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Crevasse - What is it?

crevasse is vertical hole in the ice of a glacier. A glacier in turn is perhaps best described as a moving river of ice. Glaciers form in areas when more snow falls during the winter season that can melt away during the summer, typically in cold regions such as Greenland and the Antarctic or in the high mountains. In total some 10% of the earth’s land surface (15 million square kilometres) is covered by glaciers. Over 95% of this is in the Antarctic and Greenland (see below) but glaciers still cover extensive areas of Canada, North America (principally Alaska), the Himalayas, Scandinavia and the European Alps. These are the areas of most concern to the off-piste traveller.

The snow piles up on a glacier and becomes more and more compacted until the lower layers turns into rock hard glacier ice under huge pressure from the weight of the snow above. As more and more snow falls, the sheer weight of the ice causes the glacier to gradually to deform and slide down the mountain side carrying the original snowflakes down to the warmer, lower areas when they eventually melt away. Scientists estimate that glaciers start moving when the depth reaches about 18 metres. The full process from snowflake to melt water can take thousands of years. Scientists have estimated that a snowflake landing on the top of Mont Blanc might take several thousand years before it eventually reaches the bottom of the mountain and melts away in the summer sunshine. The Antartic ice cap is known to have existed for over 40 million years!

When the ice is moving smoothly under pressure it flows like a very dense liquid, retaining its shape and causing no real problems for glacier travellers. At many resorts such as Zermatt or Saas Fee, skiers trail merrily down the “piste” blissfully unaware that their sedate runs are perched on top of a glacier! The problems arise when the ice has to flow around an obstacle, round a corner, down a steep drop, or out into a widening plateau. At such times, the ice comes under tension and cracks forming vertical slots in the surface. These slots are crevasses. They can vary from a few inches up to tens of metres wide and vary in depth from a few feet up to tens or even (reportedly) hundreds of metres!

In summer, lower altitude surface snow melts away leaving just the hard glassy surface of the glacier ice with most or all of the crevasses wide open and fairly easy to see and avoid. In winter it is a very different matter. Snow forms bridges which covers the surface of even quite wide crevasses so that an apparently smooth snow field can contain dozens or hundreds of hidden traps which the unwary traveller can plunge into when the snowbridge fails. These bridges tend to be weakest at the start of the winter (when not enough snow has fallen) and in the late Spring (when the bridges have begun to weaken and melt). This is one of the main reasons why the European ski touring season does not really start until March or even April when the crevasse danger is at its lowest.

There are two main risks with crevasses:

Trauma: a sudden fall of tens of metres onto the rock hard ice of a crevasse can easily be fatal. Louis Lachenal was killed in just such a fall whilst skiing on the Valle Blanche.

Hypothermia: crevasses are unbelievably cold. Little or no sun penetrates and their lower depths can remain well below zero all the year round. On a sunny winter’s day, a fall into a crevasse can plunge the victim in an instant from feeling warm and cosy to a very real risk of freezing to death.