SAN FRANCISCO --- Over the last two decades, scientists have generated many pictures of Earth's interior by mapping the behavior of seismic waves, those waves of energy that shake the ground during earthquakes. These seismic images show some areas where waves travel faster than average and some where they travel more slowly through the mantle---a plastic-like layer between Earth's crust and core that flows under pressure, lifting or lowering features on the surface.
Interpreting exactly what such differences in "seismic velocity" mean has been a challenge, says Carolina Lithgow-Bertelloni, assistant professor of geological sciences at the University of Michigan. But such understanding is essential to knowing "how the mantle has evolved, how it works dynamically, and what we would expect to happen at Earth's surface as a result," she explains.
In an invited presentation to the American Geophysical Union today, Lithgow-Bertelloni and Lars Stixrude, U-M assistant professor of geological sciences, unveiled a model that relates seismic velocity differences to properties of the mantle. Previous models had assumed that the speed at which seismic waves travel through a particular region in the mantle depends only on the temperature of that region. But Lithgow-Bertelloni and Stixrude reasoned that other factors, such as chemical composition and physical structure of the minerals in the region also contribute.
Preliminary results of work based on their model show that phase transitions---changes in the crystal structure of minerals that result from pressure and temperature changes inside Earth---can suddenly speed up or slow down plate movement at Earth's surface. Such plate movement causes mountain-building, earthquakes and volcanoes.
"While this is a new and exciting result, even more surprising is the fact that when we converted the velocity differences of some seismic models to temperature, the results suggested temperatures high enough to cause melting in some regions of the Earth," says Lithgow-Bertelloni. "One such region that we found down to a depth of 200-600 km is right under the East African rift and might possibly be the source of the volcanism in the area." The next step will be to learn how much of the material has melted and to determine its composition, she adds. Comparing its composition to rocks at the surface will help confirm that the melting is responsible for the volcanic activity.
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