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Holes in glaciers

What with new observations from space of the flow of water beneath the Antarctic Ice Sheet, and against a backdrop of long-standing knowledge that glaciers go faster in the summertime, holes in glaciers have seen an upsurge of interest. We are not talking about cracks in glaciers — crevasses — although the holes usually owe their genesis to pre-existing cracks. These holes, or moulins, are tubes of crudely circular cross-section, they are made by meltwater, and all the evidence suggests that we need to know a lot more about them if we want to understand glacier flow properly.

One way to find out more about natural holes in glaciers is to drill artificial ones. Among the more useful but fundamentally simple technologies in glaciology is borehole video. You lower a camera down your borehole and shoot. You may find, as did Luke Copland and his fellow-workers on Haut Glacier d’Arolla in Switzerland some years ago, that your own hole has intercepted a hole made by the glacier itself. That is, there is a hole in the wall of your own hole.

The forces at work inside glacier ice are varied, but only one can produce this kind of hole: transfer of thermal and mechanical energy from flowing meltwater. The borehole video is showing us conduits. It is reasonable to conclude that the water is coming from the glacier surface. But where is it going?

One thing we have learned from holes in the walls of boreholes, a few centimetres in diameter at most, is that some of the conduits of the englacial drainage system are small. We also know that some are not so small, because boreholes sometimes penetrate larger voids, and sometimes with disconcerting results. If you tap into a void that is filled not just with water but with water under pressure, you get a geyser.

So boreholes in general, and borehole video in particular, are showing us fragments of a complicated system that conveys meltwater through the glacier. Presumably the water sometimes ends its journey by refreezing, but if it can transfer enough heat to the conduit wall it will keep the conduit open and may even enlarge it. If that happens, the water will eventually reach the margin of the glacier or, more interestingly, the bed.

Finding englacial meltwater conduits the size of your finger is a pretty impressive feat, but there is a limit to the number of boreholes we can drill, and finger-sized holes are not physically impressive. A recent study by Catania and Neumann of holes in the Greenland Ice Sheet is impressive as to both technology and physical scale.

They used ice-penetrating radar to image the holes remotely. The ice was 400-600 m thick and they made a number of assumptions, such as that the holes, with diameters of about a metre, are vertical cylinders. In their radar traverses the holes show up as strong diffractors, visually striking hyperbolical shapes, superimposed on the well-defined layering that represents the history of accumulating ice.

The layers are downwarped in association with two of their holes. They argue persuasively that this is because the meltwater delivered by the holes keeps on melting the basal ice, releasing gravitational potential energy as it flows away. Further, to achieve observable downwarping the holes must be persistent. The system of conduits is embedded in a medium that is flowing slowly downhill. Any one hole ought to be short-lived, getting pinched off as the ice carries it away from its source of surface meltwater, or simply squeezing it shut. The two persistent holes appear to have supplied meltwater to the bed for long enough, one to a few decades, to achieve about 30 m of further basal melting in one case and 15 m in the other.

Yet again we have fragments of an evidently complicated picture: two long-lived holes, several more short-lived ones (no layer downwarping), and part of the study area in which the diffractors are so numerous that they obscure the layering completely. The simplest explanation of the numerous diffractors is that they are more closely spaced, smaller holes, perhaps on the same scale as the finger-sized ones seen directly by Copland.

Evidently we still have a lot to learn about holes in glaciers.

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