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Embargoed until 2 p.m. EDT
NSF PR 02-50 - June 6, 2002
Geophysicists Find Sharp Sides to African Superplume
Scientists at the California Institute of Technology
(Caltech) have discovered that the African superplume
- a massive, hot upwelling of rock beneath southern
Africa - has edges that are sharp and distinct, not
diffuse and blurred as previously thought. Such sharp,
lateral boundaries have never been found in the Earth's
mantle before, and they challenge scientists' understanding
of the interior. The research was funded by the National
Science Foundation (NSF).
In a paper to be published in the June 7 issue of the
journal Science, a team of geophysicists at
Caltech's Seismological Laboratory used a fortuitous
set of seismic waves from distant earthquakes to show
that the boundary of the African superplume appears
to be sharp, with a width of about 30 miles. The sharp
boundary is not vertical but somewhat tilted, somewhat
like a rising plume of smoke that is tilted by the
wind. This suggests that the plume is unstable. Using
dynamic computer modeling, the scientists provide
further evidence of what they and other geologists
suspected, that the superplume has a dense chemical
core that differs from the hot rock that comprises
the surrounding mantle. The interdisciplinary team
of seismologists and geodynamicists from Caltech includes
Sidao Ni, the paper's lead author and a staff scientist
in the seismology lab. Eh Tan, Michael Gurnis, and
Don Helmberger are co-authors.
"This is an exciting new discovery that addresses the
anomalous observations of the African superplume and
has broad implications for understanding deep earth
dynamics on a global scale,"" says Robin Reichlin,
program director in NSF's division of earth sciences.
"The interdisciplinary approach was a key element
to unraveling the structure, composition and dynamics
of the African superplume."
About 20 years ago, scientists developed a way to make
three-dimensional "snapshots" of the earth's interior
using the seismic waves, or vibrations, that travel
through the earth following an earthquake. By measuring
the time it takes for these waves to travel from an
earthquake to a recording station, they can infer
the temperatures and densities in a given segment
of the mantle, the middle layer of the earth. In the
mid-1980s, they noticed a huge area under Africa where
seismic waves passed through slowly, implying that
the solid rock was at a substantially higher temperature.
Some 750 miles across and more than 900 miles high,
the region was initially thought to be a giant anomaly,
with broad, diffuse edges, that was hotter than the
mantle's surrounding rock. The so-called African superplume
was slowly rising upwards, much like the thermal convection
that occurs in a pot of boiling water. As seismic
instrumentation improved, other evidence suggested
that the structure might be more than thermal, possibly
having a different chemical composition from the surrounding
mantle rock.
It turned out, says Gurnis, that a clear pattern of
seismic waves developed that grazed the east edge
of the plume, creating a peculiar pattern that was
indicative of an incredibly sharp boundary - a boundary
that probably extends nearly 900 miles above the core.
The findings startled the researchers. "No one expected
this," says Gurnis. "Everybody thought there'd be
these very broad, diffuse structures. Instead, what
we've found is a structure that is much bigger, much
sharper, and extends further off the core mantle boundary."
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