Anyone who has traveled much through the Arctic has seen them: astonishingly perfect circles, straight-sided polygons, even stripes that crisscross the hillsides, all made out of stones, as though some infant giant had been pushing pebbles into pleasing arrangements. And while nobody has invoked aliens--the popular explanation for crop circles in temperate zones--people have wondered for centuries what strange force could have assembled these supernatural-looking formations.
The answer, as two California geologists finally explained in a paper that appeared in Science last week: it's nothing more complicated than freezing and thawing--the same process that causes roads to buckle in developed parts of the world. According to lead author Mark Kessler, it all starts with a field strewn randomly with rocks lying on top of soil. No field is perfectly flat, of course, and when the soil freezes in winter, any slight bump expands, pushing the rocks up and to the side. When things thaw out, though, the bump subsides straight down, so the rocks stay where they are. Next winter, the bump expands again--but since it's wider than it was the first time around, it expands even more, pushing the rocks further.
Over time, Kessler says--and computer simulations prove it--the rocks, under pressure from expanding bumps on all sides, are gradually forced into narrow strips that outline broad swaths of pure soil. At the same time, irregularities in the thickness of the rock lines are smoothed out by this continuous squeeze. Whether the final result looks like a circle or a polygon depends on how thick the initial stone layer is and how much the local soil expands when it freezes. Stripes turn up when the process happens on a hillside. No aliens necessary--and knowing how these remarkable shapes were formed doesn't take anything away from their striking beauty. --By Michael D. Lemonick