from asphalt to permafrost blisters

In the course of a conversation with Roger Hobbs, who at the time was Professor of Structural Engineering at Imperial College London, I mentioned my newly found fascination with pavement blisters. Roger had himself worked extensively on the subject of thermal buckling of pipelines and had even published a couple of excellent articles dealing with the upheaval buckling of heavy sheets, so I knew he would be interested in the asphalt blisters. His work on the upheaval of heavy sheets had to some extent been inspired by some problems occurring with the distortions that can arise during the hot rolling of steel sheet and also by the upheaval buckling so often experienced when wood block or laminate floors are not provided with sufficient expansion relief gaps around their edges. It was around this time that fashions in interior decoration were changing in favour of unbonded wood laminate floors over the more traditional carpet. Anyway, in the course of our conversation Roger mentioned the existence of very similar forms of blister called "pingos" known to form in ice at high latitudes.

At the time I had not heard of pingos but assumed they were large versions of the blisters I described above as forming on lake ice sheets. It was some years later that I decided to set as a project for a final year undergraduate project the task of finding out more precisely what are pingos and what are the current explanations for their formation. As shown in the following photograph of a well formed pingo, somewhere in the Mackenzie River delta of Northern Canada, there certainly appeared to be some remarkable similarities in the form of pingos and asphalt blisters. Well yes, they are a little bigger. Typical diameters are in the region of 200 to 400m and the better formed ones have heights that can reach up to 80m. And yes, they are comprised of frozen ground that emerges from the surrounding, ostensibly flat alluvial areas, underlain with permafrost. But the characteristic shape is close to that of the asphalt blister and they have a strong tendency to form domes that are essentially circular in plan.

The similarities are evident when one compares the geometries of the above pingos with those of typical asphalt blisters (see previous blogs), some of which are for convenience reproduced below. 

Furthermore, when we started to delve into the literature relating to pingos we were to find that the currently accepted explanations for their genesis and growth reflect closely those we had found for the emergence of asphalt blisters - albeit with ground water pressure taking the place of gas pressure. And just as for asphalt blisters, it seemed to me there were some pretty compelling factors about the growth of pingos that did not appear to stack-up against the existing hypotheses for their emergence. It has been the problems in having an alternative hypothesis for the emergence of pingos taken seriously by the experts in this area that has to some extent encouraged me to enter blogland. I am hoping therefore to spend the next few postings discussing:

• a little more fully what are pingos:
• the currently accepted hypotheses for pingo growth;
• what are some of my problems with these hypotheses;
• what is my credible alternative hypothesis;
• what has been the reactions of the permafrost community to this alternative hypothesis;
• and, lastly, to explain how the dialogue so far generated over pingo growth has I believe led to some other exciting new theories covering much wider classes of natural phenomena.

This latter aspect will open up a whole raft of fascinating topics ranging from a number of different forms of pavement failure, to the formation of many other periglacial and glacial morphologies, to the movement of ice sheets and glacial ice, to the processes that might be playing and have played a major part in the dynamic evolution of the Earth's crust and by analogy the surface features of many other planets and their satellites within the solar system.