The processes in periglacial areas give rise to a large variety of landforms. These landforms include ice wedges, patterned ground and pingos. There is a full explanation of pingo formation and diagrams of the other major landforms below, and you can research the explanations of the others yourself. The transect below also shows how climate, the depth of the active layer and the type of permafrost vary across this zone;
Pingos are dome shaped hills that are commonly up to 500m in diameter and up to 50 meters in height. These land features are common in periglacial areas that are characterised by permafrost and a seasonally changing active layer. At the core of the pingo is an ice lens of varying size, and the surface layer is made of soil often topped with vegetation. The surface can also contain cracks as a result of ground swelling.
There are 2 basic types of pingo, the open system type where water that forms the ice lens comes from outside of the system, and the closed system type where the water required for ice lens formation is contained within the area where the pingo is formed.
pingos are typical of the Mackenzie River Delta in Northern Canada on low lying
areas where permanent and continuous permafrost can be found.
In this area of Canada there are
hundreds of small lakes where water is trapped in small depressions during
summer because the ground underneath is permanently frozen and effectively
impermeable (won’t let water through it).
Over winter, ground water
underneath the lake sediments within the Talik (unfrozen ground) can be trapped
by ice from the lake surface as the lake freezes, and advancing permafrost
within the ground.
This decrease in temperature causes
this ground water to freeze into and ice lens, which grows over time as water
freezes to the ice lens due to the increase in hydrostatic pressure.
This causes the sediment above to
bulge upward into the characteristic pingo shape.
pingos occur in areas of discontinuous permafrost where there are interspersed
areas of permafrost (land frozen for at least 2 years) and talik.
The active layer continually
freezes and melts year on year above the permafrost and talik.
winter, as the active layer freezes down over water can become trapped between
the descending freezing plane of the active layer and the permafrost that
This promotes the growth of an ice
lens which pushes the land up above it as it expands.
Water underneath the permafrost can
move through the talik between the permafrost areas
because of capillary action (the
movement of water through the soil because of ) and hydraulic pressure.
This water migrates to the ice lens
and freezes, swelling the ground above further.
|Both of these pingos can rupture if they become too large, and if the ground cracks it exposes the ice lens to the atmosphere and the suns energy. This can then melt the ice causing the pingo to collapse, leaving a ruptured pingo with a depression in the middle that can fill with water and a lake, and a rampart around the edge that is prone to the mass wasting process of solifluction. The collapsed pingo is known as an ognip|
Ice wedges and patterned ground in Northern Canada
|Think about it|
Complete the timeline below on pingo formation
Read more at the Fundamentals of Physical Geography online textbook