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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 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 (640 kilometers) 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 PLATES COLLIDE
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 (2,800 kilometers) from Earth¡¯s core to its crust, or lithosphere. |
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¡°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 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 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.
WATCHING THE WAVES
Romanowicz and Yuancheng Gung were able to develop images that indicate
the presence of the superplumes by measuring the movement of seismic waves
through Earth. |
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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 CT 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.
EARTH¡¯S BULGES
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 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.
(C) 2002 Associated Press. All rights reserved. This material may not
be published, broadcast, rewritten or redistributed.
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