Palsas & Lithalsas

Besides pingos palsas are another landform which is typical for arctic and subarctic regions. The term comes from the Lappish and Finnish word for ‘a hummock rising out of a bog with a core of ice’. Although there is no general consensus about the correct usage of the word palsa, it is usually defined as ‘a peaty permafrost mound possessing a core of alternating layers of segregated ice and peat or mineral soil material’.

Palsas are low circular or oval mounds which may rise up to 10m above the surrounding terrain, with long axes up to 100m or more. However, most palsas are considerably smaller than this. Palsas usually occur in groups or ‘fields’, with individual structures displaying different ages of formation. As palsas consist of an ice core growing within a bog or mire (a ‘palsa mire’), their superficial covering usually consists of peat. However, some palsas have a very thin peat cover, or none at all. In this case their cover usually consists of siltysediments of lacustrine or glaciomarine origin. The latter type are sometimes referred to as ‘mineral palsas’ or lithalsas (from lithic palsas). The distinction however is not always very clear, as most researchers find that the peat usually overlies at least some mineral sediments and that the ice which forms the core of the palsa mostly occurs in these sediments; hence the confusion about the terminology.

Emerging palsa in a fen near Churchill, Manitoba, Canada.
Emerging palsa in a fen near Churchill, Manitoba, Canada.

Like pingos, palsas are now considered to be true perennial permafrost mounds as they have been tentatively described in areas of continuous permafrost. Originally, palsas were regarded as marginal to permafrost as they are mainly found in (more southerly) areas of sporadic and discontinuous permafrost.

However, pingos and palsas have a fundamentally different relationship with the permafrost they are associated with, as in some areas the palsas are the permafrost, i.e. the only parts which remain permanently frozen, whereas pingos require a surrounding permafrost environment in order to grow. Another important distinction is the fact that pingos grow below the active layer — the depth over which the annual freeze-thaw cycle operates — whereas palsas grow in the active layer.

Although their morphology can be similar, the major distinction between pingos and palsas is a genetic one. Pingos rely on a hydrostatic or hydraulic pressure system which delivers groundwater to their core. Palsas, in contrast, grow through cryosuction whereby freezing at the core draws in water from the surrounding saturated material. This process results in a growing core of segregation ice inside the peat or mineral sediments, resulting in a topographic heave and ultimately the formation a palsa mound. It is sometimes suggested that after initial growth through cryosuction, percolation and freezing of meteoric water may also add to the growing ice core.

Fine-grained sediments such as silts and clays are especially susceptible to cryosuction because capillary forces are more efficient in them. Palsa growth will therefore occur most readily in water- saturated fine-grained sediments of organic (peat) or mineral origin (although the latter are certainly the most ‘frost susceptible’).

Most workers attribute the initiation of palsa formation to the presence of a discontinuous or a variable snow cover. In areas where the snow cover is thin or absent, winter frost penetrates more deeply into the soil leading to cryosuction and heaving of the surface. These more deeply frozen areas are also more likely to survive the following summer, aided by the insulating peat cover. If this cycle is repeated in consecutive winters, surface heaving will increase and a young palsa is formed. As the elevation of the palsa above the surrounding terrain increases, its surface will be more likely to be blown free of snow, providing a positive feedback for further growth. Palsas have been experimentally created in this way by clearing snow cover from a plot during three consecutive winters.

Additionally, as the elevation increases, the peat cover will become drier, which increases its insulation value and thus further protects the palsa core from summer melting.

With these differences in topography and water content also comes a change in vegetation cover. In many cases, Sphagnum moss (or bog moss) is killed or replaced by lighter coloured heath species such as lichens (e.g. Cladonia or ‘reindeer moss’), which increases the albedo of the palsa and reduces heating by solar radiation. This plant community persists until the later stages of palsa growth and ultimate collapse. However, late-stage palsas may become colonised by trees which will trap winter snow and inhibit further palsa growth through increased insulation from winter frost.

Eventually, decay of palsas is initiated by cracking of the peat or mineral sediment cover due to tensile stresses, leading to disintegration of the insulating layer into separate blocks. These blocks will start to slide down the sides of the palsa while the increasingly exposed ice core will melt away. Ultimately, as is the case with pingos, all that is left of the palsa is a depression surrounded by a rim or rampart. In some areas, repeating growth and decay of palsas has led to the creation of a complex landform assemblage of cross-cutting rim and depression relief, which can be quite similar to that produced by thermokarst processes.

Paul De Schutter

Sources:

S. D. Gurney: Aspects of the genesis, geomorphology and terminology of palsas: perennial cryogenic mounds.
Progress in Physical Geography 25,2 (2001) pp. 249260

A.Pissart: Palsas, lithalsas and remnants of these periglacial mounds. A progress report.
Progress in Physical Geography 26,4 (2002) pp. 605-621

P. Worsley et al.: Late Holocene ‘mineral palsas’ and associated vegetation patterns: a case study from Lac Hendry, Northern Québec, Canada and significance for European Pleistocene thermokarst.
Quarternary Science Reviews Vol. 14 (1995) pp. 179-192

To top