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Pingos are intrapermafrost ice-cored hills, ranging in height from 3m to 70m and 30m to 1000m 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 1350 pingos. There is currently remarkably limited data on pingos.

History
In 1825, John Franklin made the earliest description of a pingo when he climbed a small pingo on Ellice Island, 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 honor. The term pingos, which in the Eskimo language means conical hill, has now been accepted as a scientific term in English-language literature.

Hydrostatic Pingos
Closed systems, also known as hydrostatic pingos are formed as a result of hydrostatic pressure 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 generates an upwards movement of pressure, resulting in masses of confined soil freezing which push material upwards that begins to expand.

The figure below illustrates this process and the changes that occur throughout the year. This type of closed system pingos is formed in an area where a lake has been infilled with sediment. This has meant that the ground is insulated, allowing liquid water to collect underneath the sediment. In winter months this sediment begins to freeze which leads to expansion of sediment, confining the water and increasing the pressure. This results in the formation of a mound due to the upwards pressure. However, during summer months the ice core of the pingo beings to melt which causes the mound to cave in.

Hydraulic Pingos
Hydraulic (open-system) pingos result from groundwater flowing from an outside source, i.e. sub-permafrost or intra-permafrost 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. If water pressure entering an artesian pingo is strong enough, it can lift the pingo up allowing a sub-pingo water lens to form underneath. However, if the water lens starts to leak water it can cause subsidence which can compromise the structure. 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 usually grow only a couple of 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. The base of the pingo tends to reach its maximum diameter in its early youth. This means pingos tend to grow higher rather than growing in diameter and height at the same time. The height of pingos can range anywhere from 3 to 70 metres and their diameters range from 30 to 1000 metres. The shape of pingos in usually circular. Smaller pingos tend to have curved tops whereas larger pingos usually have collapsed mounds or craters due to the melting of exposed ice.

Europe
Currently, there are no known pingos on European mainland. However, many pingos do exist in Greenland. Greenland is politically and culturally affiliated with Europe, however some refer to it as a geographical entity of North America. Within the realm of pingos, most scholars associate the pingos in Greenland with European pingo.

The landscape of Greenland contains a lot of pingos and other glacial landforms because until 18,000 years ago the area was covered in glaciers. In Western Greenland it is estimated that there are 29 pingos, whilst in Eastern Greenland it is estimated there are 71 pingos. The majority of pingos in Greenland are located within Disko Bay and Nugssuaq Peninsula within Western Greenland as well as some in Eastern Greenland in Mesters Vig. The thickness of the permafrost at Disko Bay is around 150 metres deep, making it ideal for closed system pingos to develop. There are 20 pingos located on Disko Island, with the largest located on Kuganguaq alluvial plain at 100 metres wide and 15 metres high.

Meanwhile, in Eastern Greenland the most famous pingos can be found in Nioghalvfjerdsfjorden. They are well known as they are the most Northern pingos to be found in Eastern Greenland. The largest of these pingos is 100 metres wide and 8 metres high, taking the shape of a semicircle. This pingo is still active, meaning it is increasing in elevation over time.

Tuktoyaktuk Peninsula Pingos
The Tuktoyaktuk Peninsula is an area with a marine tundra environment on the Northwest coast of Canada. This peninsula is covered in thick permafrost, which is known to be more than 50,000 years old. There are many pingos within this region, all ranging in size and diameter. The most well known pingo in this area is Ibyuk Pingo, which is the tallest pingo in Canada. The height of this pingo is 50 metres above sea level, but the pingo is still increasing in height by a few centimetres every year. This pingo is one of the younger pingos in the area, estimated at around 1,000 years old.

Alaska
Approximately 80% of Alaska is covered in permafrost, with 29% of this continuous permafrost, 35% discontinuous permafrost and the rest sporadic or isolated permafrost. Throughout Alaska, there are more than 1,500 known pingos with the majority being open system pingos. The height of pingos in Alaska ranges from 3 to 54 metres in height and 15 to 450 metres in width. The world’s tallest pingo is located in Alaska, known as the Kadleroshilik Pingo. The Kadleroshilik Pingo is 54 metres in height, but is continuing to rise in elevation by a few centimetres a year.

Siberia and Asia
In Siberia, an area containing a high density of close system pingos can be found near Yakutsk located on the Lena River. In this area there are more than 500 pingos next to the Lena River. The area is comprised of alluvial plains areas of thick permafrost, allowing pingos to form and develop.

Areas of central Asia are known to have pingos at the highest elevations in the world. For example, the Tibetan Plateau has pingos at above 4000 metres in elevation due to its permanently frozen terrain. This environment is perfect for pingo production, and the cold, dry permafrost along with cold temperatures deters pingo collapse.

Outer Space- Mars
Although no pingos have been confirmed to be located on Mars, scholars agree that there are indisputable signs of pingo-like features (PLFs). PLFs are periglacial features which have been discovered but are usually not classed as pingos. This is usually because they are not large enough to be classified as pingos, or there is not enough evidence to class them as pingos.

Differences Between Pingos and Palsas
Palsas are structurally similar to pingos; however, with heights between approx. 0.5 and 2 m and lengths between approx. 5 and 25 m, palsas are significantly smaller than pingos. Both, however, are considered to be true perennial permafrost mounds since both occur in areas of continuous permafrost. Moreover, contrary to pingos which are usually isolated, palsas usually arise in groups with other palsas, such as in a so-called palsa bog. Unlike pingos, palsas do not require surrounding permafrost to grow, seeing as palsa are permafrost. Pingos also grow below the active layer, which is the depth that the annual freeze-thaw cycle occurs, and palsa grow in the active layer.

Both palsas and pingos result from freezing of water to an ice core. Palsas, however, do not necessarily require positive hydrostatic pressure (to inject water), since the boggy soil is water-saturated and therefore has sufficient supply for the growing ice core.

Effects of Climate Change
Global warming is causing Arctic temperatures to rapidly rise, causing permafrost  to thaw. For this reason, permafrost environments are extremely vulnerable to climate change. Permafrost degradation caused by climate warming is indicated by increased mean annual ground temperature, increased active layer thickness, talik and thermokast development and disappearance of permafrost islands. The interchange between permafrost degradation and aggradation shapes sub-Arctic and Arctic lowland landscapes, and therefore contain records of past climate and landscape development.

Pingos are vulnerable to surface disturbance given the considerable amount of ground ice stored within them. Abrupt permafrost thaw processes can cause ice wedges within pingos to melt, which can result in increased pingo collapse and the formation of remnant lakes. However, there are currently few studies investigating how climate change could affect formation and growth of pingos.

Glossary
Active layer- the top layer of the ground, in an area covered by permafrost, which is subject to annual patterns of thawing and freezing

Freezeback- refreezing of previously thawed materials

Ice Pingo- a huge mass of ice forming the core of a pingo structure

Mass wasting- the downslope movement of soil or rock material, driven by gravity

Palsa- low permafrost mounds with cores of layered peat and segregated ice

Periglacial- conditions, landforms and processes which are associated with cold but non-glacial environments

Permafrost- the ground layer, which can be soil or rock, that remains at or below 0 degrees Celsius for at least two consecutive years

Pingo- a perennial mound of frost consisting of a core of ice which was produced by the injection of water, but is now covered in soil and vegetation

Pingo (closed system)- pingos formed in a cold environment in a zone of continuous permafrost

Pingo (open system)- pingos formed in boreal forest environments in a zone of discontinuous permafrost

Talik- a layer of unfrozen ground within a permafrost area

Tundra- a treeless terrain with a continuous vegetation cover which can be found in high latitudes and high altitudes