A brief introduction to crevasses

As an impressionable seven year old I learnt what a crevasse was; namely a large split in a glacier of great hazard to glacier travellers. This knowledge was imparted by a venture scout in my parents group who, on a climbing trip to the Alps, managed to end up in one, breaking several bones in the process. Years later this did not discourage me from my own forays into alpine mountaineering, so it was probably inevitable that I would have my own brush with mortality in a crevasse while researching them as part of my PhD work (see photo).

Some injuries, 3 days after falling into a crevasse (thankfully to be rescued by quick-thinking field assistants).  Not recommended
Some injuries, 3 days after falling into a crevasse (thankfully to be rescued by quick-thinking field assistants).
Not a recommended “experience”.

The research was interesting and made more so by being carried out in such a spectacular environment. Breiðamerkurjökull is a southern outlet glacier of the Vatnajökull ice cap in Iceland. It’s actually one of the more popular tourist destinations in Iceland thanks to the boats that run on the lagoon in front of the glacier, getting people up close and personal with icebergs. The icebergs are one of the reasons we chose to work there, as the rationale of my Phd project was can a crevasse depth relation be used as a parameterisation for calving in ice sheet models?

I was moved to revisit this work recently when a friend (and ace glacier/climate blogger) Liam Colgan posted about crevasse factoids.

Crevasses on Breidamerkurjokull, note figure for scale
Crevasses on Breidamerkurjokull, note figure for scale

Crevasses are extremely beautiful features to observe and they are interesting scientifically since they indicate all sorts of information about what is going on in a glacier. As they are aligned more or less with the principal stresses in a particular location we can see where a glacier is accelerating or decelerating, that is stretching or compressing respectively, based on the shape and alignment. They can also be used as a feature to track glacier velocity between two successive images taken from aircraft or satellites. Crevasses are also significant in other ways, since they are a plane of weakness that can be exploited by meltwater, channelling it away from the surface of the glacier to the bed changing the velocity of the glacier. And as proved in the case of my Phd work, when they extend deep enough in the right place, they cause large chunks of ice, namely icebergs, to fall off the front of glaciers.

Given all these interesting habits it is probably surprising to learn that the large computer models of ice sheets and glaciers don’t usually include crevasses in them, though there are some more recent honourable exceptions, mostly working with single outlets or small glaciers such as Sue Cook’s work with the Elmer model. This is because an individual crevasse is not only too small for the resolution of a model, it’s also a discontinuity, and the approximations of the physics of ice sheets do not easily allow discontinuities. To put it another way, when we model glaciers we usually assume they are really large and thick fluid bodies, and as everyone knows, fluids don’t crack. This is just another bizarre property of water, and if I get chance I’ll discuss that again in further detail in another entry. But back to crevasses.

Now I mostly work with a climate model, HIRHAM5, using it to calculate surface mass balance, that is accumulation of snow and the melt and run-off from the surface of glaciers and ice sheet. However, I am finally (loosely) involved in a project that sets out to finish in some way the work I started as a young PhD student.

At DMI we run the PISM ice sheet model, fully coupled with a global climate model EC-Earth as I wrote about in this post. We will also soon be running HIRHAM5 coupled to PISM in order to study feedbacks between ice sheet dynamics and surface climate forcing (mainly in terms of how topography and elevation of the ice sheet affects the surface mass balance). We also intend to participate in the ISMIP6 model comparison project which will compare the results of several different global climate models that also include ice sheets in a realistic fashion.

 

One of the key challenges in getting these running is how to deal with the ocean interface with the ice sheet, both in terms of submarine melt of outlet glaciers (likely a far more important process than earlier recognised) and in terms of calving icebergs. One of our main (and in my opinion most interesting) projects right now, ice2ice has allowed us to employ a PhD student to work on this specific issue. She will be using a similar idea to Faezeh Nick’s model of outlet glacier calving, which in turn was based on a long ago work (pdf) I was part of as a lowly PhD student.

By comparing the measured crevasse depths with numerical models I was able to show that simple models can be used as approximations of crevasse depth. That study is still one of the very very few where actual empirical measurements of crevasse depth, strain rate, spacing and other variables were made and compared with model output.

In my current incarnation as modeller I will be keeping very carefully away from all sharp fractures in the ice and concentrating instead on the model part. Expect updates here…

 

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