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Inner Earth Spews Superplumes
April 19, 2002 08:00 CDT

Scientists have documented two of what they call superplumes of molten rock pushing through the boundary between the Earth's upper and lower mantle, and they may be the source for volcanoes and could be affecting movement of the planet's crust.

Scientists at the University of California at Berkeley studied seismic waves and found evidence of the superplumes beneath the south central Pacific Ocean and southern Africa. Their findings are reported in Friday's issue of the journal Science.

Smaller regions of magma rising to the Earth's crust provide the force under volcanoes and other hot spots. But the superplumes come from far deeper, crossing the boundary between the upper and lower mantle about 400 miles deep, an area that had been thought by some scientists to impede the flow of material.

David Bercovici, a professor of geology and geophysics at Yale University, told the Associated Press there had been other indications of the superplumes, such as variations in the Earth's gravity field in those areas.

Researchers Barbara Romanowicz and Yuancheng Gung developed images that indicate the presence of the superplumes by measuring the movement of seismic waves through the Earth. Romanowicz said they used elastic tomography, a process that measures the movement of seismic waves to chart the interior of the planet, somewhat like a CAT scan machine uses X-rays to look inside a person.

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"Emphasis so far has been on the cold down-moving subducted plates and their critical role in mantle dynamics. We think the superplumes play an important role as well," Romanowicz said.

When large surface plates collide, one slips beneath the other in a process called subduction. This can generate earthquakes and volcanoes along the boundary. The San Andreas fault in California is such a subduction zone.

The Berkley study focuses on the hot material rising upward from the base of the mantle -- the partially molten region that extends about 1,740 miles from the Earth's core to its crust, or lithosphere. "The hot material brought under the lithosphere by the superplumes then spreads out horizontally toward mid-ocean ridges," Romanowicz explained.

The ridges are often active volcanic areas. The material heats up the region under the plates that cover the Earth's surface and thus may be an active contributor to their movement. The scientists have yet to determine the exact temperature in the plumes, however they appear to be several hundred degrees hotter than material around them.

"We do not know precisely because the images we have are still not very well resolved, and the actual temperature may depend on whether the superplumes are - like we see them now - wide, thick conduits several thousand kilometers across, or whether they are composed of several narrower plumes grouped together," she told the AP.

"Generally, it is assumed that only about 10 percent of the heat that comes out at the surface of the Earth comes from the earth's core. This number may thus be underestimated, perhaps as much as by a factor of two," she wrote.

Regions above the superplumes tend to bulge upward. The plateaus of southern and eastern Africa are about 1,600 feet higher than most old continental areas in the world, she pointed out. This is referred to as the "African superswell."

Also, she wrote, heat flow from the Earth's interior measured in a wide area of southern Africa is higher than expected, indicating that an unusually large supply of heat must be coming from underneath.

Volcanoes in Africa and in the southern Atlantic Ocean could be related to the superplume in the same way as Hawaii and other hotspot volcanoes in the southern Pacific may be related to the Pacific superswell, she said.

Source: University of California, Berkeley; AP

Cosmiverse Staff Writer

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