Understanding Crevasses: A Glacier Travel Primer for New Mountaineers
A crevasse is a crack in glacial ice that forms when different sections of the glacier move at different speeds, creating tensile stress that the ice cannot accommodate without fracturing. They range from hairline cracks a few inches wide and a few feet deep to open chasms wide enough to swallow a climber, tent, and pack simultaneously, plunging hundreds of feet into darkness. Some crevasses are completely obvious: open blue-green gashes visible from a hundred meters that any competent traveler simply walks around. Others are entirely invisible, bridged by a thin arch of compacted winter snow that appears identical to solid glacier surface but will not support a person’s weight. The second type is the primary hazard in glacier travel.
Understanding crevasses, how they form, where they tend to concentrate, how to assess the strength of a snow bridge, and what to do when someone goes through one, is the foundational safety knowledge for any mountaineer venturing onto glaciated terrain. This knowledge cannot be acquired entirely from reading, but understanding the underlying principles before your first glacier experience makes hands-on instruction significantly more productive.
Where Crevasses Form: Predicting the Hazard
Glaciers flow downhill continuously, driven by gravity and the plastic deformation of ice under pressure. When different sections of the glacier travel at different speeds, or when the glacier flows over terrain features that create differential stress in the ice, crevasses open to accommodate the strain. Learning to anticipate where crevasses will be before you can see them is a significant glacier navigation skill, particularly when spring snowpack covers the surface and the underlying ice structure is invisible.
Icefalls are the most dramatically crevassed zones: sections where a glacier flows over a sharp increase in the slope angle, causing the ice to break into a chaotic maze of towers, chasms, and fractured blocks. In addition to the crevasse density, icefalls are hazardous because of serac collapse: the towers of ice left between crevasses can topple or collapse spontaneously, often triggered by temperature changes that alter the stress balance in the ice. Minimize time in icefalls and move through them in the early morning hours when cold temperatures reduce serac instability.
Convex bends in the glacier surface, where the ice rolls over a change in the underlying terrain, create zones of tensile stress that open transverse crevasses running roughly perpendicular to the glacier’s flow direction. These are often the densest crevasse zones on moderate glaciers and should be mapped from photographs and maps before you enter them.
Glacier margins, where the ice meets the rock walls of the surrounding terrain, often contain crevasse systems running parallel to the glacier edge, created by differential movement between the slower-moving marginal ice and faster-flowing central glacier. These marginal crevasses can be partially hidden under snow or debris fallen from the adjacent rock walls.
Assessing Snow Bridge Strength
In spring and early summer, many crevasses are covered by snow bridges: arches of compacted snow that span the opening and may or may not support the weight of a person and pack. Assessing the strength of a snow bridge is one of the most difficult real-time judgment calls in glacier travel, involving several factors that interact in ways that experience develops better than any description can capture.
Snow bridges are generally stronger when the snow is cold, dense, and well-consolidated; when the bridge has been in shaded conditions that prevented diurnal warming cycles from softening and weakening the bond between snow crystals; when the bridge shows consistent width across its span suggesting it is bridging a narrow crevasse; and when probing with a ski pole or ice axe shows consistent resistance rather than a hollow thud. They are weaker and more dangerous when the snow is soft and wet from afternoon solar heating; when the bridge shows sagging or curvature suggesting it has already begun to deform under its own weight; and when probing reveals a hollow sound indicating the snow arch is thinner than the surface suggests.
When crossing a snow bridge of uncertain strength, one team member crosses at a time while the others remain in a position to arrest a crevasse fall. Cross quickly and directly without stopping in the middle to probe or assess. Cross in the early morning when snow bridges are at their strongest from overnight cold.
Rope Team Protocols
Glacier travel in crevasse terrain requires traveling roped as a team, with members spaced along the rope at distances that prevent more than one person from being in a crevasse zone simultaneously. The standard spacing for a three-person team on a moderate glacier is 20 to 25 feet between each climber, with the excess rope carried in hand coils rather than trailing on the surface. When a crevasse fall occurs, the rope team members not in the crevasse must immediately arrest to stop the fall and then build an anchor to begin the rescue operation.
Every member of a glacier rope team carries individual crevasse rescue equipment: a chest harness for the fallen climber to convert their sit harness to a full-body configuration during hauling; at least two prussik loops for ascending the rope from inside the crevasse; locking carabiners; and the components and knowledge to build a Z-pulley system providing 3:1 mechanical advantage for the surface team hauling the fallen member. The specific techniques for building this system efficiently in a real emergency cannot be adequately conveyed by written description. Take a crevasse rescue course from a qualified instructor before your first glacier objective. This is not a precaution; it is a prerequisite for traveling on glaciated terrain responsibly.
NOLS, the American Alpine Institute, and qualified guide services in regions with accessible glacier terrain all offer glacier travel and crevasse rescue courses. The Mazamas in Portland, the Mountaineers in Seattle, and the Colorado Mountain Club offer guided glacier courses that include crevasse rescue training as a core component. Take one before you travel on a glacier, not after.
