Crevasses are deep fissures or cracks in glacial ice. These openings, which can extend up to 20 meters wide and 45 meters deep, reveal the dynamic nature of glaciers. Understanding how and where these features develop offers insight into the forces shaping glacial landscapes.
The Science of Ice Cracking
Glacial ice responds to stress differently depending on depth. Deeper ice deforms plastically, flowing slowly under pressure, while the upper layers act more rigidly. When stress is applied rapidly or intensely, the ice fractures in a brittle manner, similar to how a stiff material might break. This cracking is driven by three primary types of stress: tension (pulling apart), compression (pushing together), and shear (sliding past one another). Tension and shear are the main forces forming crevasses.
Crevasse Formation in Tension Zones
Tensional stress pulls ice apart, forming crevasses. These zones often occur where a glacier’s speed increases. An icefall, where a glacier flows over a steep drop, is a prominent example. The ice’s acceleration over this incline causes stretching, resulting in numerous transverse crevasses perpendicular to ice flow.
Transverse crevasses also form on convex slopes, where the glacier flows over a bulge, causing the surface ice to extend and crack. A bergschrund, found at the head of a glacier, is another specific tensional crevasse. This deep crack forms where moving glacier ice separates from stagnant ice or rock walls at the back of a cirque. These features illustrate how changes in topography directly influence the ice’s behavior.
Crevasse Formation in Shear Zones
Shear stress develops when adjacent parts of a glacier move at different speeds or in slightly different directions. This differential movement causes the ice to slide past itself. Glacier margins are common shear zones because friction with valley walls slows the ice near the edges, while the center flows faster. This speed difference creates marginal crevasses, which often angle upstream, typically around 45 degrees relative to the valley wall.
As a glacier navigates a bend, the ice on the outer curve must travel farther and faster than the ice on the inner curve. This generates shear stress, often resulting in radial crevasses that fan out from the inside of the bend. Splaying crevasses, which can appear near the edges or where a glacier widens, are another type influenced by shear. These patterns reflect ice movement and underlying terrain.
Safety Around Crevasses
While visually compelling, crevasses pose substantial hazards to anyone traveling on glaciers. Their presence indicates an active, moving glacier. Snow bridges often conceal crevasses, making them invisible and extremely treacherous, as they can collapse without warning. Recognizing subtle depressions or irregularities in the snow surface can sometimes indicate a hidden crevasse.
Safe travel in glaciated environments requires specialized knowledge and preparation. Avoiding crevassed areas is always the safest approach when possible. For those who must traverse such terrain, proper training in glacier travel techniques, using appropriate equipment, and traveling with experienced professionals are all necessary precautions. This awareness helps to mitigate the risks associated with these powerful natural formations.