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My Favourite Glaciological Word

Speakers of English in my subject, glaciology, have never been afraid of borrowing good words from other languages. My all-time favourite glaciological word comes from Icelandic: jökulhlaup.

First things first. How do you pronounce it? Icelandic j is pronounced like English y. Then you stretch the ö into an uh noise, and emphasize the h, which should be like the ch in Scots loch. The au is roughly as the vowel in cow, or possibly slurp. You can listen to an Icelander saying it here.

Next, what does it mean? It translates literally into English as “glacier burst”, which will be more informative for most readers but is nowhere near as much fun. A jökulhlaup is a large, sudden and usually unwelcome increase in the rate of flow of a stream draining a basin in which there is an ice-dammed lake.

Glacier ice can be very effective as a dam for its own meltwater. Unfortunately it is also untrustworthy for this job. Being less dense than the water it is damming, it is vulnerable to flotation. If the water depth reaches nine tenths of the thickness of the ice dam (the ratio of ice density to water density), the ice will float.

Flotation can be avoided if the water manages to tunnel beneath the ice. A subsurface channel forms and the water starts squirting out of the lake. The flow rate grows steadily because the water enlarges the channel steadily, melting its walls. This kind of jökulhlaup ends abruptly when the supply of water runs out, that is, when the lake has emptied. The glacier carries on deforming slowly, squeezing the channel shut over the course of the winter, and the same thing happens again next summer once the lake has refilled with meltwater.

The nastier kind of jökulhlaup is the one in which flotation is sudden and on a large scale. Huge volumes of water can begin to flow almost immediately. How long the jökulhlaup lasts depends on how long the supply of water lasts, how good it is at enlarging the channel and how long it can keep the dam afloat. The nastiness lies in the unpredictability of flood onset. If you live downstream, or have invested in valuable downstream structures such as bridges or oil pipelines, you get little or no warning of the arrival of an enormous wall of water.

The biggest jökulhlaup we know of is probably the one that emptied Lake Agassiz-Ojibway and led to the disconcerting cold snap 8200 years ago. But in modern times they happen on a smaller scale every year, in hundreds of glacierized drainage basins.

Icelanders learned to live with jökulhlaups long ago. One option, in sparsely inhabited terrain, is simply to stay away from the rivers. Nepal and other Himalayan countries don’t have that option. There are many more people than in Iceland, and the rivers are too important as a resource sustaining agriculture. Here the jökulhlaups have come to be referred to as “GLOFs” – glacial lake outburst floods – which I think is not nearly as good as jökulhlaup but does cover the point that not all of these floods are due to the breaching of ice dams. Some of them come from the sudden collapse of moraine dams, and some from the drainage of lakes that are not proglacial (in contact with the glacier margin) but subglacial or supraglacial (on the glacier surface).

Whatever their etymological merits, GLOFs are a serious hazard, and have spurred the completion by ICIMOD, the International Centre for Integrated Mountain Development, of extensive glacier and glacier-lake inventories along the length of the Himalaya. Fears that the hazard is worse now than in former times, when glacier retreat was less rapid, are rational. Glacier retreat creates space for the impounding of water between the glacier and the moraine it left behind at its position of maximum extent. There is more meltwater nowadays, and more scope for it to pond in whatever embayments result from the changing relationship of the glacier to its confining walls. Call them what you will, jökulhlaups or GLOFs are worth all of the attention they are beginning to get.

Posted by Graham Cogley on August 31, 2009 3:00 PM |

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Comments (10)

  • 1 Jack D. Ives September 5, 2009 6:59 PM

    Congratulations to Professor Cogley for providing such vigorous support for the use of "jokulhlaup" in English language glaciological writing. In my view also "glof" (for glacial lake outburst flood) is an ugly term and I have been fighting it (not very successfully) since 1986. Graham Cogley's explanation of jokulhlaup mechanics is also very useful, although he omits one of the major causes of the Icelandic phenomenon - sub-glacial volcanic eruptions.

    The etymology of "jokulhlaup" is also fascinating. For more than 50 years I have used the translation "glacier leap". This was based partly upon discussions with Professor Sigurdur Thorarinsson, the famous Icelandic glaciologist and vulcanologist, in the early 1950s, and partly on the advice of my old farmer friend, Ragnar Stefansson whose family had kept a record of jokulhlaup events from the early 19th century. Ragnar related how Einar, one of his ancestors, had been caught far up on Skeidararjokull (he had been collecting wool from isolated sheep, no less)when he heard a roaring noise beneath him and was almost thrown off his feet as the glacier surface jerked upward -- hence one derivation, "glacier leap". Graham Cogley's "glacier burst" is eminently reasonable. Nevertheless,his note prompted my more stringent etymological investigation. I have been informed by the family of the famous Icelandic farmer, Hannes, from Nupsstadur on the western margins of Skeidararjokull, that "jokulhlaup" is not derived from the current Icelandic verb "hlaupa", meaning to run, to leap, to sprint. I was told that it derives from an old Icelandic noun "hlaup", meaning something that expands quickly. It apparently originated from the dairy process of making cheese, cream, and butter. In that process at a certain point the milk expands quickly in volume, for which the noun "hlaup" was used. It was then transferred to the phenomenon of sudden glacier flood whereby the glacier melt-water river (jokulsa) rapidly expanded in volume.

    A minor note on prediction, some jokulhlaup occurring with a degree of regularity every year, others several times a year, and so on: in 1954 at Skaftafell, Iceland, when I had a certain urgent appointment with the Church of England, I was anxious to "get to the church on time" yet also to witness a suspected large jokulhlaup. Ragnar warned that I would likely never see such an event in the rest of my life so the situation was tense. He then predicted that the peak flow would occur on 18th July. He was 100 per cent correct, which gave me 24 hours to spare.

    The word "hlaup" is also attached to the last syllabel of the name of the glacier in question: hence Skeidararhlaup. By far the largest documented hlaup occurred from Skeidarajokull in 1996 and was caused by a major volcanic eruption beneath Vatnajokull -- :the glacier that gives water". Undoubtedly the term jokulhlaup has been in the Icelandic lexicon for hundreds of years. It is to be regretted that Iceland rarely gets credit for its very early and very important contributions to glaciology. Dr Sveinn Palsson, for instance, observed in 1793 that glaciers ". . . flowed down in a semi-melted or thick viscous state. . . . glacier ice, without actually melting, has some kind of fluidity, like several resins . . ."

    Thanks once again to Professor Cogley

    Apologies for being unable to use Icelandic diacriticals