what are the current hypotheses for the development of pingos?

I am currently preparing this posting while on holiday and do not have access to scanning facilities which would allow me to incorporate sketches. On the basis that a sketch can often convey more meaning than many thousands of words, this makes descriptions much more difficult, but let me try. Later I will try to add some sketches to illustrate the following more clearly.

It is widely accepted that the formation of a “pingo” largely results from the development of an excess of pore water pressure in the unfrozen ground, usually referred to as the “talik”, underlying the permafrost layer. This explanation for their formation was seemingly first suggested by Porsild (1938), who is also credited with the name “pingo” derived from the Inuit word for hill. The mechanics believed to account for the initiation and subsequent growth of pingos has been greatly elaborated in the many works of Mackay - see for example his extensive summary of his own work, covering a period of nearly 50 years of continuous research into pingos, as well as a comprehensive summary of the contributions of others, which is contained in Mackay (1998). There are also very useful summaries of pingos in their wider periglacial context in the works of for example Washburn (1979) and French (1996).

What most of the explanations have in common is that pore water pressure build-up, beneath the usually recently frozen and normally aggrading lower boundary of the permafrost, pushes up a locally weakened area into the characteristic dome or, less commonly, ridge formation. The local weakening is often attributed to the presence of a locally thinned area of permafrost, which is thought to sometimes arise as a result of any remaining shallow pond water. The build-up of pore water pressure in the underlying unfrozen “talik” is said to result from one of two different causes.

In the hydraulic model, see French (1996), Mackay (1998), Washburn (1979), sometimes referred to as an “open system”, a ground water flow and artesian pressure beneath the permafrost layer is thought to be induced by the hydraulic head associated with the elevated water tables in the surrounding higher landforms. Mackay (1998) however, observes that “despite the decades of field studies by many individuals on hydraulic (open) system pingos, particularly in Greenland and Spitzberen, there are no surveyed growth data, anywhere, on even one pingo”.

More typical of the ranges of pingo occurring in the areas extensively studied by Mackay and his associates are those that result from an hydrostatic model, otherwise known as the “closed system” – see for example Mackay (1998). In these cases the permafrost will have enclosed the underlying unfrozen talik, so that any downward growth of the underside of the permafrost ice layer will result in a roughly 10% volume expansion when the enclosed ground water turns to ice. As a consequence of the confinement this expansion will result in an increase in the hydrostatic pressure within the underlying ground water - in much the same way that a bottle of wine left unintentionally in a freezer will as the wine freezes develop very high liquid pressure, often enough to push the cork out or in more extreme situations burst the bottle.

A common precondition for the development of a closed system pingo is the existence of a shallow lake or estuary overlying a saturated, fluvioglacial sand and gravel, in what is otherwise a well developed permafrost region. This shallow water will act as a thermal buffer to the downward penetration of permafrost, preventing the permafrost from developing under the lake or estuary bed. This will result in a bowl of unfrozen ground beneath the lake or estuary, possibly overlying a much deeper and well formed layer of permafrost. The genesis of the formation of pingos is commonly the sudden draining of the lake or a change in sea level relative to the estuarine sediments. This exposes the relatively flat lake or estuarine bed to the development of permafrost that over a period of years gradually propagates downward to form a thickening layer, that could enclose the unfrozen talik. Any residual ponds will cause the growth of this upper layer of permafrost to be retarded, providing it is argued a relatively flexible thin layer at which the underlying pore water pressure can be relieved by inducing an upward bulge. Water will be extruded into the space created by the upward deformation of the locally bulged permafrost layer as a result of the pore water pressure in the underlying unfrozen talik. Much of this water will subsequently freeze to form the ice lens, which is an additional feature of the developing pingo.

This process will continue over a period of many years, perhaps centuries, until some sort of stasis is reached between the downward growth of the lower permafrost boundary, the fluctuations of the underlying hydrostatic pressure within the unfrozen talik, and the upward deformation of the pingo bulge. While the causal mechanics has been less well developed, surges in the growth of open system pingos are also believed to result from surges in ground water pressures.

Some references mentioned above and in other blogs that might be helpful in understanding more about the nature of pingos and the currently accepted hypotheses for their development are:

French, H. M. (1996) The periglacial environment, 2nd Edition, Longman, 341pp.
 
Mackay, J. R. (1998). Pingo growth and collapse, Tuktoyaktuk Peninsula area, Western Arctic Coast, Canada: a long-term field study, Geographie physique et Quarternaire, 52, 271-323.
 
Porsild, A. E. (1938). Earth mounds in unglaciated Arctic nothwest America. Geographic Review, 28, 46-58.


Washburn, A. L. (1979) Geocryology: A survey of periglacial processes and environments, Edward Arnold, 406pp.