Locating Crevasses on a Glacier
by Michael Powers, IFMGA
AAI Senior Guide & Director for Staff Development
Question:
From the July 2006 edition of AAI's E-newsletter
How do I know where the crevasses are on glaciers?
- Laura Henderson (Pasadena, CA)
Answer:
Dear Laura,
There's no way to know where crevasses are with certainty unless you are able to visit that glacier in mid to late season when most of the snow melts and reveals the bare ice underneath. Then you will see patterns typical for that glacier, but remember, the precise configuration of slots changes every year because of the movement of the glacier down the mountain. Depending on the altitude of the peak and regional temperatures, late season visibility may just apply to glaciers lower on the mountain. On many big peaks (or moderately high peaks with deep snowpacks), the snows never melt fully away high on the mountain - that's key to the glaciers being able to maintain themselves. On a typical late July day in the Alps or Cascades, a glacier at low altitude will look "late season" and a glacier at high altitude will look "early season."
You can make these generalizations about crevasee locations: 1) In areas where slopes lose their steepness, the glacier is likely to slow in its movement down the mountain, and therefore crevasses are likely to diminish in size and number; and 2) in areas where slopes steepen, the glacier is likely to accelerate in its descent and present more and larger crevasses.
In the early to mid-dry season (early to mid-summer is what we call it in most mountain ranges), you can't detect all crevasse patterns because of snow cover, so there are two essential components to a safe approach:
- Use proper glacier travel technique to minimize the harm done if you do end up punching part way into or falling all the way into a crevasse.
- Travel at a time when snow bridges are at their best: seasonally, this means that making your ascents early in the climbing season is better than later (because crevasses are filled in or their snow bridges are at maximum size and strength), and on a day-to day basis, this means climbing during the coldest period of the 24-hour clock. Climbers often get up at midnight and complete the climb - and the descent - by noon in order to cross snow bridges while the snow is still well frozen in place. Keep in mind, the ultimate value of this is relative to the temperatures on the mountain; if you are climbing in a hot spell and there are no freezing temperatures at night, you will still get better conditions by climbing early, but you won't get the degree of bridge strengthening that you would get from freezing night-time temperatures).
You didn't ask about them, but other primary hazards in glacial terrain are ice and serac fall, avalanches, and climber falls on steep slopes. I'll just address the first of these here.
To avoid ice and serac fall (which is more a function of glacier movement and gravity than daily temperature fluctuations), it's best to travel quickly through areas of vulnerability and avoid the time of exposure to the danger. Try to know what's above your slope. The use of a topo map will reveal steep slopes (as well as those less hazardous, large compression zones) and can help you choose the safest line to take. When it is necessary to traverse under an icefall (as opposed to going around it, which is preferable) move fast, fast, fast.
To summarize and simplify, the two most common hazards of glacier travel are crossing crevasses and traversing under icefalls. Cross crevasses when the snow is at its firmest, and minimize the time under icefalls by moving very quickly.
- Mike Powers
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