ANN ARBOR --- For centuries people have been fascinated by enormous columnar rock formations like the Devil's Postpile in California. The Celts, for example, were convinced that Giant's Causeway, in Northern Ireland, was the work of the famous giant, Finn McCool. Later, in 1693, the Philosophical Society of London heard a report from an expedition to the site, concluding that the mysterious rock formation was in fact the result of natural causes, though the authors declined to speculate on precisely which ones were involved. Modern scientists know that, in fact, the great columns of basalt were formed by cooling lava, but the actual process that produced their very regular polygonal shapes has never been completely understood.
A pair of physicists, working on the cusp between physics and geology, have developed an explanation for these splendid formations. Alberto G. Rojo of the University of Michigan and Eduardo A. Jagla of the Centro Atómico Bariloche in Argentina postulate that as the lava cooled, fractures formed at the surface and moved downward. At the top the fracture pattern looked like the familiar arrangement that we see in drying mud or in paint---a random pattern of curved lines meeting at 90-degree angles. But as the fractures moved downward they began to take the path of least resistance. The configuration that minimizes the energy required to penetrate the interior turns out to be quasi-hexagonal---a regular pattern of hexagons, pentagons, and heptagons, as in the Giant's Causeway. The results of their research will appear in the Jan. 1, 2002, issue of the American Physical Society journal, Physical Review E.
The researchers support their theory with a computer model of the process, along with a simple cornstarch experiment that anyone can do at home. Mix equal amounts of cornstarch and water and spread the mixture 1-2 cm thick on a flat, glass-bottomed surface, like a Pyrex pie-plate. After a few hours, as the mixture dries, it will start fracturing. First a series of large cracks will form from top to bottom. Then a small, irregular array of fractures will be seen at the top surface. But after a few days, when you turn the plate over, you will see that by the time they have reached the bottom, the small cracks have formed a patchwork, quasi-hexagonal pattern. Keeping the dish warm (under a 40W light bulb, for instance) helps the process along.
"Many scientists believe that the cutting edge of physics exists only at the very small scale (string theory, for example) or the very large (cosmology and astrophysics)," Rojo says. "I disagree. Many fundamental open questions and mysteries still remain at the scale of our everyday experience. Patterns, both in inanimate and in living natural objects represent just one of those questions."
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