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.