A crevasse is a deep, open crack or fissure that forms within a moving body of ice, such as a glacier, ice cap, or ice sheet. These fractures manifest the intense stress and strain the ice mass undergoes as it flows across the landscape. They are a dynamic feature of the glacial environment, constantly opening, shifting, and closing. Understanding these cracks is fundamental to glaciology and the assessment of hazards in cold regions.
Defining Crevasses and Their Habitat
Crevasses appear as near-vertical, V-shaped fissures that can be quite substantial in size. They typically reach widths of up to 20 meters and can extend downward to a depth of approximately 45 meters, or about 150 feet. The depth limit is determined by a transition point within the ice mass.
Glacier ice at the surface is relatively brittle, forming the brittle zone, where the ice fractures easily. Below this upper layer, usually exceeding 30 to 50 meters in depth, the immense pressure causes the ice to deform and flow plastically instead of cracking. This deeper region, the plastic zone, prevents crevasses from propagating to the glacier’s base, confining them to the upper, less pressurized ice.
The Mechanics of Crevasse Formation
Crevasses form when the internal stress within the ice exceeds the ice’s tensile strength, effectively pulling it apart. The primary force responsible for this fracturing is tensile stress, which stretches the ice mass. This stretching is a result of differential movement, where one part of the glacier moves significantly faster than an adjacent part.
One common trigger is the glacier accelerating as it flows over a convex slope or a steepening incline in the underlying bedrock. This change in velocity causes the upper, rigid ice to stretch too rapidly for the underlying plastic ice to accommodate. This forces the brittle surface layer to crack.
Major Classifications of Crevasses
Crevasses are categorized based on their orientation relative to the glacier’s flow and the specific stress environment that creates them. Transverse crevasses are among the most common type, running perpendicular to the direction of ice flow. They typically form in areas where the glacier is moving rapidly, such as over a sudden steepening of the slope, causing the ice to stretch longitudinally.
Marginal crevasses develop along the outer edges of the glacier, forming an angle of about 45 degrees with the valley wall. These are caused by the friction between the moving ice and the stationary rock of the valley sides, which creates a shear stress that pulls the ice apart at an oblique angle. The faster-moving central ice pulls away from the slower ice near the margins.
In contrast, longitudinal crevasses run roughly parallel to the direction of ice movement. They often appear where the glacier is expanding laterally, such as when the valley floor widens. This lateral spreading creates a tensile stress perpendicular to the flow, resulting in cracks that align with the glacier’s path.
The bergschrund is a specialized form of crevasse found at the head of an alpine glacier. This large, deep fissure separates the moving ice of the main glacier from the relatively stagnant ice and snow clinging to the headwall of the cirque above. It is a persistent feature marking the separation point between the actively flowing ice and the fixed terrain.
Hazards and Detection
The greatest hazard associated with crevasses is the formation of a snow bridge, a temporary layer of snow that drifts and settles across the opening. These bridges can completely obscure the deep void beneath, making the crevasse invisible to a traveler. A snow bridge’s strength is highly variable, often being too thin or weak to support the weight of a person or vehicle, leading to unexpected collapses.
Glacier travelers employ several methods to mitigate this hidden danger. Roping up ensures that if one person falls through a snow bridge, their teammates can anchor them and execute a rescue. Probing involves using long poles to systematically feel the snow ahead for hollow spots that indicate a void beneath. More advanced techniques include the use of ground-penetrating radar (GPR), which is towed across the surface to image the subsurface ice layers and locate the air-filled cavities of hidden crevasses.