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A Superplume Is the Reason Africa Is Splitting Apart

Primordial gases confirm the cause of the East African Rift

Olduvai Gorge in Great Rift Valley, Tanzania

EMORY KRISTOF National Geographic

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Africa is splitting in two. The reason: a geologic rift runs along the eastern side of the continent that one day, many millions of years in the future, will be replaced with an ocean. Scientists have argued for decades about what is causing this separation of tectonic plates. Geophysicists thought it was a superplume, a giant section of the earth's mantle that carries heat from near the core up to the crust. As evidence, they pointed to two large plateaus (one in Ethiopia and one in Kenya) that they said were created when a superplume pushed up the mantle. Geochemists were not able to confirm that theory. Instead they thought there might be two small, unrelated plumes pushing up the plateaus individually. The theories did not align, says David Hilton, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “There was a mismatch between the chemistry and the physics.”

So in 2006 and 2011 Hilton headed to East Africa to see whether he could lay the argument to rest. He and his team decided to use gases emanating from the rift to determine how it was created. Donning gas masks, they hiked to the tops of volcanoes in Tanzania and Ethiopia and climbed into mazuku (the Swahili word for “evil wind”)—geothermal vents and depressions where deadly gases accumulate and often kill animals. At these locations, the team collected samples of rocks deposited during eruptions, including olivines, crystals that trap volcanic gases like a bottle.

Back home in California, Hilton crushed the rocks inside a vacuum to release their gases. He was looking for helium 3, an isotope of helium present when the planet was forming that was trapped in the earth's core. Hilton figured that if rocks around both the Ethiopian and Kenyan plateaus contained this primordial gas, that would at least confirm that underground mantle plumes created them. The readings showed that, indeed, both plateaus contained helium 3. But Hilton and his group still had to wonder: Was one superplume behind it all? Or were there a couple of lesser plumes?

To answer this question, they turned to another primordial gas trapped in the mantle: neon 22. They found that neon 22 existed in both plateaus and that the ratios of helium to neon in those locations matched, results published in April in Geophysical Research Letters. That meant that the plume underneath both plateaus was of the same material and of the same age. Hence, there was one common superplume. The geophysicists, it turns out, had been right all along.

“The ‘naysayers’ who claim that the rifting and plume activity are unconnected—and some who would even deny a mantle plume is present—no longer have a leg to stand on,” says Pete Burnard, a geochemist at the French National Center for Scientific Research, who was not involved in the latest work.

The African superplume will provide scientists with easier access to study the earth's inner workings (another lies underneath the Pacific Ocean). Hilton and his team are now measuring how much carbon the mantle in East Africa is releasing, how old it is and if it has been recycled from carbon originally captured on the surface billions of years ago. This information, Hilton says, will help geologists figure out how the earth's layers interact on a longer time scale, including the hundreds of millions of years it takes for continents to form—and split.

This article was originally published with the title "Africa's Great Divide."

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