The Unsolved Mystery of the Earth Blobs

February 28, 2019

Researchers peering into Earth’s interior found two continent-sized structures that upend our picture of the mantle. What could their existence mean for us back on Earth’s surface?

Some 2,000 kilometers beneath our feet, there are enormous masses of hot mantle material that have baffled scientists for the last 4 decades.

The blobs, as some scientists have taken to calling them, are the length of continents and stretch 100 times higher than Mount Everest. They sit at the bottom of Earth’s rocky mantle above the molten outer core, a place so deep that Earth’s elements are squeezed beyond recognition. The blobs are made of rock, just like the rest of the mantle, but they may be hotter and heavier and hold a key to unlocking the story of Earth’s past.

Scientists first spotted the blobs in the late 1970s. Researchers had just invented a new way to peer inside Earth: seismic tomography. When an earthquake shakes the planet, it lets out waves of energy in all directions. Scientists track those waves when they reach the surface and calculate where they came from. By looking at the travel times of waves from many earthquakes, taken from thousands of instruments around the globe, scientists can reverse engineer a picture of Earth’s interior. The process is similar to a doctor using an ultrasound device to image a fetus in the womb.

“Ultimately, a lot of people believe plate tectonics are one of the reasons why we have life on Earth,” said geophysicist Harriet Lau at Harvard University. Scientists believe these blobs play a role in many of the processes of the deep Earth, including plate tectonics and volcanism.

Once researchers began to form a picture of inner Earth, they started to see things they never imagined.

“It was very clear in those models from the get-go that at the bottom of Earth’s mantle, nearly halfway to the center, there were these huge zones where the waves traveled more slowly,” said Ed Garnero, professor of Earth and space exploration at Arizona State University.

The slow-wave velocity zones are concentrated in two locations: One lies under the Pacific Ocean, and the other sits under Africa and part of the Atlantic Ocean. They appeared like “massive mountains on the core-mantle boundary,” said seismologist Sanne Cottaar from the University of Cambridge. Other researchers describe them as conical pits of gravel sitting “all on top of each other” or like giant sand piles. The blobs are so large that if they sat on Earth’s surface, the International Space Station would need to navigate around them.

“They’re basically unmissable,” said seismologist Karin Sigloch at the University of Oxford. “They just show up on everybody’s pictures.”

There is little doubt that the blobs exist, yet scientists have no idea what they are. A recent paper said the blobs “remain enigmatic.” Scientists can’t even decide on what to call them. They go by many names, most commonly LLSVP, which stands for large low shear velocity provinces.

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