Sea Floor Tectonics in a Tub

By slowly pulling apart a rind of wax atop a liquid layer, researchers have duplicated geologic patterns seen at Earth's mid-ocean ridges. Here, a spiral pattern arises at an offset along the wax rifting zone, similar to small rotating "microplate

ATLANTA--It's easy to wax philosophic about the beauty of patterns in nature. It's harder to mimic those patterns in a tub of wax. Yet that's just what researchers have done, using a seemingly simple laboratory model of rifting on the seafloor. The slabs of wax split apart in ways that resemble jagged faults and other shapes where new slabs of Earth's crust are born, according to work presented here yesterday at a meeting of the American Physical Society.

The seafloor forms along great cracks in Earth's skin called mid-ocean ridges. Molten rock wells up from below, hardens, and creeps from the ridges at a few millimeters per year. Geologists can't watch that process, but they can map the patterns it forges on the ocean bottom. In 1972, scientists reported seeing something similar to the clearest patterns, so-called "transform faults" that make staircase-shaped cracks, by stretching a tray of melted wax that was hardened gradually by a fan. But no one tried to see whether a tub of wax could spawn more complex, Earth-like structures.

Now, a team led by geomorphologist Eberhard Bodenschatz of Cornell University in Ithaca, New York, has revisited the problem. The team uses a tub of paraffin 3 feet long, 1 foot wide, and 4 inches deep. Cold air freezes a rind on top of the molten wax. Then, mechanical arms at either end rift it apart at rates ranging from 1 to 100 micrometers per second. A laser probe reveals the shapes and heights of the resulting patterns.

Slow rifting speeds carve a trough along the divide, while faster speeds leave a raised ridge, just as seen in the ocean. Transform faults arise spontaneously, as do curiously rotating spirals of wax at intersections of the jagged faults. Those whorls look just like odd 100-kilometer-wide bits of Earth's crust called "microplates," Bodenschatz says. "The Earth might be different in detail, but nature creates similar patterns in both systems," he says. "The physics is too complex for computer models to reproduce."

Geophysicists greet the work skeptically, Bodenschatz says, until they see movies of his wax in action. "Then their positions change dramatically," he smiles. Nonlinear dynamicist Daniel Lathrop of the University of Maryland, College Park, agrees. "It's exciting and visual research," he says. "It has the potential to teach us about the material properties and dynamics of the crust."