http://www.scientificamerican.com/article/a-superplume-is-the-reason-africa-is-splitting-apart/
A Superplume Is the Reason Africa Is Splitting Apart
Primordial gases confirm the cause of the East African Rift
Jul 15, 2014 |By Erin Biba
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."
Received on Sat Aug 02 2014 - 20:31:08 EDT