Glacier Travel & Crevasse Rescue

by Andy Selters

  over the steeper drop, and this “differential flow,” plus the brittleness of surface ice, causes the ice to split.

  This simple sort of tension zone creates transverse crevasses, slots that form perpendicular to the glacier's flow. A smooth drop will make a series of parallel crevasses form from a bit above the drop-off to somewhere above the bottom. In theory, crevasses at the bottom of the drop-off are sealed shut by the pooling, compressive flow; this area is known as a compression zone (see fig. 1.1).

  Crevasse-forming tension also occurs when a glacier's margins drag and press against the adjacent mountain slopes and valley walls. Here, the center of a glacier is less affected—the central stream of ice courses on faster and more freely, as with the leading arm of an amoeba. This condition subjects the glacier's margins to two forces: compression from the adjoining rock and tension from the faster, central stream. These forces combine to create a classic pattern of herringbone crevasses angling up-glacier at 45 degrees, as shown in figure 1.2. These are called marginal crevasses. Usually the flow differential is greater toward the central end of marginal crevasses; therefore, they tend to widen toward the free-flowing center of the glacier.

  Turns generate tension as well. The ice on the outside of a turn has farther to travel, and thus is pulled with tension creating radial crevasses. Ice along the inside edge suffers a confusion of tensions from all sides and can break into crisscross crevasses. Similar confusion results when glaciers with different drainages converge. Compressive forces from the merging flows create pressure that on the one hand seals crevasses but also creates pressure that speeds the flow and generates crevasses nearby.

  Figure 1.1 Cross-section of tension and compression zones

  Figure 1.2 Marginal crevasses (top view)

  In heavily glaciated regions like the Alaska Range, the tops of ridges can accumulate enough snow and ice to initiate flow off either side of the ridge line, creating a tension zone atop the ridge. Thus, crevasses can form along broad, relatively flat ridges. Those used to windswept ridge tops in a moderately glaciated region like the Cascades never expect to see crevasses here. In an analogous situation, the perennial cornices along ridge tops in heavily glaciated terrain can teeter downslope at glacial slowness, leaving shallow crevasses as they slowly crack away from the ridge line.

  Finally, there is the uppermost crevasse in a glacier; the bergschrund (see fig. 1.3). This is the gap where the glacier pulls away from stationary ice and snow above. Above most bergschrunds, there rises a steep mountain headwall that sloughs off most snowfall; headwalls are generally too steep to collect enough mass to become part of the moving glacier. Because of all the snow that fills in from the headwall above, bergschrunds generally aren't as deep as normal crevasses, although they can be enormously wide and long.

  Late summer travel on the Coleman Glacier, Mount Baker, North Cascades

  Figure 1.3 Cross-section of a bergschrund

  Cousins to crevasses, moats are the melt-gaps between a glacier's edge and its surrounding rock walls. Moats can be fatally deep and overhung by weak snow just like crevasses. Moats often run deeper than expected, because the adjacent rock warms in the sun, channeling heat and melt-water down to deepen the hole. Moats can be the most dangerous feature of small glaciers and snowfields like those in the Sierra or Tetons. Rock anchors often must be placed to belay across a moat.

  Most crevasses are generally linear; extending perpendicular to the tension in the ice. They range from incipient slits to gigantic chasms capable of swallowing entire neighborhoods. Larger; thicker glaciers and the colder glaciers of the polar regions have larger and deeper crevasses. On small glaciers, in say the southern Cascades, the largest crevasses may be no more than 15 feet across. The big glaciers in Alaska and the Karakoram have yawning gaps of up to 60 feet (20 meters) across. Even though legends abound of terrifying slots hundreds of feet deep, a crevasse on a temperate glacier is almost never deeper than 115 feet (30 meters). Polar glaciers generate crevasses up to 165 feet deep (50 meters). Because the plasticity of ice increases with depth, and because flow tension decreases with depth, crevasses tend to narrow gradually from the surface downward. Thus, getting tightly wedged is a common hardship to those who fall into crevasses.

  Depending on its temperature and flow rate, glacier ice splits partly in slow deformation and partly in distinct lurches. A 6-foot-wide crevasse may open over the period of a couple months, widening an inch or so at a time, mostly in small but audible “ice quakes.” Temperate glaciers form crevasses with a moderate percentage of recrystallization flow, whereas brittle polar glaciers pop and shudder constantly (see fig. 1.4).

  The more active a glacier, the more crevasses it generates, so that small, stagnating glaciers essentially are snowfields with a bergschrund and a crevasse or two, while active sections of large glaciers might have more crevasses than solid ground in between. Many glaciers spill over cliffs or very steep sections, and here the ice periodically calves off in hunks of various proportions. The resulting chaos of crevasses and towers (called seracs) is known as an icefall.

  Figure 1.4 Glacier Overview

  This outline gives us a general idea of glacier flow and how crevasses form, but, as with fluid studies in general, science has never fully comprehended the dynamics of glaciers. Combinations of dropping, turning, dragging, colliding, pooling, and so on can create crevasses angling in any direction. Also, a glacier's flow often deforms crevasses as it carries them out of their original tension zone. Whatever the combined forces, however they tend to be sustained over a given area, and, therefore, it's normal to find crevasses running fairly parallel within that area.

  SNOWBRIDGES

  The previous discussion can give the glacier traveler an abstract idea of where crevasses are most likely to be found, but it's the instability of snow that bridges crevasses that makes them dangerous. Thus, the most important clues to assessing the crevasse hazard are a sharp eye for the dips and extensions of snowbridges, and an understanding of and subtle feel for the changing strength of the snow underfoot.

  One characteristic of snow is that it tends to stick together The “glue” is simply tiny necks of ice that freeze the individual crystals into an intricate network. When snow falls on a crevassed glacier instead of dribbling into the crevasses like sand through a sieve, many of the crystals accrete on the crevasse lips, especially if the temperature is not far below freezing and if there is a wind. In this way cornices build out from the edges, and eventually they meet and partially or entirely bridge the crevasse. Also, during a storm season, continuing snowfall keeps the gaps bridged even as they widen with glacial movement.

  When snowbridges are strong, they allow mercifully straightforward travel over crevasse-riddled terrain. But when they're weak, they unexpectedly drop climbers into the crevasse's dark depths. Whether a bridge gives the boon of access or the bane of collapse depends partly on its thickness, but more on the strength of its bonding network. With the nearly infinite types of snow, from new-fallen powder to old wind crust, this bonding network can be amazingly strong or hopelessly weak. The interacting forces of temperature, humidity, solar radiation, wind, and the snow's own weight determine the characteristics of snow; so the strength of an undisturbed snowbridge is determined by its climatic history and the current weather.

  Because of seasonal climate patterns, most snowbridges are temporary affairs. Generally, they form and thicken during a stormy season, then progressively collapse during a fair-weather season, the season that climbers naturally favor: Let's take a closer look now at the main processes affecting snow and, hence, snowbridges.

  Figure 1.5 Important processes of snow on a glacier

  Newly fallen snow is rather weak, but as a rule the delicate crystals start to settle as soon as they land and the accumulated layer gradually strengthens. Compaction, sintering (the rounding of crystals with vapor transfer), and wind battering—collectively known as age hardening—all combine to consolidate intricate, ind
ividual snowflakes into small, rounded grains held together by stronger, more interconnected bonds (see fig. 1.5). This process goes on in temperatures below the melting point, although it slows to insignificance below -40 degrees Fahrenheit or so. By this process, snow can strengthen considerably within a couple of days to a couple of weeks after falling. Deeper snow compresses and strengthens itself more, so thicker snowbridges are stronger not only from their greater mass but also from a sturdier bonding network.

  When this “new” (unmelted) snow warms to the melting point (generally in the spring), the bonding network quickly collapses. In the first thaws, the necks between crystals melt first, and the snow can swiftly deteriorate into an insubstantial slurry of crystals and melt-water The low-density, dry snow of continental and subarctic regions has weaker bonds that break down faster and deeper than do those of the denser, wetter snow of maritime climates. But in any case, when the melt-water refreezes (generally at night) the bonds re-form more thickly, the crystals grow larger and rounder; and the whole network consolidates itself stronger than before. Typically what happens during late springtime then, is that daytime heat melts the snowbridges into weak slush and nighttime cold freezes them into strong “Styrofoam-snow” or melt-freeze snow. When the melt-freeze cycle continues for many weeks the snow gradually consolidates into very coarse-grained, very firm “summer snow.” Late summer sun softens only the surface of this old snow, and even a plunging ice-ax shaft might penetrate just a few inches. Old snow that makes it through a full melt season and into the next one becomes a very dense, transitional snow-ice known as firn, or névé.

  For the typical summer excursion, then, it can be invaluable to get a feel for the strength of snow after a period of melt-freeze cycles, and to continually assess the ongoing cycle each day.

  Besides gauging the general snow conditions, climbers of course want to know exactly where the hidden crevasses lie. This is especially difficult when snowbridges are of “new” snow as the unmelted snow tends to cover a glacier's structure with a smooth, level blanket. But a bridge begins to sag after it goes through some melt-freeze cycles. These sags can become quite dramatic over time. Also, as spring progresses, low-lying sags tend to collect more dust. They also tend to collect more of the new powder from a late spring or summer dusting of snow. For these reasons, it's common to see snowbridges accentuated by dustier snow or, conversely, by cleaner whiter snow.

  Also, when snowbridges collapse they rarely do so all at once. Rather sections of the bridge progressively melt and drop away, leaving a hole that hints at a wider and longer underlying crevasse. So when you see a hole in the glacier's surface it's wise to assume there's a crevasse extending well beyond the visible abyss, especially lengthwise.

  Climbers on Yerupajá Glacier, Cordillera Huayhuash, Peru

  Even though crevasses often exhibit visual clues as to their existence, many do not. The history of mountaineering includes the obituaries of many who found this out the hard way. Prudent climbers assume that anywhere there's snow on a glacier there might be a crevasse.

  GLACIER SEASONS IN VARIOUS CLIMATES

  Knowing the pattern of snow evolution, we can now generalize about the annual cycles of surface conditions on glaciers in various climates.

  MARITIME, TEMPERATE CLIMATES

  Most glacier travelers climb in temperate latitudes, where the mountains have generally winter-wet and summer-dry climates like the Cascades and the European Alps. Here most glaciers range in length from a mile or two to maybe 10 miles.

  Typically, winter in the temperate latitudes is characterized by a westerly storm track that buries glaciers with deep snowfall. Mountain slopes near the western coasts of the continents receive a particularly heavy load of wet, maritime snowfall. Slightly more continental, the Alps receive somewhat less winter snow than the Cascades. Aside from meaning a lot of poor weather for travel, the heavy snows of midwinter present glacier travelers with good news and bad news. The bad news is that most crevasses are undetectable under the deep, unmelted snow and since temperatures persist below the freezing point the snow cannot melt and refreeze into a stronger network. The good news is that although crevasses are hidden, most bridges become thick enough to bear plenty of weight. For this reason, most climbers consider middle to late winter to be a relatively safe time to travel in temperate regions, especially on skis (see fig. 1.6 for the complete snowbridge cycle).

  This relative safety continues until sometime in the spring when long days, a high sun and warm air start melting the snowpack. Depending on the depth of the pack and the heat of the season, within a week or two the bridges start to sag and collapse, becoming quite unsafe. Strong sun makes snowbridges most dangerous in the afternoon and evening, with nighttime freezes bringing relative safety during the late night and early morning. But spring conditions can be especially treacherous if nighttime cloud cover or generally warm air keeps the snow from refreezing at night, so that the following day melt-water percolates much deeper into the snowpack and deteriorates the bridges that much more. Down in the ablation zone the spring thaw starts early, gradually progressing up the glacier as the season warms. This means that when the lower part of a glacier has its “spring,” the upper reaches are still in “winter.”

  Figure 1.6 Typical snowbridge seasons on a temperate glacier

  As spring turns to summer, the lengthening “melt” portion of the daily melt-freeze cycle collapses many snowbridges, and the crevasses open up. After a few weeks of this cyclic decay, those bridges that persist withstand melt-weakening more and more and are more likely to remain firm into late morning or even midday. However, a late spring or early summer snowfall can rebridge the newly opened crevasses with a scanty and treacherous few inches of new snow. Summer snowfalls are common in the cooler, higher elevations of the Pennine and Bernese Alps and many bridges persist through the season. Overall, though, warm summer weather on a temperate glacier gradually opens the crevasses, leaving fewer, but more trustworthy, snowbridges. By late September, there might not be a single snowbridge left on a Cascade glacier, and the primary challenge of travel can be to wend a route through mazes of wide-open crevasses.

  As temperatures drop, autumn snowfalls dangerously veil open crevasses with a thin layer of powder As little as 4 or 5 inches of new snow can lay an even, white blanket that obscures all signs of crevasses, but of course this snow will not support the most featherweight climber. It is almost a certainty at this time that any bridge or overhang will give way. Further, it can be very difficult to stop a partner's crevasse fall because the hard summer surface underneath can be quite difficult to ice-ax arrest on, and a stiff crevasse edge will slow the rope very little (see the chapters on travel and rescue techniques). When conditions are like this, it's wise to just stay off glaciers. The snow accumulations of late fall and early winter though, make it easier to hold a partner's fall, then gradually stack the bridges high enough to bear weight, bringing a return to the relative safety of midwinter.

  The temperate glaciers of the Southern Hemisphere—found in the New Zealand Alps and Patagonia—form in a stormier climate than do those of the Northern Hemisphere. Thus, Patagonia has the two largest temperate icecaps (large “icefields” where a plateau of ice sends off flow in all directions) in the world, even though the latitude is similar to that of southern British Columbia, and the peak altitudes are similar to those of the North Cascades. Some glaciers in both New Zealand and Patagonia extend more than 20 miles. Austral summers are not frigid, however so the accumulation zones in both regions collect rain as well as snow. This means that snowbridges cycle through degrees of sloppiness throughout the summer.

  SUBARCTIC CLIMATES

  Typified by those in the Alaska Range, subarctic glaciers have the always-frozen characteristics of polar glaciers in their upper reaches, while their ablation zones are temperate, with melt-water and moderate temperatures easing their flow. The Alaska Range has very large glaciers, many over 30 miles long. These glaciers receiv
e much of their snow in the late summer and early fall, which means that crevasses can be fairly well covered as the bitter, but relatively dry winter moves in. The dry, intense cold of winter keeps these bridges from strengthening as much as on temperate glaciers. In subarctic climates, it's mostly wind that age-hardens the snow during the winter; exposed spots become stiff with a few inches of “wind board.” Pockets protected from the wind remain powdery into the spring thaw.

  When thawing temperatures arrive in April, May, or June depending on the elevation, the subarctic days are so long that melt-water suddenly percolates well into the snowpack, making it deeply treacherous for weeks. The bridges, built of cold, light snow and suddenly infused with melt-water can show little or no sag or stress, and even surprisingly thick ones can readily collapse, especially in the warm afternoons. As a result, climbers in the Alaska Range fall through far more frequently and more unexpectedly than do climbers on temperate glaciers.

  Midsummer opens the crevasses and deteriorates bridges with appalling speed, but firmer, more-trustworthy bridges usually don't develop, because occasional or frequent snowfalls keep renewing them with untrustworthy snow. This oscillation between thawing temperatures and accumulating snow sets up ideal conditions to prolong instability. Like any glacier condition, this prolonged instability moves up-glacier with the advance of the season, and so unstable bridges are a serious problem somewhere on the giant Alaskan glaciers throughout the spring and summer Moderate to severe instability can persist until the colder days and heavier snows of late summer and autumn rebuild the bridges to relative strength.

  The subarctic glaciers of southeastern Alaska and western Yukon form under a similar but more maritime regime. The combination of high peaks near the coast in a strong storm track makes for incredible snowfalls—30 feet can fall in a week, even in May—and the “winter” accumulation season is some 10 months long. The result is the third most extensive glacial terrain on the planet after Antarctica and Greenland. Glaciers here are seas of ice up to 15 miles wide and 90 miles long that fill great valleys with ice more than 1,500 feet deep. There are also large icefields. The usual climbing season, April through July, starts out bitterly cold but quickly warms, with thawing conditions progressing above 10,000 feet by the summer solstice.