Periglaciation: processes and landforms

There wasn't much demand to understand processes in periglaciated areas, until the search for oil and gas in the 1960s. I will now discuss several processes that occur in periglaciated areas into two parts. Sorry for the lack of illustrations.


Ground ice 
Frost heave- fine grained soils such as silts and clays expand to form small domes. This process is the result of thermal conductivity of stones, which is greater than that of soil to be moved to the surface. The area under a stone becomes colder than the surrounding soil and ice crystal form. Further expansion by the ice, widens the capillaries in the soil, allowing more moisture to rise and freeze. Large crystals force the stones above them to rise until they reach the surface. Stones are unable to return to their original position. A pattern of freezing and thawing sorts material to form patterned grid, as shown below.

                                                         

Ground contraction
Refreezing of the active layer during the severe winter cold causes the soil to contract. Cracks open or fill with meltwater and wind blown deposits. When the water refreezes during the following winter the cracks widen and form ice wedges.

Freezing of groundwater -pingos
Pingos are domeshaped, isolated hills which interrupt the flat tundra plains. They can have a diameter of up to 500m and may rise 50m in height.

                                     Pingos near Tuktoyaktuk, Northwest Territories, Canada


Open system (hydraulic) pingos
These occur in valley bottoms and in areas of thin or discontinous permafrost. Surface water is able to infiltrate into the upper layers of the ground where it can circulate in the unfrozen sediment before freezing. When water freezes, it expands and forms localised masses of ice. Ice forces an overlying sediment upwards into a domeshaped feature.


Closed system (hydrostatic) pingos 
These are more characteristic of flat low lying areas where the permafrost is continuous. It forms on the sites of small lakes where water is trapped by freezing from above and by the advance of the permafrost inwards from the lake margins . Water below freezes, forcing the ground above to rise upwards into a dome shape.


Frost weathering 
This is the main process of weathering, scree often develops at the front of steep slopes and is responsible for turning well jointed rocks such as granite into tors.