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. 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. Water percolating through the soil builds up underneath blocks. When it freezes, blocks are pushed up towards the surface. When the soil thaws, the blocks do not return to their original location because finer particles fill in voids. The process may continue until the blocks are completely unearthed.
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