environmentalresearchweb blog

« Going offshore (part 2) | Main | Oslo: Polar vision meets Eurovision »

How calving works

We all know something about icebergs, even if only that one of them sank the Titanic. Working back from what you know about that, you will probably recall that icebergs come from glaciers that reach the sea, and that they are produced in calving events. But that is as far as most people’s knowledge goes.

It may come as a surprise that until recently glaciologists didn’t know much more than this either. Even now we cannot predict the rate of production, still less single calvings. But our ideas have taken some steps forward lately.

The calving rate is the velocity at which bits drop off the glacier at the calving front. One complication is that the calving front need not be immobile. The calving rate is actually the rate at which ice arrives at the front — the ice velocity — minus the rate at which the front advances or retreats — the rate at which the glacier’s length changes.

Why doesn’t the ice just keep going? One answer, only partial, is that sometimes it does. Instead of a calving front we have a grounding line. But this is turning one question into two (or more).

The first question, Why do some glaciers end in floating tongues?, is rather easy to answer, although much harder to model. The glacier stays in contact with the solid bed until it satisfies the condition for flotation, which is that there must be just enough water to support the weight of the ice.

The second question, What makes the iceberg finally decide to calve?, has been harder to answer. Early observers noted an apparent relationship between calving rate and water depth: the deeper the water, the faster the rate. But this fails to account for variations of the calving rate with time. Kees van der Veen found that during its rapid retreat the calving front of Columbia Glacier was where the glacier was 50 metres thicker than required for flotation. This was an advance, but still did not relate the calving rate to what we know about glacier dynamics. The same can be said of the argument of Andreas Vieli and co-authors that the glacier-specific 50 m should be replaced by some constant fraction of the water depth.

That missing link between observed behaviour and understanding of dynamics may finally have been forged by Doug Benn, Nick Hulton and Ruth Mottram. They argue that calving happens when a transverse (across-flow) surface crevasse deepens to a state in which its base reaches water level. It is a very fruitful idea.

The tip of the crevasse is at that depth at which the weight of overlying ice is just able to keep the crevasse walls together against the longitudinal tensile stress that is pulling them apart. So there is our link to dynamics, because the longitudinal stress is just the rate at which the ice is accelerating towards the terminus, and is something we can have a shot at predicting.

But there is more. A crevasse that propagates down as far as water level is likely to fill with water, the weight of which will help to resist closure and make it much more likely that the crevasse will propagate all the way to the bottom of the glacier, or in other words to produce an iceberg.

There may be yet more. No ice shelves are known to exist where the mean annual temperature is warmer than about —5°C. Perhaps that is because ice colder than that keeps going when it reaches the sea, and perhaps in turn that is because its crevasses fail to make contact with water when they propagate downwards. But that is an idea awaiting exploration.

The Benn work is certainly not the last word on this subject. For example in the latest issue of Journal of Glaciology Jaime Otero and co-authors make the new idea more numerically versatile by improving the calculation of the crevasse depth and of the stress field. Relying on measured ice velocities, they reproduce numerically the stable position of the calving front of a small tidewater glacier in the South Shetlands.

We still can’t stop icebergs hitting ocean liners, but there are plenty of other reasons for wanting to understand calving rates, and the dynamicists are starting to make progress.

Posted by Graham Cogley on June 7, 2010 3:00 PM | Permalink