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Deep Earth
Afar from close-up

The Afar Depression in northeastern Ethiopia is a scientific treasure trove. To anthropologists, Afar is home to hominid fossils like the 3.2 million-year-old Lucy. To geologists, it is the site of a triple junction, where three pieces of Earth’s crust are simultaneously pulling away from each other (see Geotimes, February 2006). The powerful stresses in the crust lead to frequent earthquakes and volcanism, and produce deep cracks and fissures in the region. Researchers now say that the source of this activity could be a deep upwelling of hot rock far to the southwest, revealing a possible new connection in Earth’s interior.

Scientists have traditionally thought that Afar’s hotspot was the result of one or more shallow plumes of hot rock rising through Earth’s mantle. However, some global-scale seismic models, which help reveal the structure of the mantle, hint at a connection between other regional hotspots below Ethiopia and Tanzania and the deeper African superplume — implying a possible connection with the Afar hotspot as well, says Andrew Nyblade, a seismologist at Pennsylvania State University in University Park. But those models lack the resolution to accurately map the upper mantle.

For a higher resolution picture of the Afar hotspot, Nyblade, Margaret Benoit, a seismologist at MIT in Cambridge, Mass., and their colleagues analyzed seismic data from hundreds of earthquakes recorded on 27 monitoring stations in Ethiopia between 2000 and 2002. Seismic waves travel more slowly through hotter rock, and by measuring how long it takes for a wave to travel through the ground to multiple stations, seismologists can construct 3-D maps of plumes and magma in the subsurface.

Using this technique, called seismic tomography, the team mapped the upper mantle beneath Afar to a depth of 500 kilometers. If the source of the hotspot is a single plume rising through the mantle, it would spread out in a thin layer once it hits the base of the lithosphere, at around 100 kilometers below the surface, Nyblade says. In that case, researchers would expect to see a region of slower seismic waves no more than about 100 kilometers below the base of the lithosphere, he says.

But “the results were startling,” says Benoit, lead author, who at the time of publication was also at Penn State. Instead of a thin layer, the data revealed a zone of slower waves more than 500 kilometers deep — a depth that “is very difficult to explain,” she says. Furthermore, the low-velocity zone is not level, but dips to the southwest as it goes deeper, the researchers report in the May issue of Geology.

That anomaly is the second such zone Nyblade and colleagues have found. In the April 7 Geophysical Research Letters, Nyblade and graduate student Yongcheol Park reported mapping a similar low-velocity zone that also dips to the west under Kenya and Tanzania. Instead of independent anomalies, the researchers say, both hotspots may instead be connected to the massive African superplume, thought to arise from the boundary between the mantle and the core, 2,950 kilometers deep.

Although the dipping low-velocity zone suggests a physical connection between the Afar hotspot and the deeper plume, no clear mechanism exists to explain how that would happen, Nyblade says. “There are a lot of issues surrounding how you get a feature that would slant up through the mantle dynamically,” he says. “but an upwelling that extends from the core to the surface would strongly influence the debate about how mantle convection works.”

“They make a convincing case” that the anomaly under Afar doesn’t resemble a standard plume, says Robert van der Hilst, a seismologist at MIT. “That’s a powerful conclusion,” but the possible connection to the superplume “is still a little bit tenuous,” he adds. Additional seismic data is necessary to reveal whether the low-velocity zone continues below Benoit’s team’s measurements and connects to the deeper superplume, van der Hilst says.

Exploring that structure is a primary objective of the AfricaArray project, a National Science Foundation-funded collaboration to install seismic monitoring stations across the continent (see Geotimes, October 2005). Nyblade, who is principal investigator on the project, hopes to further define the anomaly, possibly tracing it through the mid-mantle and into the lower mantle. “That’s the prize,” he says.

Carolyn Gramling

"A far-off new ocean," Geotimes, February 2006
"Developing a Scientific Workforce in Africa," Geotimes, October 2005

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