Geophysicists Propose A New Model Of Earth's Mantle

Sandwiched between Earth's crust and molten outer core, the vast mantle accounts for 83 percent of the planet's volume. It is filled with solid rock but, heated by the core and by its own radioactive decay, it circulates like a pot of impenetrable soup. That circulation is the driving force behind the surface motion of tectonic plates, which builds mountains and causes earthquakes.

For almost 50 years scientists have debated how the mantle circulates -- in one layer or two? Recent work by seismologists shows that tectonic plates sink from the Earth's surface deep into the lower mantle, suggesting that the mantle overturns as a single layer. However, the Earth's heat budget and geochemists' studies of the chemical composition of rocks erupted at volcanic islands indicate that large sections of the mantle have remained isolated from the surface since the Earth formed, suggesting that there must be two layers.

Writing in the March 19 issue of the journal Science, UC Davis geophysicist Louise Kellogg and her MIT collaborators try to resolve the conflict.

They propose that there are indeed two mantle layers. In their model, tectonic plates do dive very deep into the mantle, but encounter a geological barrier at about two-thirds of the distance to the outer core. This jibes with the seismic records.

When the sinking plate reaches this barrier, it is deflected and largely prevented from mixing with the deep-mantle material. But at a few spots on that geological boundary, hot plumes rise up, pulling a bit of deep-mantle material along with the ancient slab to the surface, erupting in volcanic islands. That would explain the geochemists' findings.

"This is where I come in. I try to merge the time history of the geochemist and the snapshot of the seismologist," says Kellogg, who attacked the problem by computer modeling of fluid dynamics.

Kellogg's MIT co-authors are Brad Hager and Rob van der Hilst. The work was funded by the National Science Foundation.

Kellogg, 39, was one of the first recipients of a $500,000 national Presidential Faculty Fellowship. She is an associate professor of geology and vice chair of the UC Davis Geology Department. She received bachelor's degrees in philosophy and engineering physics in 1982, a master's in engineering physics in 1985 and a Ph.D. in geological sciences in 1988, all from Cornell University.