Washington DC
ONE of the great mysteries of plate tectonics鈥攖he theory that the
Earth鈥檚 crust is a shifting jigsaw of rocky plates鈥攊s why the plates are
so vast. Two researchers in California now claim to have solved the puzzle by
simulating the behaviour of the Earth鈥檚 interior using a powerful
supercomputer.
Tectonic plates crawl along at a rate of several centimetres a year because
they hitch a ride on the Earth鈥檚 mantle. This layer under the crust churns
slowly along, driven by the heat of the planet鈥檚 core. The problem is that the
plates average 6000 kilometres across, yet the laws of fluid dynamics suggest
that the biggest should be only half this size. Giants like the Pacific plate,
which is 10 000 kilometres wide, just should not exist.
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Other researchers have tried鈥攁nd failed鈥攖o reproduce the mantle鈥檚
behaviour and the pattern of tectonic plates seen at the surface, using
increasingly complex computer simulations. But Mark Richards and Hans-Peter
Bunge of the University of California at Berkeley, say that the problem can be
solved by making a simple assumption about the mantle鈥攖hat it gets stiffer
with increasing depth.
The mantle, which is some 2900 kilometres thick, behaves much like water when
it is heated in a large, shallow pan. The hotter material near the bottom rises,
while the cooler material at the surface falls, creating a pattern of convection
currents. But because rock is so much more viscous than water, the currents in
the mantle take not seconds, but tens of millions of years, to rise or fall.
In normal convection systems, the currents are only about as wide as they are
deep. This means that currents in the mantle should be no more than 3000
kilometres across鈥攚hich in turn sets the maximum size for the tectonic
plates riding on the mantle.
Richards saw a possible solution to the paradox in studies of gravitational
variations over the Earth鈥檚 surface, which suggest that the viscosity of the
mantle increases by a factor of 30 or more from top to bottom鈥攚ith much of
the increase occurring at a point about 660 kilometres down. So he and Bunge,
working with a computer model originally developed by John Baumgardner of the
Los Alamos National Laboratory in New Mexico, spent three weeks running a
simulation of such a mantle on a supercomputer. It is as if the water in the
heated pan is sitting on top of a layer of syrup, Richards says.
In a paper that will appear next month in Geophysical Research
Letters, the researchers show that the change in stiffness alone made the
mantle currents almost as wide as the crustal plates. When they added the
assumption that the Earth鈥檚 crust is slightly sticky, so that pieces of it tend
to cling together, their model Earth looked almost exactly like the real thing.
鈥淭his was not an expected result,鈥� Richards says. No one had predicted that a
change in mantle stiffness would produce larger currents.
鈥淚 think it鈥檚 very interesting and very important,鈥� says Richard O鈥機onnell, a
geophysicist at Harvard University. Only recently have most researchers begun to
accept that the mantle鈥檚 viscosity may change quite sharply. But in retrospect,
he says, Richards and Bunge鈥檚 findings already seem almost obvious.