NSF PR 96-52 - October 1, 1996
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Robin Reichlin |
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Model of Earth's Interior Predicts
Size and Shape of Tectonic Plates
With a simple assumption and lots of supercomputer
time, two National Science Foundation (NSF)-supported
geophysicists have solved a long-standing problem
in geology -- why the jigsaw puzzle of crustal plates
on the Earth's surface looks the way it does.
The problem, which has bedeviled the theory of plate
tectonics since it was proposed nearly a half century
ago, is that basic theories of fluid heating and convection
say the planet's surface should be broken into many
small puzzle pieces, none larger than about 3,000
kilometers across. Instead, scientists see a smaller
number of huge plates. One of these, the Pacific plate,
spans nearly 13,000 kilometers at its widest.
The University of California at Berkeley geophysicists
found that by making a simple but fundamental assumption
-- that the viscosity or stiffness of the hot rock
in the Earth's interior increases by a factor of 30
from top to bottom -- they could predict what is observed
on the surface, says Robin Reichlin, program director
in NSF's division of earth sciences, which funded
the research. "This includes not only the size of
the plates but also the geometry of plate boundaries
and even the stability of so-called hot spots that
underlie island arcs such as the Hawaiian Islands."
In the new model, upwelling of hot rock from the deep
mantle and downwelling of cool rock from near the
surface -- analogous to the upward movement of hot
air and the downward flow of cool air in the atmosphere
-- create a cyclic flow or convection cell with dimensions
close to the dimensions of the tectonic plates. Because
convection in the mantle is assumed to nudge the continents
around on the surface of the Earth and break them
up into plates of roughly the same size as the convection
cell, this model provides an explanation for why the
plates are the size they are. Geophysicist Mark Richards
and graduate student Hans-Peter Bunge describe the
model in a cover article scheduled for publication
in the October issue of Geophysical Research Letters.
What Richards and Bunge did in their model was simplify
Earth's interior to include only one major physical
effect -- that the viscosity of the mantle increases
with depth. The effect has only recently been established
from seismic studies. "Assuming a 30-times increase
in viscosity causes a dramatic change over what you
get when you assume a uniform viscosity in the mantle,"
Bunge says. "Instead of isolated point-like cold blobs
dropping into the interior, the pattern changes to
long, linear structures sliding into the interior
that look like subduction zones." Subduction zones
are places where tectonic plates dive under one another
into the mantle. "Once we included the effects of
changing viscosity, we got pretty much the Earth as
we know it," Richards says.
Their model also explains the stability of Earth's
hotspots, upwellings of hot molten rock that remain
constant for billions of years. The Hawaiian and Reunion
Islands, as well as Yellowstone and Iceland, are examples
of hot spots that have remained in the same place
for much of the Earth's history. The reason, Richards
says, is that these upwellings are rooted solidly
in the very viscous deep mantle, near where it borders
the core, and can't move.
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