from pingos to ice-wedge polygons

This post was prepared 1.5.10 but for some reason I failed push the publish button. Anyway, for what it is worth here it is to provide the background to how it was I developed an interest in wider aspects of periglacial processes, some of which KI hope to again take-up in future posts. 

Fairly early in my thinking about the alternative model for the development of pingos I was in correspondence with Professor Chris Burn at Carleton University in Ottawa. My own thinking about the pingo developments was based upon the seasonal changes in temperature causing massive in-plane compressive forces when the expansion during the warming period is constrained and tensile forces when the contraction during cooing is constrained. A form of upheaval buckling failure seemed to be the result of the compressive phase while the cracking during tension meant that the uplift deformations would not be fully recovered during the tensile cooling phase. Drawing inspiration from what happens in lake ice when it develops cooling cracks, I reasoned that the tension cracks would fill with water and moisture which would undergo a phase change to form ice. In this way the permafrost sheet would be again integral and ready for another increment of compressive uplift during the next cycle of warming. And so it would go on; a little additional uplift occurring every season the compressive forces are sufficiently high to overcome the elastic resistance to an extra increment of uplift.

Chris Burn was kind enough to point out to this novice in the area of permafrost research that a very similar mechanism was widely considered to be responsible for the development of ice wedge polygons. This picture shows a typical area of well developed ice-wedge polygons. A given region will have fairly regular arrays of either orthogonal or hexagonal crack networks whose characteristic dimensions are fairly robust. The borders of the polygons have cracks penetrating deep into the underlying permafrost. These cracks are widest at the top and narrow with depth. The cracks open a little each cold season and the moisture getting in then freezes to form an ice vein parallel to those of previous seasons. Over long periods the resulting wedges of ice can become very wide and exhibit vertical laminations reflecting the seasonal growth of the crack. Often there are raised ramparts of soil either side of the border cracks, so that during wet periods small ponds can form in the centrally depressed areas.

It was comforting to learn that the mechanism being advanced for pingo growth was not too off the wall and that closely related phenomena were widely regarded to be a work in the development of ice-wedge polygons. But as I became more familiar with the literature relating to ice-wedge polygons I was surprised to see there was apparently little recognition of the effects the compressions developed during the warming phase could exert on the geometry of the polygons. .


Depending upon the climatic conditions, the nature of the ground cover and the underlying permafrost, seasonal cooling cause cracks to develop. These crack will penetrate down into the permafrost with the crack widths reflecting the pattern of temperature attenuation with depth - wide at the top and very fine cracks at the base. The depth of the cracking is a direct function of the depth to which significant tesile stress arise which is in turn directly related to the depth of penetration of the seasonal temperature changes. Simple considerations of the St Venant principle would indicate that a crack will relieve the tensile stress to a distance from the crack that is 2 to 3 times the crack depth. Hence, the nearest location for a parallel crack would be 2 to 3 times the depth of the cracking. It is this that seems to control the characteristic horizontal spacing between the cracks. Being a 2-dimensional stress field similar cracking occurs in other directions.   form ion the permafrost.   and specifically the seasonal range of surface temperatures,  especially the thermal conductivity,