Palsa
Encyclopedia
Palsas are low, often oval, frost heaves
occurring in polar
and subpolar climates, which contain permanently frozen
ice lenses
. Like pingo
s, palsas consist of an ice core with overlying soil, but they are generally smaller than pingos, often occur in groups and may develop from ground water without additional hydrostatic pressure. Palsas are characteristically found in areas with discontinuous permafrost, and in such areas they may be the only reliable surface evidence of permafrost. Palsas need large quantities of water for the formation of their ice lenses, and for this reason they occur particularly in bog
s.
. The lack of thermal insulation provided by thick snow permits much deeper freezing in winter. This ice may then last through the summer with a persistent 'bump' of up to several cm due to frost heave. The elevated surface of a palsa will tend also to have thinner snow cover, allowing greater winter cooling, while in summer the surface material (especially if organic) will dry out and provide thermal insulation. Thus the interior temperature is consistently lower than that of adjacent ground. This contributes to the formation of an ice lens which grows by drawing up surrounding water. The expansion of the ice upon freezing exerts pressure on the surrounding soil, further forcing water out of its pore spaces which then accumulates on and increases the volume of the growing ice lens. A positive feedback
loop develops. Changes in surface moisture and vegetation will then be such as to preserve the newly-formed permafrost.
The overlying soil layer is gradually lifted up by frost heaving
. In cross-section, the ice cores of a palsa show layering, which is caused by the successive winter freezing intervals. The pressing out of water from the pores is not crucial, however, since the boggy soil is water-saturated and thus always provides enough water for ice core growth.
Palsas appear to go through a developmental cycle that eventually leads to thawing and collapse. Open cracks that commonly accompany palsa growth and the water that tends to accumulate around palsas, probably as a result of their weight depressing the adjacent bog surface, are important factors in this process. The fact that palsas in various stages of growth and decay occur together shows that their collapse is not necessarily indicative of climatic change. All that is usually left after a palsa collapses is a depression surrounded by a rim.
Palsa forms include mounds, moderately straight ridges, and winding ridges. Palsas in Iceland have been described as hump shaped, dike shaped, plateau shaped, ring shaped, and shield shaped, those in Norway as palsa plateaus, esker palsas, string palsas, conical ore dome-shaped palsas, and palsa complexes.
Widths are commonly 10–30 m, and lengths 15–150 m. However, lengths of up to 500 m have been reported for esker-like palsa ridges running parallel to the gradient of a bog
. Heights range from less than 1 m to about 10 m at a maximum above the surrounding area. Large forms tend to be considerably less conical than small ones. In places, palsas combine to form complexes several hundred meters in extent. The permafrost core contains ice lenses no thicker than 2–3 cm, though locally lenses up to almost 40 cm thick have been described.
Palsas form in groups or "fields" and the individual palsa can show its age difference from the others.
Palsa surfaces are frequently traversed by open cracks, caused by doming (dilation cracking), frost cracking, or desiccation. The vegetation of a palsa may comprise low shrubs and lichen in addition to the sedges characterizing the peat. Also, the higher a palsa grows the dryer the peat covering the palsa becomes leading to more insulation which protects the inner core from melting.
There are two types of palsas, those with a peat core and those with a core of mineral soil, usually silt
. The peat
-core type is the most common, the other being regarded as exceptional by some, but more common than formerly thought.
regions and are therefore found in Subarctic
regions of northern Canada
and Alaska
, Siberia
, northern Fennoscandia
and Iceland
. They are almost exclusively associated with bogs and commonly occur in areas where the winters are long and the snow cover tends to be thin. In some places palsas extent into underlying permafrost; in others they rest on an unfrozen substratum.
In the southern hemisphere palsa remains from the last glacial maximum
have been identified on the Argentine side of Isla Grande de Tierra del Fuego
just north of Cami Lake. Remainders of Ice-Age palsas are to be found also in Hochmooren of Central Europe, such as Hohen Venn in the German-Belgian border area.
s, 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 palsamoor. 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.
meaning "a hummock rising out of a bog with a core of ice," which in turn is a borrowing from Northern Sami
, balsa. As palsas particularly develop in moorland
s they are therefore also named palsamoors. Bugor and bulginniakhs are general terms in the Russian language
(the latter of Yakut
ian origin) for both palsas and pingos.
Frost heaving
Frost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...
occurring in polar
Polar climate
Regions with a polar climate are characterized by a lack of warm summers . Regions with polar climate cover over 20% of the Earth. The sun shines 24 hours in the summer, and barely ever shines at all in the winter...
and subpolar climates, which contain permanently frozen
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
ice lenses
Ice lens
An Ice Lens or ice lenses are formed when moisture, diffused within soil or rock, accumulates in a localized zone. The ice initially accumulates within small collocated pores or pre-existing crack, and, as long as the conditions remain favorable, continues to collect in the ice layer or ice lens,...
. Like pingo
Pingo
A pingo, also called a hydrolaccolith, is a mound of earth-covered ice found in the Arctic and subarctic that can reach up to in height and up to in diameter. The term originated as the Inuvialuktun word for a small hill. A pingo is a periglacial landform, which is defined as a nonglacial...
s, palsas consist of an ice core with overlying soil, but they are generally smaller than pingos, often occur in groups and may develop from ground water without additional hydrostatic pressure. Palsas are characteristically found in areas with discontinuous permafrost, and in such areas they may be the only reliable surface evidence of permafrost. Palsas need large quantities of water for the formation of their ice lenses, and for this reason they occur particularly in bog
Bog
A bog, quagmire or mire is a wetland that accumulates acidic peat, a deposit of dead plant material—often mosses or, in Arctic climates, lichens....
s.
Genesis
Palsas may be initiated in areas of a moor or bog where the winter freezing front penetrates relatively faster than surrounding areas, perhaps due to an unusually thin cover of snowSnow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
. The lack of thermal insulation provided by thick snow permits much deeper freezing in winter. This ice may then last through the summer with a persistent 'bump' of up to several cm due to frost heave. The elevated surface of a palsa will tend also to have thinner snow cover, allowing greater winter cooling, while in summer the surface material (especially if organic) will dry out and provide thermal insulation. Thus the interior temperature is consistently lower than that of adjacent ground. This contributes to the formation of an ice lens which grows by drawing up surrounding water. The expansion of the ice upon freezing exerts pressure on the surrounding soil, further forcing water out of its pore spaces which then accumulates on and increases the volume of the growing ice lens. A positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
loop develops. Changes in surface moisture and vegetation will then be such as to preserve the newly-formed permafrost.
The overlying soil layer is gradually lifted up by frost heaving
Frost heaving
Frost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...
. In cross-section, the ice cores of a palsa show layering, which is caused by the successive winter freezing intervals. The pressing out of water from the pores is not crucial, however, since the boggy soil is water-saturated and thus always provides enough water for ice core growth.
Palsas appear to go through a developmental cycle that eventually leads to thawing and collapse. Open cracks that commonly accompany palsa growth and the water that tends to accumulate around palsas, probably as a result of their weight depressing the adjacent bog surface, are important factors in this process. The fact that palsas in various stages of growth and decay occur together shows that their collapse is not necessarily indicative of climatic change. All that is usually left after a palsa collapses is a depression surrounded by a rim.
Morphology
Widths are commonly 10–30 m, and lengths 15–150 m. However, lengths of up to 500 m have been reported for esker-like palsa ridges running parallel to the gradient of a bog
Bog
A bog, quagmire or mire is a wetland that accumulates acidic peat, a deposit of dead plant material—often mosses or, in Arctic climates, lichens....
. Heights range from less than 1 m to about 10 m at a maximum above the surrounding area. Large forms tend to be considerably less conical than small ones. In places, palsas combine to form complexes several hundred meters in extent. The permafrost core contains ice lenses no thicker than 2–3 cm, though locally lenses up to almost 40 cm thick have been described.
Palsas form in groups or "fields" and the individual palsa can show its age difference from the others.
Palsa surfaces are frequently traversed by open cracks, caused by doming (dilation cracking), frost cracking, or desiccation. The vegetation of a palsa may comprise low shrubs and lichen in addition to the sedges characterizing the peat. Also, the higher a palsa grows the dryer the peat covering the palsa becomes leading to more insulation which protects the inner core from melting.
There are two types of palsas, those with a peat core and those with a core of mineral soil, usually silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
. The peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
-core type is the most common, the other being regarded as exceptional by some, but more common than formerly thought.
Permafrost plateau
Permafrost plateaus are structures of coalesced palsas that form a continuous elevated flatland area usually in a peat bog. Permafrost plateaus may have parts suffering decay while other growing at the same time. Permafrost plateaus are usually surrounded by wetlands but have often internal water pools during summer due to uneven relief, partial decay and thaw.Geographic distribution
Palsas are typical forms of the discontinuous permafrost zonePermafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
regions and are therefore found in Subarctic
Subarctic
The Subarctic is a region in the Northern Hemisphere immediately south of the true Arctic and covering much of Alaska, Canada, the north of Scandinavia, Siberia, and northern Mongolia...
regions of northern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
, Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
, northern Fennoscandia
Fennoscandia
Fennoscandia and Fenno-Scandinavia are geographic and geological terms used to describe the Scandinavian Peninsula, the Kola Peninsula, Karelia and Finland...
and Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
. They are almost exclusively associated with bogs and commonly occur in areas where the winters are long and the snow cover tends to be thin. In some places palsas extent into underlying permafrost; in others they rest on an unfrozen substratum.
In the southern hemisphere palsa remains from the last glacial maximum
Last Glacial Maximum
The Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
have been identified on the Argentine side of Isla Grande de Tierra del Fuego
Isla Grande de Tierra del Fuego
Isla Grande de Tierra del Fuego is an island near the southern tip of South America from which it is separated by the Strait of Magellan...
just north of Cami Lake. Remainders of Ice-Age palsas are to be found also in Hochmooren of Central Europe, such as Hohen Venn in the German-Belgian border area.
Differences and commonalities between pingos and palsas
Palsas are morphologically and genetically similar to pingoPingo
A pingo, also called a hydrolaccolith, is a mound of earth-covered ice found in the Arctic and subarctic that can reach up to in height and up to in diameter. The term originated as the Inuvialuktun word for a small hill. A pingo is a periglacial landform, which is defined as a nonglacial...
s, 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 palsamoor. 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.
Terminology and synonyms
Palsa (plural: palsas or palsen) is a term from the Finnish languageFinnish language
Finnish is the language spoken by the majority of the population in Finland Primarily for use by restaurant menus and by ethnic Finns outside Finland. It is one of the two official languages of Finland and an official minority language in Sweden. In Sweden, both standard Finnish and Meänkieli, a...
meaning "a hummock rising out of a bog with a core of ice," which in turn is a borrowing from Northern Sami
Northern Sami
Northern or North Sami is the most widely spoken of all Sami languages. The speaking area of Northern Sami covers the northern parts of Norway, Sweden and Finland...
, balsa. As palsas particularly develop in moorland
Moorland
Moorland or moor is a type of habitat, in the temperate grasslands, savannas, and shrublands biome, found in upland areas, characterised by low-growing vegetation on acidic soils and heavy fog...
s they are therefore also named palsamoors. Bugor and bulginniakhs are general terms in the Russian language
Russian language
Russian is a Slavic language used primarily in Russia, Belarus, Uzbekistan, Kazakhstan, Tajikistan and Kyrgyzstan. It is an unofficial but widely spoken language in Ukraine, Moldova, Latvia, Turkmenistan and Estonia and, to a lesser extent, the other countries that were once constituent republics...
(the latter of Yakut
Yakut
Yakut may refer to:* Yakuts, the Turkic people associated with the Sakha Republic* Yakut language, a Turkic language also known as Sakha.* Ruby in Turkish language* Yakut , a breed from Russia*Yakut Pony, horse breed from Siberia, Russia...
ian origin) for both palsas and pingos.
See also
- CryoturbationCryoturbationIn gelisols , cryoturbation refers to the mixing of materials from various horizons of the soil right down to the bedrock due to freezing and thawing.Cryoturbation occurs to varying degrees in most gelisols...
- Frost heavingFrost heavingFrost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...
- GelifluctionGelifluctionGelifluction, very similar to solifluction, describes the seasonal freeze-thaw action upon waterlogging topsoils which induces downslope movement. Gelifluction is prominent in periglacial regions where snow falls during six to eight months of the year...
- Gelisol
- Ice wedgeIce wedgeAn ice wedge is a crack in the ground formed by a narrow or thin piece of ice that measures up to 3-4 metres wide at ground level and extends downwards into the ground up to several metres. During the winter months, the water in the ground freezes and expands...
- Patterned groundPatterned groundPatterned ground is a term used to describe the distinct, and often symmetrical geometric shapes formed by ground material in periglacial regions...
- ParaglacialParaglacialParaglacial means unstable conditions caused by a significant relaxation time in processes and geomorphic patterns following glacial climates. Rates of landscape change and sediment output from the system are typically elevated during paraglacial landscape response.When a large mass of ice melts,...
- PeriglacialPeriglacialPeriglacial is an adjective originally referring to places in the edges of glacial areas, but it has later been widely used in geomorphology to describe any place where geomorphic processes related to freezing of water occur...
- PermafrostPermafrostIn geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
- PingoPingoA pingo, also called a hydrolaccolith, is a mound of earth-covered ice found in the Arctic and subarctic that can reach up to in height and up to in diameter. The term originated as the Inuvialuktun word for a small hill. A pingo is a periglacial landform, which is defined as a nonglacial...
- SolifluctionSolifluctionIn geology, solifluction, also known as soil fluction, is a type of mass wasting where waterlogged sediment moves slowly downslope, over impermeable material. It occurs in periglacial environments where melting during the warm season leads to water saturation in the thawed surface material ,...
Further reading
External links
Pictures of palsas and further information:- http://www.isgs.illinois.edu/shilts/shilts-palsa.shtmlPalsa, a Fennoscandian term for a round or elongated hillockHillockA hillock or knoll is a small hill, usually separated from a larger group of hills such as a range. Hillocks are similar in their distribution and size to small mesas or buttes. The term is largely a British one...
or mound, maximum height of about 10 m, composed of a peatPeatPeat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
layer overlying mineral soil.] - William W. Shilts Geologic Image Gallery (Illinois State Geological Survey)
- Field trip guide on periglacial (cryogenic) geomorphology (html)
- Field trip guide on periglacial (cryogenic) geomorphology (pdf)
- Interpretation guide of natural geographic features: Palsa bog (index)
- Interpretation guide of natural geographic features: Palsa bog (aerial photographs)
- €U(RO)CK article from a 2005 issue