178
Climatic Geomorphology
subsea platforms. It is not known whether they have grown under the sea or have formed
on land and have been invaded latterly by marine waters. The highest documented pingo in
emerged land above sea level is the 48-m high Ibyuk pingo, located close to the
Mackenzie Delta in Canada. Most of the pingos are less than 20 m high and their diameters
range between 30 and 600 m. There is an inverse relationship between the diameter and
height of the pingos. In some cases they are elongated, more than a kilometre in length
and maximum height of 9 m. Because they keep a constant diameter, the gradient of the
slopes increases during the growth and never exceeds 45 ~ (Pissart, 1988). Pingos show two
types of cracks. Dilation radial cracks converge in the top and result from the upward push
produced by the ice core during its growth stage (Washburn, 1979). The opening of the
cracks may lead to the partial thawing of the underlying ice giving place to a crater-like
subsidence depression in the centre. The concentric cracks, not so conspicuous as the
former ones, result from the thawing of the ice core during the negative growth stage of
the pingo (Mtiller, 1959). In contrast to the palsas, pingos have a massive ice core that
penetrates several meters beneath the land surface (Lundquist, 1969). The cover material
is made up of loose sediments like gravel, sand and silt, but pingos with a rock cover of
sandstone and shale are also known. This cover layer may reach up to 14 m thick. The
growth rate varies from very low values to 1.5 m/yr (Mackay, 1973). On the other hand, all
the known pingos are younger than 10,000 years old and some of them are a few hundred
years old. Finally, pingos disintegrate progressively due to a rise in temperature giving
place to thaw lakes.
Two methods of pingo formation have been suggested. The so-called closed-system
origin explains the pingos of the Mackenzie Delta (Canada) and Central Yakutia (Siberia)
developed in areas with thick continuous permafrost. They form in relation to lake
depressions. During the development of the permafrost, the freezing of entrapped water
within a lake creates a massive ice core by downward percolation and aggradation. The
volume increase generates a cryostatic pressure that domes the lake sediments and may
even cause the extrusion of water towards the surface. If the water reaches the surface it
may give place to aufeis and when it contains gas it may produce explosive activity.
Generally, the water does not reach the surface but it freezes generating a massive ice core
(Mackay, 1979) (Figure 8.8). The cryostatic origin of the pingos in the Mackenzie Delta is
supported by the fact that 98% of the 1380 mapped pingos are located at the edge or inside
of contemporaneous or old lakes (Stager, 1956).
A great proportion of the open-system pingos are located in areas of relatively thin
permafrost of Alaska and Greenland. They are generated by groundwater flowing under
artesian pressure through a thin permafrost or in taliks within the permafrost. This water
freezes as it forces its way upwards forming an ice core that domes the surface. The
hydrostatic pressure result from height differences (Holmes et al., 1968) (Figure 8.9) so
that they form in topographic lows like valley bottoms or distal sectors of low angle
slopes (Mtiller, 1959).
3. Slope morphology and evolution
The slopes in periglacial environments show a wide range of morphologies because they
develop under variable conditions of temperature, moisture, lithology and vegetation
Climatic Geomorphology
subsea platforms. It is not known whether they have grown under the sea or have formed
on land and have been invaded latterly by marine waters. The highest documented pingo in
emerged land above sea level is the 48-m high Ibyuk pingo, located close to the
Mackenzie Delta in Canada. Most of the pingos are less than 20 m high and their diameters
range between 30 and 600 m. There is an inverse relationship between the diameter and
height of the pingos. In some cases they are elongated, more than a kilometre in length
and maximum height of 9 m. Because they keep a constant diameter, the gradient of the
slopes increases during the growth and never exceeds 45 ~ (Pissart, 1988). Pingos show two
types of cracks. Dilation radial cracks converge in the top and result from the upward push
produced by the ice core during its growth stage (Washburn, 1979). The opening of the
cracks may lead to the partial thawing of the underlying ice giving place to a crater-like
subsidence depression in the centre. The concentric cracks, not so conspicuous as the
former ones, result from the thawing of the ice core during the negative growth stage of
the pingo (Mtiller, 1959). In contrast to the palsas, pingos have a massive ice core that
penetrates several meters beneath the land surface (Lundquist, 1969). The cover material
is made up of loose sediments like gravel, sand and silt, but pingos with a rock cover of
sandstone and shale are also known. This cover layer may reach up to 14 m thick. The
growth rate varies from very low values to 1.5 m/yr (Mackay, 1973). On the other hand, all
the known pingos are younger than 10,000 years old and some of them are a few hundred
years old. Finally, pingos disintegrate progressively due to a rise in temperature giving
place to thaw lakes.
Two methods of pingo formation have been suggested. The so-called closed-system
origin explains the pingos of the Mackenzie Delta (Canada) and Central Yakutia (Siberia)
developed in areas with thick continuous permafrost. They form in relation to lake
depressions. During the development of the permafrost, the freezing of entrapped water
within a lake creates a massive ice core by downward percolation and aggradation. The
volume increase generates a cryostatic pressure that domes the lake sediments and may
even cause the extrusion of water towards the surface. If the water reaches the surface it
may give place to aufeis and when it contains gas it may produce explosive activity.
Generally, the water does not reach the surface but it freezes generating a massive ice core
(Mackay, 1979) (Figure 8.8). The cryostatic origin of the pingos in the Mackenzie Delta is
supported by the fact that 98% of the 1380 mapped pingos are located at the edge or inside
of contemporaneous or old lakes (Stager, 1956).
A great proportion of the open-system pingos are located in areas of relatively thin
permafrost of Alaska and Greenland. They are generated by groundwater flowing under
artesian pressure through a thin permafrost or in taliks within the permafrost. This water
freezes as it forces its way upwards forming an ice core that domes the surface. The
hydrostatic pressure result from height differences (Holmes et al., 1968) (Figure 8.9) so
that they form in topographic lows like valley bottoms or distal sectors of low angle
slopes (Mtiller, 1959).
3. Slope morphology and evolution
The slopes in periglacial environments show a wide range of morphologies because they
develop under variable conditions of temperature, moisture, lithology and vegetation