In polar landscapes, but also in other areas with intensive frost action, more or less regular patterns often develop in soils. Repeated freezing and thawing processes shift the surface sediment in such a way that fascinating shapes emerge. An international team of scientists has now found the explanation for how the rearrangement of the Earth’s surface happens in cold regions.
The research team, consisting of scientists from China, Japan, the Netherlands and the U.S., conducted a series of experiments to track the phenomenon of frost patterns in soils. In their study, published in early October in the journal Proceedings of the National Academy of Sciences, they were able to show how ice needles growing randomly on frozen ground can gradually move small stones into regular, repeating patterns. The researchers used a combination of novel experiments and computer modeling to describe these striking features with new theoretical insights.
“The presence of these amazing patterns that develop without any intervention from humans is pretty striking in nature,” said co-author Bernard Hallet, a professor emeritus of Earth and space sciences at the University of Washington and a member of the Quaternary Research Center. “It’s like a Japanese garden, but where is the gardener?”
Hallet specializes in studying the patterns that form in polar regions, high-mountain and other cold environments. One of the reasons for the patterns is needle ice. As the temperature drops, the moisture contained in the soil grows into spikes of ice crystals that protrude from the ground. “When you go out in the backyard after a freezing night and you feel a little crunch under the foot, you’re probably walking on needle ice”, Hallet said.
As needle ice forms it tends to push up soil particles and, if there are any, small stones. More needle ice can form on patches of bare soil compared to rock-covered areas, Hallet said. The ice needles will slightly displace any remaining stones in the barer region. Over years, the stones begin to cluster in groups, leaving the bare patches essentially stone-free. “That kind of selective growth involves interesting feedbacks between the size of the stones, the moisture in the soil and the growth of the ice needles,” Hallet explains.
Senior author Quan-Xing Liu at East China Normal University uses fieldwork and lab experiments to understand self-organized patterns in nature. For this study, the experimental setup was a flat square of wet soil a little over 1 foot on each side (0.4 meters) that began with stones spaced uniformly on the surface. The researchers ran the experiment through 30 freeze-thaw cycles. By the end of that time, regular patterns had started to appear
Movies of the development of a stone pattern by needle ice growth and decay at 30 percent (left) and 60 percent stone cover (right) on level ground from top view. Video: Li et al. 2021
“The videos are pretty striking, and they show that the ice just comes up and in a single cycle it pushes up stones and moves them slightly to the side,” Hallet said. “Because of those experiments and the abilities of the individuals involved to analyze those results, we have much more tangible, quantitative descriptions of these features.”
Further experiments looked at how the pattern changed depending on the concentration of stones, the slope of the ground, and the height of the ice needles, which is also affected by the stone concentration. Based on those results, the authors wrote a computer model that predicts what patterns will appear depending on the concentration of stones on the frost-prone surface.
Julia Hager, PolarJournal / Original text: Hannah Hickey, University of Washington
Link to the study: Anyuan Li, Norikazu Matsuoka, Fujun Niu, Jing Chen, Zhenpeng Ge, Wensi Hu, Desheng Li, Bernard Hallet, Johan van de Koppel, Nigel Goldenfeld, Quan-Xing Liu. Ice needles weave patterns of stones in freezing landscapes. Proceedings of the National Academy of Sciences, 2021; 118 (40): e2110670118 DOI: 10.1073/pnas.2110670118