IN SOME remote polar regions, the ground is decorated with mysterious circles, polygons, islands and stripes made from stones. Puzzled scientists have proposed many mechanisms to explain their origin, but no one could account for all of the different shapes.
Now Mark Kessler and his colleagues at the University of California, Santa Cruz, have developed a computer model that can reproduce all the patterns (Science, vol 299, p 380). 鈥淭he processes were identified before, but no one had successfully put them together,鈥� says Rorik Peterson of the University of Alaska, who also studies the patterns.
Starting with bare soil randomly scattered with stones, the computer model simulates hundreds of years of winter freezing and summer thawing. Kessler explains that any initial bumps in the soil grow with each cycle. As ice forms on the surface, water is sucked up from the underlying soil by capillary action and more soil is pulled upwards to take its place. As a result of this process, known as frost-heave, the bumps push upwards and displace loose stones lying on the surface. The stones collect in a circle up to two metres wide around each bump.
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Kessler has studied stone circles as they emerge on the Norwegian island of Spitsbergen. 鈥淚n World War II they created a little gravel landing strip by ploughing through a load of these stone circles,鈥� he says. More than 50 years later, the circles are just starting to regrow, but Kessler estimates that it takes up to 500 years for a pattern to form from scratch.
The more complicated patterns arise when a second mechanism interacts with the frost-heave. The stone piles are squeezed when the soil around them freezes and swells. If the squeezing is strong, avalanches of stones form lines and polygons. Different conditions create the other patterns: if the stones are sparse, they collect in islands, if the ground is sloped, the stones gather into stripes.
