Palsa

Palsa (Plural: Palsas, also Palsen, from Finnish "palsa", which in turn is a borrowing from Northern Sami "balsa"), designates a low oval elevation in areas with Permafrost, frequently peat bogs, where a perennial ice lens has developed within the soil. A palsa consists, similarly to a pingo, of an ice core and overlying soil materials, in case of a palsa usually boggy soil. Palsas are characteristically found in areas with discontinuous permafrost and in such areas may be the only reliable surface evidence of permafrost. A palsa needs large quantities of water for the formation of its ice core (i.e. ice lens), which is why they frequently emerge from and are bound by boggy soils, which can store enormous quantities of water in their pores. Palsas develop particularly in moorlands and are therefore also named "palsamoors". "Bugor" and "Bulginniakhs" are general terms in Russian (the latter of Yakutian origin) for both palsas and pingos.

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 snow. 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 vegetatino will then be such as to preserve the newly-formed permafrost.

The overlying soil layer is gradually lifted up by the progressive volumetric expansion of the freezing ice. In cross-section, the ice cores of a palsa show layering, which is caused by the successive winter freezing intervals. In case of palsas, 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.

Locales

Palsas are typical forms of permafrost regions and are therefore found in Subarctic regions of Northern America and Alaska, Siberia, Northern Scandinavia, Greenland and Iceland, Spitsbergen, and areas of the Antarctic. 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.

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.

Morphology

Palsa forms includ emounds, 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 7 m, or about 10 m at a maximum. 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.

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.

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.

Differences and Commonalities between Pingos and Palsas

Palsas are morphologically and genetically 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. Moreover, contrary to pingos which are usually isolated, palsas usually arise in groups with other palsas, such as in a so-called palsamoor.

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

References

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Websites

Pictures of Palsas and further information:
* http://www.isgs.uiuc.edu/shilts/key_pinpal.htm
* http://www.geo.wvu.edu/~kite/Geol221_2001Lect22Periglacial/sld061.htm
* http://www.cits.rncan.gc.ca/guide/bog_pals/pg07_e.html