MIT Researchers Propose New Model For Convective Circulation Within Earth's Mantle

March 19, 1999

Almost two years after convincing the scientific community that most of the Earth's mantle is uniform in composition, National Science Foundation (NSF)-funded researchers at the Massachusetts Institute of Technology propose a model that may explain why the mantle seems to comprise two dissimilar and separate sections. The scientists have published their results in this week's issue of the journal Science.

The MIT researchers hope that their new "hybrid convection model" will lay to rest questions about the nature of the mantle -- a 3,000-kilometer-thick layer of rock between Earth's crust and core -- and introduce new ways of thinking about the planet's heat-transfer system as a whole. "We expect that this model will form a new framework for further investigations of the chemical and thermal evolution of our planet," says Robert van der Hilst, an earth scientist at MIT.

"This work shows that we live in exciting times because of major advances in our capabilities in seismology and geochemistry," says James Whitcomb, director of NSF's geophysics program, which funded the research. "The nature of convection of the earth's mantle is one of the great outstanding questions in earth science, and these results are a major step in solving the puzzle."

For almost 50 years, scientists have debated whether the heat transfer called convection occurs throughout the entire mantle at once -- creating a huge mixing pot of essentially the same stew -- or separately in the upper mantle, which extends from near the surface to about 660 kilometers in depth, and the lower mantle, from 660 to about 2,880 kilometers. The second scenario would mean that like oil and water, there are two chemically distinct sections of the mantle that almost never mix.

Using computer simulations and mountains of data to create a kind of CAT scan of the Earth, the MIT researchers demonstrate that previous evidence for separate upper and lower mantles may be explained by processes in the very depths of the mantle, an area about 1,000 kilometers from the molten core.

In this area, shifts in densities due to increased quantities of iron and silicon, partially offset by skyrocketing temperatures, may account for minute, previously unexplained differences in the composition of magmas. Researchers have long noted these differences in mid-ocean ridges and ocean islands, where, after being heated deep within the planet, the mantle reveals itself in volcanic eruptions.

On the basis of a wide range of evidence from geophysics and geochemistry, the researchers argue that a transition in the mantle's structure and composition occurs in the middle of the lower mantle, at about a depth of 1,700 kilometers, and that elusive "reservoirs" of high radioactive heat production and distinctive chemical composition reside in the bottom 1,000 kilometers of the mantle.

"We realize that this is a first-order model, but it's more realistic than ones in the past," says van der Hilst. "A lot of evidence can be explained with this model, but we still don't know much about the ultimate origin and nature of this layer. We hope to provoke a lot of interest on the topic in multiple scientific disciplines."

The research is also funded by the David and Lucile Packard Foundation.
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National Science Foundation
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