Pingos are ice-cored hills (Figure P63), typically conical in shape, covered by soil and vegetation with a core of massive ice produced primarily by the injection of water to the base of aggrading permafrost. They grow and persist only in a permafrost environment. Porsild (1938) first proposed the term pingo, which is a local Inuit word for an ice-cored conical hill in the Mackenzie Delta. In Siberia, similar forms are referred to as bulgannyakh. Pingos vary considerably in size (anywhere from 2 to 50 m high) and active examples show steady increases in height during their lifetime.
Perhaps the greatest concentration of pingos occurs in the Tuktoyaktuk Peninsula to the west of the Mackenzie Delta in the North West Territories of Canada, where over 1,350 examples have been identified. This is a significant concentration given that the number of active pingos world-wide is estimated at around 5,000, with examples occurring in the Yukon, Alaska, Svalbard, Greenland, Siberia, Mongolia and at altitude on the Tibetan Plateau (Mackay, 1998). There are two basic types of pingos, they are:
Open (hydraulic) system pingos. These occur in hydraulic systems where intra- or sub-permafrost groundwater flows under a hydraulic gradient to the pingo site and maintains a sub-pingo water lens that freezes to form a body of injection ice (hydrolaccolith) that uplifts the surface permafrost to form a conical hill (Figure P64). The key element in this process is the presence of artesian water pressures, which maintain a steady but slow supply of groundwater to the growing ice mound. Open system pingos tend therefore to be concentrated in high relief areas, such as on lower hill-slopes, at the base of alluvial fans, in alluvial valley bottoms or in front of glaciers where subglacial groundwater rise beneath proglacial permafrost. The hydrology of these systems is poorly understood, and other growth mechanism may be involved. In particular, their formation appears to require a delicate balance between three variables all of which may be subject to seasonal or annual oscillations (French, 1996), namely water pressure, overburden or permafrost strength, and the rate of freezing. If water is injected toward the base of the growing pingo too fast, water pressure will rise until the pingo ruptures and a surface spring is formed. Alternatively, if the water is injected too slowly then the sub-pingo water lens will freeze and the pingo will cease to grow. These variables are unlikely to remain in perfect balance for long and consequently other mechanisms may be involved in sustaining the growth of the ice-core, such as the formation of segregation ice.
Closed (hydrostatic) system pingos. In this case, the groundwater flow necessary to maintain the growth of the ice core results from pore water exclusion caused by permafrost aggradation within saturated sands found beneath drained lakes or in areas of permafrost talik (Figure P64). Hydrostatic pingos tend to occur in regions where thaw lakes, in areas of permafrost, are drained catastrophically via either coastal retreat or the headward erosion of rivers. Closed system pingos have been widely studied and perhaps the most comprehensive data set exists for those examples on Tuktoyaktuk Peninsula (Mackay, 1998).
Relict pingos occur extensively within areas of former permafrost, and consist of a circular rampart surrounding a shallow depression. Relict examples are considered by many to provide good evidence of former extent of permafrost in the past and Pleistocene examples have been described from across Northwest Europe (Watson, 1977; De Gans, 1988), and North America (Bik, 1969). In addition, fossil examples have been recorded from glacial deposits of late Ordovician age in Jordan (Abed et al., 1993).