Crevasses are deep cracks or fissures that form within glaciers and ice sheets. They are significant features, varying greatly in size from narrow fractures to wide chasms. These openings on the ice surface are a result of the forces acting on the ice as it moves.
Understanding Crevasses
Crevasses form when the ice within a glacier experiences stress that causes it to fracture. Glacial ice, while appearing solid, behaves in both brittle and plastic ways depending on conditions. Near the surface, ice is brittle, meaning it will crack when subjected to tensile stress, similar to how a rigid material might break if pulled apart. This stress arises from the glacier’s movement over uneven terrain, such as bumps in the bedrock, or from changes in its flow speed.
For instance, when a glacier speeds up as it flows down a valley, the ice can stretch and pull apart, leading to cracks. Stress also builds where the glacier’s flow is constrained by valley walls or where the ice spreads out over a wider area. These different stress patterns result in various types of crevasses, reflecting specific dynamics within the ice flow.
What Limits Their Depth
The depth to which a crevasse can extend is limited by the physical properties of ice under pressure. As depth increases within a glacier, the weight of the overlying ice exerts increasing pressure. This pressure causes a change in how the ice behaves, transitioning from a brittle state, where it cracks, to a plastic or ductile state, where it deforms and flows.
This change is known as the brittle-ductile transition zone. Below this zone, ice can no longer sustain open fractures; instead, it slowly flows and deforms under stress, much like a very thick, slow-moving fluid. The pressure essentially squeezes any potential cracks shut, preventing them from propagating further. This transition means that while the surface ice can crack, the deeper ice simply flows around the stresses that would cause fracturing at shallower depths.
Reaching Maximum Depths
Crevasses reach a maximum depth of approximately 50 meters (about 160 feet) due to the brittle-ductile transition. Modern understanding places the maximum around this depth, where ice pressure becomes too great for cracks to remain open. This depth limit is consistent across various types of glaciers, whether in polar regions or mountain ranges.
However, the appearance of depth can be deceiving. A crevasse might seem deeper if it is covered by a snow bridge, which can obscure the actual base of the fracture. Additionally, water-filled crevasses, or moulins, can extend much deeper, as the water pressure can help force the crack open further than it would be in dry ice. Most visible crevasses encountered on glaciers are often shallower, with average depths ranging from 10 to 15 meters.
Navigating Crevasse Hazards
Crevasses pose hazards to anyone traversing glaciated terrain due to their depth and often concealed nature. Snow bridges, layers of snow that accumulate over a crevasse, are a particular danger, hiding them from view. These bridges can be unstable and collapse unexpectedly under weight, leading to falls. The risk increases during warmer weather when snow bridges weaken.
For safety in glaciated areas, precautions are necessary. Traveling in roped teams ensures that if one person falls into a crevasse, their companions can arrest the fall and assist in a rescue. Using crevasse rescue training and carrying appropriate gear, such as ropes, harnesses, and ice axes, are standard practices. Awareness of terrain, continuous assessment of snow conditions, and probing the snow ahead to detect hidden crevasses are important for reducing risks.