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.