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Patterned ground is the distinct, and often symmetrical geometric shapes formed by ground material in periglacial regions. Typically found in remote regions of the Arctic, Antarctica, and the Australian outback, but also found anywhere that freezing and thawing of soil alternate; patterned ground has also been observed on Mars. The geometric shapes and patterns associated with patterned ground are often mistaken as artistic human creations. The nature of patterned ground had long puzzled scientists but the introduction of computer-generated geological models in the past 20 years has allowed scientists to relate the formation of these features to phenomena associated with frost heaving, which refers to expansion that occurs when wet, fine-grained, and porous soils freeze.
Types of patterned ground
Patterned ground can be found in a variety of forms. Typically, the type of patterned ground in a given area is related to the prevalence of larger stones in local soils and the frequency of freeze-thaw cycles.
Polygons can form either in permafrost areas or in areas that are affected by seasonal frost. The rocks that make up these raised stone rings typically decrease in size with depth. In the northern reaches of the Canadian Boreal forests, when bogs reach a eutrophic climax and create a sedge mat, Tamarack Larch and Black Spruce are often the early colonists within such a polygonal climax sedge mat.
Circles range in size from a few centimeters to several meters in diameter. Circles can consist of both sorted and unsorted material, and generally occur with fine sediments in the center surrounded by a circle of larger stones. Unsorted circles are similar, but rather than being surrounded by a circle of larger stones, they are bounded by a circular margin of vegetation.
Steps can be developed from circles and polygons. This form of patterned ground is generally a terrace-like feature that has a border of either larger stones or vegetation on the downslope side, and can consist of either sorted or unsorted material.
Stripes are lines of stones, vegetation, and/or soil that typically form from transitioning steps on slopes at angles between 2° and 7°. Stripes can consist of either sorted or unsorted material. Sorted stripes are lines of larger stones separated by areas of smaller stones, fine sediment, or vegetation. Unsorted stripes typically consist of lines of vegetation or soil that are separated by bare ground.
Formation of patterned ground
As water freezes, it expands and takes up about 10% more volume. This expansion generates enough force to transform small highway cracks into potholes and to break apart enormous boulders along fractures in the rock through the process known as ice wedging. Pressures associated with ice wedging are known to reach nearly 30,000 lbf/in² (200 MPa), a pressure close to that required to crush granite.
In periglacial areas and areas affected by seasonal frost, repeated freezing and thawing of groundwater forces larger stones toward the surface as smaller soils flow and settle underneath larger stones (see "Cryoturbation"). At the surface, areas that are rich in larger stones contain much less water than highly porous areas of finer grained sediments. These water saturated areas of finer sediments have a much greater ability to expand and contract as freezing and thawing occur, leading to lateral forces which ultimately pile larger stones into clusters and stripes. Through time, repeated freeze-thaw cycles smooth out irregularities and odd-shaped piles to form the common polygons, circular, and stripes of patterned ground.
Frost also sorts the sediments in the ground. Once the mantle has been weathered, finer particles tend to migrate away from the freezing front, and larger particles migrate through the action of gravity.
Patterned ground forms mostly within the active layer of permafrost.
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- Media related to Patterned ground at Wikimedia Commons