2 gigantic blobs of magma menace from far below

WASHINGTON - Two superplumes of molten rock appear to be powering through the boundary between the Earth's upper and lower mantle, perhaps feeding volcanoes and affecting movement of the planet's crust.

New evidence of the superplumes - beneath the south- central Pacific Ocean and southern Africa - comes from studies of seismic waves conducted by scientists at the University of California at Berkeley and reported in today's issue of the journal Science.

Smaller regions of magma rising to the Earth's crust power 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.

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 study seeks to focus attention on the hot material rising from the base of the mantle - the partially molten region that extends about 1,750 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 and thus may contribute to their movement.

David Bercovici, a professor of geology and geophysics at Yale University, said there had been other indications of the superplumes, such as variations in the Earth's gravity. He was pleased to see the plumes identified using seismic tools.

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

Romanowicz and Yuancheng 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 exact temperature of the plumes has not been determined, but they may be several hundred degrees hotter than the surrounding material.

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