Pingos are intrapermafrost ice-cored hills, high and in diameter. They are typically conical in shape and grow and persist only in permafrost environments, such as the Arctic and subarctic. A pingo is a periglacial landform, which is defined as a non-glacial landform or process linked to colder climates. It is estimated that there are more than 11,000 pingos on Earth. The Tuktoyaktuk peninsula area has the greatest concentration of pingos in the world with a total of 1,350 pingos. There is currently remarkably limited data on pingos.
In 1825, John Franklin made the earliest description of a pingo when he climbed a small pingo on Ellice Island in the Mackenzie Delta. However, it was in 1938 that the term pingo was first borrowed from the Inuvialuit by the Arctic botanist Alf Erling Porsild in his paper on Earth mounds of the western Arctic coast of Canada and Alaska. Porsild Pingo in Tuktoyaktuk is named in his honour. The term pingos, which in Inuvialuktun means conical hill, has now been accepted as a scientific term in English-language literature.
Permafrost
Pingos can only form in a permafrost environment. Evidence of collapsed pingos in an area suggests that there was once permafrost. Pingos can collapse due to the melting of the supporting ice and give rise to a depression in the landscape showing an inverse shape (horizontal mirror).
Closed systems, also known as hydrostatic pingos are formed as a result of hydrostatic pressure that has built up within the core of pingos due to water. They occur in regions of continuous permafrost where there is an impermeable ground layer. These pingos are found in flat, poorly drained areas with limited groundwater available such as shallow lakes and river deltas. The formation of these landforms occurs when layers of permafrost generate an upwards movement or pressure, resulting in masses of confined soil freezing, which pushes material upwards due to expansion.
The figure above illustrates this process and the changes that occur throughout the year.
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Palsas are peat mounds with a permanently frozen peat and mineral soil core. They are a typical phenomenon in the polar and subpolar zone of discontinuous permafrost. One of their characteristics is having steep slopes that rises above the mire surface. This leads to the accumulation of large amounts of snow around them. The summits of the palsas are free of snow even in winter, because the wind carries the snow and deposits on the slopes and elsewhere on the flat mire surface.
Frost heaving (or a frost heave) is an upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface, upwards from the depth in the soil where freezing temperatures have penetrated into the soil (the freezing front or freezing boundary). Ice growth requires a water supply that delivers water to the freezing front via capillary action in certain soils. The weight of overlying soil restrains vertical growth of the ice and can promote the formation of lens-shaped areas of ice within the soil.
Periglaciation (adjective: "periglacial", referring to places at the edges of glacial areas) describes geomorphic processes that result from seasonal thawing and freezing, very often in areas of permafrost. The meltwater may refreeze in ice wedges and other structures. "Periglacial" originally suggested an environment located on the margin of past glaciers. However, freeze and thaw cycles influence landscapes also outside areas of past glaciation. Therefore, periglacial environments are anywhere when freezing and thawing modify the landscape in a significant manner.
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