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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.
"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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