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NSF PR 98-8 - February 11, 1998
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Evidence Found for Molten Rock Two Thousand Miles
Beneath Earth's Surface
Deep inside the earth, two thousand miles beneath
its surface, pockets of molten rock can be found in
a region where many scientists did not expect to find
anything but solid rock. Researchers funded by the
National Science Foundation report the finding in
this week's issue of the journal Nature.
"One of the most exciting puzzles in the earth is
the nature of mantle convection, which is the driving
force of earthquakes," says Jim Whitcomb, NSF program
director for geophysics. "This study is an important
step in characterizing the lower boundary of that
convection in a newly discovered region above the
core-mantle boundary."
Based on their analysis of seismic waves measured
in Norway, scientists John Vidale of Univeristy of
California, Los Angeles and Michael Hedline of University
of California, San Diego have produced the most convincing
data yet for the existence of molten rock deep within
the earth's mantle.
"Just as temperature varies tremendously at different
parts of the earth's surface, there also seem to be
large temperature differences 2,000 miles beneath
the earth, above the core-mantle boundary," says Vidale,
who studies the region between the earth's mantle
-- a thick layer of rock -- and its outer core. "For
example, areas with many volcanoes and earthquakes,
like the western United States, have hotter rock underneath
than regions with 'less exciting' geology, like the
midwest or southeastern U.S. We may be seeing signs
of a similar variety of activities deep within the
earth."
Vidale and Hedlin analyzed seismic waves from 25 earthquakes
of magnitude 6.0 or higher that have struck the southwest
Pacific island of Tonga -- the locale with more major
earthquakes than anywhere on earth. The measurements
were generated by a network of 132 seismometers spread
over 60 miles in Norway, instruments that were originally
deployed in the 1960s to monitor the Soviet Union's
nuclear weapons testing. Seismic vibrations from Tonga
radiate through the earth, and reach seismometers
in Norway in about 20 minutes.
The scientists' analysis reveals that seismic waves
hit an obstacle that causes the waves to change direction,
or scatter. By studying this scattering, they were
able to draw conclusions about the boundary between
earth's core and mantle. "The best and perhaps only
explanation for the large amount of scattering in
the seismic waves is that part of the rock in the
mantle is melted," explains Vidale.
The new study detected a slurry of molten rock across
a 300-by-600-mile region deep beneath Tonga.
Closer to home, volcanic plumes that erupt in Hawaii
and thermal phenomena like the geysers in Yellowstone
may originate in the core-mantle boundary, but that
remains to be proven, says Vidale. A goal of his research
is to learn whether material is transported from the
core to the mantle, and vice versa.
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