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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.
Inner Earth Spews
Superplumes 
"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..gif)
