It sounds like the stuff of science fiction: stones arranging themselves into perfect circles or elaborate labyrinths. But the forces behind these mysterious patterns, which are commonly found in many polar and high alpine environments, are much more pedestrian--simple cyclic freezing and thawing of the surrounding ground. A numerical model that can account for the unusual patterns is described in the current issue of the journal Science.

Over the years, scientists have put forth a number of explanations for the strange stone configurations, but accounting for their variety proved difficult. "The patterns form by self-organization, and the same fundamental processes are at work in the formation of all these different patterns," says Mark Kessler of the University of California at Santa Cruz, who authored the study with Brad Werner of University of California at San Diego. Their model, Kessler explains, "is essentially a hypothesis about what is important in the formation of patterned ground." The team found, using computer simulations, that the two main mechanisms are lateral sorting, which moves stones and soils to regions that have high concentrations of similar particles, and squeezing, which stretches stones into longer lines by causing movements within a pile of rocks.

Freezing and thawing of the ground influence both of those processes, and their relative strengths determine what the final pattern looks like. For instance, polygons arise when squeezing is strong enough to counteract the effects of lateral sorting. "If these patterns were on the ground around here, I think we would have figured them out a long time ago," Kessler says. "These landscapes are so amazing, it's the kind of thing that really calls out for an explanation."