Cirque Glaciers – alternatives to rotational slip (and BBC Bitesize…)

I vividly remember preparing for my GCSEs by carefully re-drawing a high (possibly 9?) mark question that about cirque glaciers.  From memory it looked something like this:

Apologies for the lack of colour, it's shameful for a geographer I know...

Bosh, A* material without a doubt. It’s an appealing diagram, a semi-circle of ice held within a rocky amphitheatre. The visualisation of it slowly rotating, eroding and spilling over the edge is simple and seemingly logical. It also handily explains the bowl the glacier erodes out of the mountainside. It’s based on a paper from 1949 and features in quite a few GCSE/A-Level guides.

Unfortunately though it bears little to no similarity to how cirque glaciers actually move. A study published in 2010 set out to examine this well held truth through field and modelling studies.  They’ve found that little to no sliding occurs in a cirque, rather the majority of movement occurs through ice deformation.  The modelling determined that even with the ice set at an unrealistically low viscosity (e.g. really stiff and hard to deform), there wasn’t enough driving stress to force sliding rather than deformation except at the very lowest part on the stoss side of the riegal.  There was absolutely no sliding at the centre of the cirque.

So that rather debunks our diagram…It seems a bit of a shame as it’s pretty much the first bit of glaciology I ever did.  But what I should have realised is that it was fundamentally flawed from the start.  All good diagrams should contain a scale, and although this was never picked up on by the teachers/examiners, its addition would have quickly raised questions about the rotational sliding hypothesis.  Let’s draw the diagram again, but to a realistic scale:

(accuracy not entirely guaranteed still...)

Although we initially drew it as a semi-circle, cirques are generally nothing of the sort.  Typical dimensions might be more like 1km by 0.2km.  Now, looking at this new diagram, does rotational sliding seem likely? No, the alternative theory seems much more plausible, and importantly for young glaciologists it’s also easier to visualise.

However, I’d leave it a few years before you stick it in your GCSE paper…

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