundary. This suggests that the edges of the blobs mark a transition between materials, not just temperature.
In this view, the blobs are so-called thermochemical piles, clumps of dense rock with a distinct chemical composition. Because of their prolonged contact with the core, they are hotter than the rest of the mantle, causing plumes to sprout.
Assuming that the blobs are distinct, they could be old—the last surviving remnants of the infant Earth. One leading idea is that they formed when the entire lower mantle was an ocean of magma, shortly after the planet’s birth. Rock began to cool and crystallize, but iron stayed melted in the magma ocean, says Nicolas Coltice at the École Normale Supérieure in Paris. Then, when the last dregs of magma crystallized, they were so dense and iron-rich that they sank to the bottom of the mantle, forming the blobs.
Down there, they would have held out through the early planet’s greatest cataclysm: a hypothesized impact with a Mars-size body called Theia that ultimately birthed the moon. Or, Garnero speculates, the dense, distinct piles might even be fragments of Theia itself, forever interred in the deep Earth.
Read: The moon may be formed from many tiny moons
In the thermal-only view, plate tectonics are the true movers and shakers of the world, dictating where upwelling happens. But the thermochemical-piles camp believes that hot, heavy, stable blobs would have more of a back-and-forth dialogue with the tectonic system on the surface. Cold currents from sinking plates would push the blobs around like Silly Putty; in turn, upwelling heat from the warm blobs would push the plates right back.
To test how much the blobs are helping to pilot the geophysical ship, scientists looked to Hawaii. In the past year, researchers invoked the blobs to solve two long-standing puzzles there.
Consider first the Hawaiian–Emperor chain, a stretch of islands and underwater mountains. The chain starts at the still-growing Big Island and spans 6,200 kilometers, ending near Russia. Geologists have long explained the chain as a hot spot: As the Pacific plate slides over a fixed mantle plume, the plume pushes up new volcanic islands from below.
The only trouble is the bend. Smack in the middle of the chain is a 60-degree kink. The bend, geophysicists thought, came from a long-ago shift in the plate’s motion.