Often, soils from cold regions, such as Arctic soils, show polygonal forms in their surface. These polygons are formed because of the freeze-thaw cycle, characteristic of permafrost.
What is permafrost?
Permafrost is a subsurface soil layer which stays permanently frozen (below 0 oC) during long periods of time, usually more than two consecutive years.
Most extensive permafrost areas can be found in circumpolar areas from North America (Canada and Alaska), Asia (Siberia), Europe (Norwich), cold continental areas (Tibet) and some islands (South Georgia and the Sandwich Islands, in the Atlantic Ocean). And on Mars!
The soil layer above the permafrost (known as the active layer) is the part of soil that thaws during the warm season and freezes again at the beginning of the cold season, because the influence of air temperature is greater in the first centimeters of soil. Commonly, the thickness of the active layer may vary between 10 and 100 cm depending on the season, aspect, vegetation, soil texture and proximity to water bodies.
How do polygons form?
Cryoturbation is one of the main processes in the soil active layer. As a consequence, the soil surface in these cold areas often show polygons, circles, steps and stripes formed by stones and fine sediments.
Repeated groundwater freezing/thawing cycles causes contraction/expansion of soil material, that forces the displacement of coarse gravels and stones over the soil surface. Areas with fine sediments (with low porosity) show larger water contents than those areas where coarse fragments accumulate.
As a result, water-saturated, finely-textured soil expands and contracts more easily during freezing/thawing cycles than coarsely textured stony areas. In the long term, stone polygons and other patterns may appear on the soil surface. Fine sediments in the center of polygons usually form ponds and small bogs.
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