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Scientists Explain Formation Of Stone Circles

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    NHNE News List Current Members: 760 Subscribe/unsubscribe/archive info at the bottom of this message. ... SCIENTISTS EXPLAIN FORMATION OF STONE CIRCLES AND
    Message 1 of 1 , Jan 17, 2003
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      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,
      Kessler said.

      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
      those areas.

      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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