'Superplumes' of magma may have quake role

Sunday, April 21, 2002

WASHINGTON -- Hundreds of miles beneath southern Africa and the south Pacific, massive plumes of molten rock rise toward the Earth's surface, material that may affect the movement of the rocky lands above.

Magma bursts forth as volcanoes in many parts of the world, but these superplumes originate so much deeper that they cross the boundary between Earth's upper and lower mantle 400 miles below.

Scientists from the University of California, Berkeley, reported new evidence of the plumes in Friday's issue of the journal Science.

Researcher Barbara Romanowicz said earthquake studies until now have emphasized the dynamics of collisions between the planet's massive surface plates. When two of them crash together, one slips beneath the other in a process called subduction, and earthquakes and volcanoes can follow. "We think the superplumes play an important role as well," Romanowicz said.

The new study seeks to focus attention on the hot material rising upward from the base of the mantle, a 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 midocean 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 actively contribute to their movement.

Developing images

David Bercovici, a professor of geology and geophysics at Yale University, said other indications of superplumes have been detected, such as variations in the Earth's gravity field in those areas. He was pleased to see the plumes identified through seismic measurements.

"It's not hugely surprising to see upwellings at these regions, but it's nice to see they are distinct," he said.

Romanowicz and Yuan-cheng Gung were able to develop 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.

She said the plumes' temperature has not been determined, but they may be as much as several hundred degrees hotter than the surrounding material.

"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 said.

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 said. This is referred to as the "African superswell."

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