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 hot rock that comprises the surrounding mantle. The interdisciplinary team of seismologists and geodynamicists from Caltech includes Sidao Ni, the paper's lead author and a staff scientist in the seismology lab. Eh Tan, Michael Gurnis, and Don Helmberger are co-authors.
"This is an exciting new discovery that addresses the anomalous observations of the African superplume and has broad implications for understanding deep earth dynamics on a global scale," says Robin Reichlin, program director in NSF's division of earth sciences. "The interdisciplinary approach was a key element to unraveling the structure, composition and dynamics of the African superplume."
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 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 high, 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.
It turned out, says Gurnis, that a clear pattern of seismic waves developed that grazed the east edge of the plume, creating a peculiar pattern that was indicative of 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."