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NSF PR 99-18 - March 18, 1999
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MIT Researchers Propose New Model for Convective Circulation
Within Earth's Mantle
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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