Superplumes rumble inside Earth UC Berkeley scientists track flow of molten rock

By Randolph E. Schmid The Associated Press
Friday April 19, 2002

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 — located 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 Friday'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. 

"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," researcher Barbara Romanowicz said. 

When two of the planet's large surface plates collide, one slips beneath the other in a process called subduction. This can generate earthquakes and volcanoes along the boundary. 

The study seeks to focus attention 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. 

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 field in those areas. He was pleased to see the plumes identified using 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 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 that the exact temperature of the plumes 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. 

"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 added. 

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 said, 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. 

There are volcanoes in Africa and in the southern Atlantic Ocean that 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.