Periglacial landforms
177
2.1. Palsas
These mounds occur in boggy areas with discontinuous or sporadic permafrost (Zoltai,
1971). Their surface is criss-crossed with open fissures caused by dilation or desiccation
(Embleton and King, 1975b). At the present time they are occur in areas with a mean
annual temperature below 0~ as in Canada, Iceland, Sweden, and alpine regions
(Kershaw and Gill, 1979). Palsas may be isolated or form in groups and are differentiated
based on the morphology of dome-shaped, ribbon-shaped, and platform palsas. The former
ones are between 0.5 and 7 m in height, 10 to 30 m wide and 15 to 150 m long. The larger
palsas tend to be less conical than the smaller ones. The ribbon-shaped palsas have a lower
height and may be parallel or perpendicular to the contour lines, reaching several hundred
meters in length. The platform palsas rise 1 to 1.5 m above the surrounding bog and may
cover several square kilometres. Their generation is related to differential thawing.
Finally, the paisa complexes are formed by several types of palsas at different growth
stages and show numerous closed depressions generated by thawing (Sepp/il/i, 1988).
Some authors use the term string bogs to designate the association of different types of
palsas.
Based on their internal structure, two types of palsas are differentiated. The palsas with
a peat core have an outer layer of peat that thaws and dries in summer. Around 80 to 90%
of the core is composed ice in small crystals and lenses of segregated ice whose thickness
increases with depth from 5 to 10 mm to 5 to 10 cm. The base is generally formed by siltsized mineral particles. The silt core palsas also have an outer peat layer and a silt and clay
core with ice lenses and veins with a reticular arrangement (Sepp~il/i, 1988). Lithalsas are
similar mounds, but without any peat cover (Harris, 1998). We can see that no definitive
agreement exists with regard to terminology (Pissart, 2000).
The origin of the palsas is largely related to the lower heat conductivity of the dry peat
with respect to the saturated and frozen peat, with a conductivity around twenty times
higher (Washburn, 1979). In winter the peat wets and increases its heat conductivity
favouring the inhomogeneous penetration of the freezing and the formation of segregated
ice. The consequent differential cryostatic pressure gives place to elevations in the surface.
The outer peat layer that dries in summer with low heat conductivity has an insulating
effect preventing the thawing of the inner part of the paisa. The palsas may disintegrate
gradually from the margins due to a rise in temperature. Finally, they may become shallow
ponds and a new stage of peat accumulation may take place (Friedman et al., 1971).
The remnants of lithalsas present encircling ramparts (Pissart, 2000). Seasonal palsas
linked to oozes have been reported in the Paramera of Avila Province (Central Spain) at
1200 m in altitude (Molina and Pellitero, 1982). Although most of the palsas of the
Scandinavian countries formed 1000 to 3000 years ago, absolute datings indicate that
some of them were formed recently (Sepp~il~i, 1988).
2.2. Pingos
"Pingo" is the Eskimo term for mound. They are also called hydrolaccoliths and in Siberia
are called bulgunniaks. The pingos are domed perennial ice-cored mounds that generally
bulged out in large plains. Recently, submarine pingos have been recognised on some
177
2.1. Palsas
These mounds occur in boggy areas with discontinuous or sporadic permafrost (Zoltai,
1971). Their surface is criss-crossed with open fissures caused by dilation or desiccation
(Embleton and King, 1975b). At the present time they are occur in areas with a mean
annual temperature below 0~ as in Canada, Iceland, Sweden, and alpine regions
(Kershaw and Gill, 1979). Palsas may be isolated or form in groups and are differentiated
based on the morphology of dome-shaped, ribbon-shaped, and platform palsas. The former
ones are between 0.5 and 7 m in height, 10 to 30 m wide and 15 to 150 m long. The larger
palsas tend to be less conical than the smaller ones. The ribbon-shaped palsas have a lower
height and may be parallel or perpendicular to the contour lines, reaching several hundred
meters in length. The platform palsas rise 1 to 1.5 m above the surrounding bog and may
cover several square kilometres. Their generation is related to differential thawing.
Finally, the paisa complexes are formed by several types of palsas at different growth
stages and show numerous closed depressions generated by thawing (Sepp/il/i, 1988).
Some authors use the term string bogs to designate the association of different types of
palsas.
Based on their internal structure, two types of palsas are differentiated. The palsas with
a peat core have an outer layer of peat that thaws and dries in summer. Around 80 to 90%
of the core is composed ice in small crystals and lenses of segregated ice whose thickness
increases with depth from 5 to 10 mm to 5 to 10 cm. The base is generally formed by siltsized mineral particles. The silt core palsas also have an outer peat layer and a silt and clay
core with ice lenses and veins with a reticular arrangement (Sepp~il/i, 1988). Lithalsas are
similar mounds, but without any peat cover (Harris, 1998). We can see that no definitive
agreement exists with regard to terminology (Pissart, 2000).
The origin of the palsas is largely related to the lower heat conductivity of the dry peat
with respect to the saturated and frozen peat, with a conductivity around twenty times
higher (Washburn, 1979). In winter the peat wets and increases its heat conductivity
favouring the inhomogeneous penetration of the freezing and the formation of segregated
ice. The consequent differential cryostatic pressure gives place to elevations in the surface.
The outer peat layer that dries in summer with low heat conductivity has an insulating
effect preventing the thawing of the inner part of the paisa. The palsas may disintegrate
gradually from the margins due to a rise in temperature. Finally, they may become shallow
ponds and a new stage of peat accumulation may take place (Friedman et al., 1971).
The remnants of lithalsas present encircling ramparts (Pissart, 2000). Seasonal palsas
linked to oozes have been reported in the Paramera of Avila Province (Central Spain) at
1200 m in altitude (Molina and Pellitero, 1982). Although most of the palsas of the
Scandinavian countries formed 1000 to 3000 years ago, absolute datings indicate that
some of them were formed recently (Sepp~il~i, 1988).
2.2. Pingos
"Pingo" is the Eskimo term for mound. They are also called hydrolaccoliths and in Siberia
are called bulgunniaks. The pingos are domed perennial ice-cored mounds that generally
bulged out in large plains. Recently, submarine pingos have been recognised on some