Over the years, better maps kept showing the same bloblike features. One huddles under Africa; the other is beneath the Pacific.
They lurk where the planet’s molten iron core meets its rocky mantle, floating like mega-continents in the underworld. Their highest points may measure over 100 times the height of Everest. And if you somehow brought them to the surface, God forbid, they contain enough material to cover the entire globe in a lava lake roughly 100 kilometers deep.
“It would be like having an object in the sky, and asking, ‘Is that the moon?’ And people are like, no. ‘Is that the sun?’ No. ‘What is it?’ We don’t know!” said Vedran Lekić, a seismologist at the University of Maryland. “And whatever it is, it is intimately tied to the evolution of the Earth.”
The first mystery of these hulking, hidden seismic features is whether they’re made of different stuff than the rest of the Earth’s mantle. The second: How do these patterns in the deep leave traces on our surface world?
Neither case is settled. But in recent years, many earth scientists have begun to make the case that these vague shapes are piles of dense, smoldering rock that date to the dawn of the planet. And multiple studies in the past year have argued that their persistent influence might be responsible for long-puzzling patterns in volcanic hot spots like Hawaii.
“These are the largest things on the planet,” said Ed Garnero, a seismologist at Arizona State University. “Only recently have I started thinking, ‘Wow, this is potentially super profound.’”
Core of the Matter
If an omnipotent scientific illustrator halved the Earth, they would first need to cut through the thin crust we live on, which is broken into shifting tectonic plates. Then they’d pass through the rocky mantle. Only at 2,900 kilometers down, about halfway to the very center, would they hit the core-mantle boundary.
To map that part of the Earth, seismologists use the waves released by earthquakes. As the waves rattle outward, they change speed depending on what material they pass through. That causes them to arrive at different monitoring stations at different times. In 1984, the Harvard researcher Adam Dziewonski first integrated data from many different earthquakes into a global map. The two blobs showed up immediately, attached to the core on either side like Princess Leia side buns.
In these regions, earthquake waves seem to slow down, suggesting that the blobs are hotter than the surrounding mantle. How do we know this? Rock expands when heated. That causes waves to travel sluggishly through warm regions, said Garnero, like the slower vibrations moving through a loose guitar string.
The slowing waves gave these features their formal name: large low-shear-velocity provinces, or LLSVPs — an unmagical abbreviation that may have contributed to the topic’s low profile. “We are also to blame,” said Sanne Cottaar, a seismologist at the University of Cambridge, “for misnaming this feature so badly.”