Pingo
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A pingo is a mound of earth-covered ice found in the Arctic, subarctic, and Antarctica that can reach up to 70 metres in height and up to 2 kilometres in diameter. The term originated as the Inuit word for a small hill. The plural is 'pingos'. Pingos are a periglacial landform, which is defined as a nonglacial landform or process linked to colder climates. Periglacial landforms often involve frozen ground.
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[edit] Locations
Tuktoyaktuk in the Mackenzie Delta of the Northwest Territories has one of the highest concentrations of pingos, with some 1,300 examples. Pingo National Landmark protects eight of these features. Other places with pingos include Alaska, Greenland, and the Norwegian island of Spitsbergen.
In Siberia, pingos are known as bulganniakh, from the Yakut language.
[edit] Types
Rounded tops are common for smaller pingos, but larger ones often have breaks in the ice at the top. These larger pingos can have craters that have cones resembling those from volcanoes. This is due to the ice breaking and from melting of the inner ice core. Beds will often dip outward from the center when they occur in stratified sand or silt, a lot like being adjacent to an intrusive body. Pingos that form in bedrock can show similar deformation. The pingo's ice core usually originates from segregation or injection of fluid water, and can be massive. Tension fractures are normal for the mound's summit, but pingo ice expansion is brief and rare. A small freshwater lake can occupy the summit where a crater has formed from the ice melting.
Pingos are generally classified as hydrostatic (closed-system) or hydraulic (open-system). Hydrostatic-system pingos result from hydrostatic pressure on water from permafrost, and commonly form in drained lakes or river channels.
Relict hydrostatic (closed-system) and hydraulic (open-system) pingos may be distinguished from each other by determining if lacustrine (lake) deposits are associated with the formation.
[edit] Formation
Pingos can only form in a permafrost environment. Evidence of collapsed pingos (ognip) in an area suggests that there was once permafrost.
Pingos usually grow only a few centimetres per year and the largest take decades or even centuries to form. The process that creates pingos is believed to be closely related to frost heaving.
Pingos are generally classified as hydrostatic (closed-system) or hydraulic (open-system). Hydrostatic-system pingos form as a result of hydrostatic pressure on water from permafrost, and commonly form in drained lakes or river channels. Permafrost rises to the drained body's former floor. Pore water is expelled in front of the rising permafrost, and the resulting pressure causes the frozen ground to rise and an ice core to form. The shape and size of a hydrostatic or closed system pingo is often similar to the body of water that it originated from. They can vary from symmetrical conical domes to asymmetric, elongate hills.
Hydraulic-system pingos result from water flowing from an outside source, subpermafrost or intrapermafrost aquifers. Hydrostatic pressure initializes the formation of the ice core as water is pushed up and subsequently freezes. Open-system pingos have no limitations to the amount of water available unless the aquifers freeze. They often occur at the base of slopes and are commonly known as Greenland type. The groundwater is put under artesian pressure and forces the ground up as it makes an expanding ice core. It is not the artesian pressure itself that forces the ground up, but rather the ice core that is being fed the water from the aquifer. These are often formed in a thin, discontinuous permafrost. These conditions allow an ice core to form, but also provide it with a supply of artesian ground water. These pingos are often oval or oblong shaped. It is still not entirely understood why open system or hydraulic pingos normally occur in unglaciated terrain.
Pingos eventually break down and collapse. The current estimate is that pingos can last about 1000 years.
[edit] History
The term pingo was first borrowed from the Inuit by the arctic botanist AE Porsild in 1938. Porsild Pingo in Tuktoyaktuk is named in his honor.
[edit] References
Easterbrook, Don. Surface Processes and Landforms. Second Edition. 1999, 1993. Prentice-Hall, inc. P. 412-416.
