Devilish polygons speak of past stress

From Minneapolis, at a meeting of the American Physical Society

The colorful names remain, such as Devil’s Postpile and Giant’s Causeway. Long abandoned, however, are fearsome notions that supernatural beings made these spectacular rock formations in California, Northern Ireland, and elsewhere.

These close-packed, orderly arrays of rock columns, some of them hundreds of meters tall, formed as lava slowly cooled and contracted, scientists believe. However, the exact mechanism that yielded shafts with polygon-shaped cross sections remains obscure.

A pair of physicists intrigued with the arrays have presented a new explanation. Computer simulations and a simple experiment with cornstarch support their view, they contend.

Alberto G. Rojo of the University of Michigan in Ann Arbor and Eduardo A. Jagla of the Abdus Salam International Center for Theoretical Physics in Trieste, Italy, draw upon earlier studies of how fractures of the lava forming these structures penetrated downward. That research indicated that as the lava cooled, cracking occurred, layer by layer, beginning at the top.

Rojo and Jagla modeled the forces on each thin layer as it cracks. Stresses applied by already fractured rock above and the unfractured material beneath the layer encourage polygon-shaped breakage patterns. That shape provides the lowest energy arrangement given the constraints, the scientists argue.

By contrast, the scientists note, rupturing of the relatively unconstrained top surface produces not polygons but fissures that usually meet at right angles. Erosion has erased such surfaces, revealing the polygons seen in rock formations today, Rojo says.

The cornstarch test, first devised by a scientist in Germany, involves mixing equal weights of the fine powder and water and then drying a half-centimeter-thick layer of the glop in a rectangular glass pan. The patchwork of polygons visible in a few days through the bottom of the pan closely matches patterns that emerge in rock and computer simulations, the researchers report.

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