Scientists Explain Formation Of Stone Circles
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SCIENTISTS EXPLAIN FORMATION OF STONE CIRCLES AND OTHER STRANGE PATTERNS IN
University of California - Santa Cruz / EurekAlert!
January 16, 2003
SANTA CRUZ, CA--Perfect circles of stones cover the ground in parts of
Alaska and the Norwegian islands of Spitsbergen. Elsewhere in the far north,
stones form other striking patterns on the ground: polygons, stripes,
islands, and labyrinths. No, pranksters are not at work in these remote
areas, nor are aliens, elves, or any other outside forces moving the stones
around. According to scientists who have studied the phenomenon, cyclic
freezing and thawing of the ground drives simple feedback mechanisms that
generate these remarkable patterns.
"The patterns form by self-organization, and the same fundamental processes
are at work in the formation of all these different patterns," said Mark
Kessler, a postdoctoral researcher in the Earth Sciences Department at the
University of California, Santa Cruz.
As a graduate student working with Brad Werner, a professor of geophysics at
UC San Diego, Kessler developed a numerical model of the processes involved
in the self-organization of these patterns, known to geologists as "sorted
patterned ground." Kessler and Werner reported their findings in a paper
published in the January 17 issue of the journal Science.
Over the years, other scientists have proposed various explanations for
these unusual patterns of stones and soil. But until now, no single
explanation has been able to account for the full range of patterns seen in
"The model I developed is essentially a hypothesis about what is important
in the formation of patterned ground," Kessler said. "When you run the model
on a computer, you can see the evolution of the pattern over time, and you
can also see how small changes in key parameters result in a transition from
one pattern to another."
According to Kessler, the patterns result primarily from the interaction of
two mechanisms: lateral sorting, which moves soil toward areas of high soil
concentration and stones toward areas of high stone concentration; and
squeezing of stone domains, which causes stones to move within linear piles
of stones and lengthens these lines of stones. The relative strengths of
lateral sorting and squeezing, plus the slope of the ground and the ratio of
stones to soil, are the factors that determine which pattern will emerge,
Driving the mechanisms of lateral sorting and squeezing is the phenomenon of
frost heave--the expansion of fine-grained soils during freezing of wet
ground. Frost heave results from the formation of discrete ice lenses in the
soil. The soil near the surface expands because water flows up through the
soil toward the ice lens as it forms (and to a lesser extent because the
water expands as it freezes).
"If you start with a random layer of stones over a layer of soil, frost
heave makes the soil layer unstable and deforms the interface between stones
and soil," Kessler said.
As an ice lens grows near this interface, it pushes outward on the stones
and also dessicates and compresses the soil below it. Where the interface
between stones and soil is inclined, this causes lateral displacement of
both stones and soil. When the ground thaws, the compressed soil reabsorbs
water and expands. But the expansion occurs vertically, so it does not
reverse the lateral displacement of soil by frost heave. Furthermore, the
greater compressibility of soil-rich areas results in soil transport toward
Other processes are also involved in lateral sorting, but the end result is
a positive feedback loop in which cycles of freezing and thawing cause
soil-rich areas to attract more soil and stone-rich areas to attract more
Once stones have been sorted into concentrated areas, or "stone domains,"
frost heave also squeezes and uplifts the stone domains. Differential uplift
causes stones to migrate along the axis of a linear stone domain and
lengthens the domain.
"The pattern depends on the relative strengths of lateral sorting, which
actually brings stones back into areas of high stone concentration, and
squeezing, which moves stones along," Kessler said. "One of the real
mysteries to me was how you can get labyrinths or islands of stones in one
location and polygons in another, when the ratio of stones to soil is the
same in both places. Our model indicates that you get polygons when the
squeezing is strong enough to counteract the effects of lateral sorting."
There are a variety of factors that can lead to differences in the relative
strengths of squeezing and lateral sorting, he said. These include the
compressibility of the soil and the size of the stones.
Another important factor is the extent to which the stone domains are
confined by the soil, which determines whether squeezing will mainly cause
stones to move along the stone domains or roll back onto the soil domains.
In their model, Kessler and Werner can vary the degree of confinement, the
concentration of stones, and the slope of the ground to produce circles,
labyrinths, islands, stripes, and polygons of stones.
The researchers compared the patterns generated by the model with those
observed in nature, using low-elevation aerial photographs of polygon
networks in Alaska. Quantitative measurements of the natural and
computer-generated polygons showed they were consistent. For example, one of
the interesting features of polygon patterns, both in the model and in
nature, is the tendency to form three-way intersections with equal angles
between the intersecting lines, Kessler said.
One reason these patterns have remained unexplained for so long may be their
occurence in remote areas, far from the temperate zone where most scientists
live, Kessler said. "If these patterns were on the ground around here, I
think we would have figured them out a long time ago. These landscapes are
so amazing, it's the kind of thing that really calls out for an
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