What are Pingos?

Pingos are low isolated dome-like hills generally found in permafrost regions. Obviously, they are distinct from patterned ground as physical features but are associated with the latter in distribution. They may be as high as 100 meters but the usual elevation is not more than a few dozen meters. Their diametrical extent on the ground may go up to 300 meters. They occur in northern Siberia and northern Canada and Alaska. The elevation of these hills is due to hydrostatic pressure. As pressed water happens to rise and frozen into ice the injected body raises the ground surface as pingos.

Such hills occur in the regions of continuous and discontinuous permafrost. The smaller ones may be flat-topped while the larger ones may have punctured tops resembling craters. Their interior is composed of silt and sand. The larger hummocks or hills better deserve the title ‘pingo’.

The larger pingos are of two categories—Greenland type, and Mackenzie type. The former results from hydrostatic pressure of water. Such water may be in the permafrost or beneath. The pressure of water pushes up the frozen silt and sand layer nearer the ground as well as the ice layer beneath it. The second type is related to lakes. Because of the unfrozen water of a lake there is no permafrost immediately beneath it but as sediments accumulate in it these become frozen. The lake contracts. Thus the water of the unfrozen ground is trapped by the growing permafrost beneath the lake and that already in existence. The hydrostatic pressure causes a bulge in the giound. The trapped water freezes into massive ice.

Smaller hummocks may also result from the formation of segregation ice if segregation-favouring sediments like silt and clay are present. If there is saturated sand between impervious rock below and encroaching permafrost above, the seasonal freezing of the sand would mean an increase in volume and hence an upward bulge.

There is an excellent description, illustration and interpretation of pingos by Muller in Fairbridge’s Encyclopedia of Geomorphology.

The permafrost is raised by the freezing of ground water into a lens of ice. The ground water below permafrost rises under hydrostatic pressure (under artesian conditions) and gets gradually converted into ice lens by contact with permafrost which surrounds it. Thus, the rising and freezing water assumes the shape of an upward convex lens.

It must be emphasized that pingo refers to a perennial feature resulting from the formation of ice-lens within the permafrost and is different from mounds resulting from seasonal freezing of ground water.

The zone of pingos is approximately between 65-75 degree north with discontinuous permafrost and thin continuous permafrost. Two types of pingos have been identified. One is ‘open system’ type where because of the thinner permafrost or discontinuity of permafrost water can go down from the surface. This is characteristic of hilly areas where there is sufficient head for hydrostatic (or artesian) pressure. The second type is a ‘closed system’ pingo, which forms in the site of lakes, described above. Radiocarbon dating has indicated the age of some pingos as up to 10,000 years while some of them are in the process of formation now.

While sorted and unsorted circles, nets and polygons are features of level ground; the ground patterns are modified into steps and stripes where the slope of the surface is 2 degree or more. Stripes are formed along the slope. Terraces and steps are formed transverse to the slope.