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EGU 2011: “Blocks and springs” technique analyses glacier break-off

By Liz Kalaugher at the EGU General Assembly, Vienna

Back in 1895, the sudden collapse of the Altels cold (high-altitude) hanging glacier brought around five million cubic metres of ice crashing down onto the valley below. The event, the largest known ice avalanche in the Alps, killed six people and 170 cows, as well as causing the valley’s entire summer harvest to fail.

Although there were various theories as to the cause – chief amongst them the increases in summer temperatures over the last few years – the exact mechanism that led a roughly semicircular region of ice to detach from the bedrock beneath was unclear. But now Jerome Faillettaz from ETHZ in Switzerland and colleagues have used a new numerical tool to show that the avalanche must have been due to a local decrease in the friction coefficient between the ice and bedrock, probably because of meltwater entering via a crevasse.

Applicable to landslides and rockfalls as well as ice on steep slopes, the tool uses a simple “blocks and springs” approach for modelling gravity-driven instability. As the blocks begin to slide, the brittle springs fail; the model accounts for factors such as creep, friction and glacier boundary conditions.

The researchers found that while altering the glacier geometry in the model or changing the support provided by the peak’s side glacier did not cause ice break-up, a uniform change in friction coefficient across the whole of the glacier base made all of the ice fall. But when the team initiated a progressive decrease in friction in just one area of the glacier, a crown crevasse opened up and the ice failed with a similar semi-circular pattern to the 1895 event.

The research isn’t only relevant to the past – it’s likely that climate change will affect the stability of cold hanging glaciers around the world. Ice from the Glacier de Taconnaz on Mont Blanc could, for example, crash down onto the resort town of Chamonix. Faillettaz says that we need detection methods such as seismic monitoring to provide an early warning of such disasters.

Indeed Faillettaz and colleagues have trialled the use of seismic geophones on the Weisshorn cold glacier, which breaks up on a roughly ten-to-fifteen-year cycle and can disturb road and train access to Zermatt, Switzerland. The scientists believe they can detect acceleration of the ice in this way up to two weeks before any fracture. Although it’s also possible to spot surface acceleration visually, the technique doesn’t work in the bad weather conditions when such events often take place.

Faillettaz is cautious about the general applicability of the seismic method, however. In the case of the Weihorn, trapped water is not believed to be a factor behind the break-up; it may be harder to use seismic techniques to predict an Altels-type failure.

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