Scientists at the California Institute of Technology have discovered that the African superplume-a massive, hot upwelling of rock beneath southern Africa-has edges that are sharp and distinct, not diffuse and blurred as previously thought. Such sharp, lateral boundaries have never been found in the Earth's mantle before, and they challenge scientist's understanding of the interior.
In a paper to be published in the June 7 issue of the journal Science, a team of geophysicists at Caltech's Seismological Laboratory used a fortuitous set of seismic waves from distant earthquakes to show that the boundary of the African superplume appears to be sharp, with a width of about 30 miles. The sharp boundary is not vertical but somewhat tilted, somewhat like a rising plume of smoke that is tilted by the wind. This suggests that the plume is unstable. Using dynamic computer modeling, the scientists provide further evidence of what they and other geologists suspected, that the superplume has a dense chemical core that differs from the scalding hot rock that comprises the surrounding mantle.
The team of scientists from Caltech includes Sidao Ni, the paper's lead author and a staff scientist in the seismology lab; graduate student Eh Tan; Michael Gurnis, professor of geophysics; and Don Helmberger, the Smits Family Professor of Geophysics and Planetary Science and director of the Caltech seismology lab.
About 20 years ago, scientists developed a way to make three-dimensional "snapshots" of the earth's interior using the seismic waves, or vibrations, that travel through the earth following an earthquake. By measuring the time it takes for these waves to travel from an earthquake's epicenter to a recording station, they can infer the temperatures and densities in a given segment of the mantle, the middle layer of the earth. In the mid-1980s, they noticed a huge area under Africa where seismic waves passed through slowly implying that the solid rock was at a substantially higher temperature.
Some 750 miles across and more than 900 miles tall, the region was initially thought to be a giant anomaly, with broad, diffuse edges, that was hotter than the mantle's surrounding rock. The so-called African superplume was slowly rising upwards, much like the thermal convection that occurs in a pot of boiling water. As seismic instrumentation improved, other evidence suggested that the structure might be more than thermal, possibly having a different chemical composition from the surrounding mantle rock.
If there were heavy and dense material associated with this anomalous mantle, the scientists reasoned, then it would either lie underneath or within the vast majority of the hot, rising African superplume. "So we said if that's the case, there should actually be a sharp boundary between the two materials, instead of a diffuse boundary," says Gurnis. The researchers went looking.
By pure chance, other unrelated work had placed a series of seismic detectors in southern Africa. This allowed the Caltech team to study and interpret the fine-scale structure of earthquake seismic waves recorded by the arrays. The energy from the earthquakes emanated from South America and passed through the African superplume.
It turned out, says Gurnis, that a clear pattern of waves developed that grazed the east edge of the plume, creating a peculiar pattern that was indicative of being an incredibly sharp boundary-a boundary that probably extends nearly 900 miles above the core. The findings startled the researchers. "No one expected this," says Gurnis. "Everybody thought there'd be these very broad, diffuse structures. Instead what we've found is a structure that is much bigger, much sharper, and extends further off the core mantle boundary."
They also found that the structure, instead of having a dome-like appearance predicted by their computer models, tilts toward the northeast. Gurnis speculates that's probably due to its dynamic state-"It's a completely different observation from what we expected to see," he says.
At this point, the team can only speculate on the causes. "One of the ideas, and it's not perfect, is that the rock composition of the plume is more iron rich, and thus denser," Gurnis suggests. "It will be interesting to see what observations other scientists can make. The idea of sharp, near-vertical edges was not on people's agendas before now, so this may change people's perspectives on the interior.
"I don't particularly like this idea," Gurnis admits; "it's strange. I guess that's why we find it so interesting."
The interdisciplinary team of researchers was funded by the National Science Foundation's Cooperative Studies of Earth's Deep Interior program.