Crevasses are deep cracks that form in the surface of a glacier or ice sheet, resulting from the stresses generated by the movement of the massive ice body across varied terrain. Glaciologists study the patterns of these fractures to understand the internal flow and strain rates of a glacier. Crevasses are dynamic indicators of a glacier’s health and activity, playing a significant role in both scientific analysis and the practical realities of polar and mountain exploration.
What is a Crevasse? Defining the Ice Fracture
A crevasse is a deep, wedge-shaped fissure that forms in the upper, brittle layer of a glacier. This fracture is created when the internal stress within the moving ice exceeds the material’s strength, causing it to crack instead of flow smoothly. The typical shape is a near-vertical fracture that narrows toward the bottom, often described as a V-shape. Crevasses can reach widths of up to 20 meters (about 65 feet) and depths exceeding 45 meters (about 150 feet).
The depth of a crevasse is limited by the pressure of the overlying ice. Below approximately 30 to 50 meters (100 to 165 feet), the immense pressure causes the ice to behave plastically, which squeezes any cracks shut. The sheer walls of an open crevasse can expose the glacier’s stratigraphy, allowing scientists to view the layers of snow and ice from previous years.
The Mechanics of Formation: Why Glaciers Crack
Glaciers are slowly flowing rivers of ice, and this movement is the fundamental cause of crevasse formation. The ice mass is divided into two zones: a brittle zone and a ductile zone. The brittle zone is the upper layer, typically the top 50 meters of ice, where the pressure is insufficient to prevent fracturing.
Below the surface layer is the ductile or plastic zone, where high pressure allows the ice to deform and flow without cracking. Crevasses form only in the brittle zone when the ice is subjected to intense tensile stress (a stretching force) or shear stress that exceeds its ability to deform slowly. When a glacier flows over an uneven bed or changes speed, the differential movement creates strain, forcing the upper layer to crack. For example, if the ice accelerates over a convex slope, the surface must stretch to keep up, similar to quickly pulling a piece of thick putty until it snaps. This sudden stretching results in a fracture.
Zonal Distribution: Where Different Crevasse Types Form
The location and orientation of a crevasse are determined by the specific stress fields present in that zone. Transverse crevasses are a common type, forming across the glacier, perpendicular to the direction of flow, in areas of longitudinal extension. These often appear where the glacier passes over a steepening slope or convex area, causing the ice to stretch and accelerate. They are frequently observed in the accumulation zone.
Conversely, longitudinal crevasses run parallel to the direction of ice flow. These typically develop where the glacier is spreading out sideways, such as when the valley widens. Marginal crevasses form near the edges of a valley glacier due to friction with the valley walls. Since the center moves faster than the edges, this shearing action causes the crevasses to angle up-glacier at about 45 degrees. A specialized fracture is the bergschrund, a massive crevasse that separates the moving ice of the glacier from the stagnant ice and rock near the head of the glacier.
Hidden Hazards and Glacier Travel
Crevasses represent a major hazard for anyone traversing glaciated areas, especially when they are concealed. A snow bridge is a fragile arch of snow that forms across the opening of a crevasse, created by wind-drifted snow or the settlement of accumulation. These bridges can make a crevasse completely invisible from the surface, creating an illusion of solid ground.
The strength of a snow bridge is highly variable and diminishes during warmer months due to melting. An unseen crevasse covered by a snow bridge is the most common danger associated with glacier travel, as the bridge can collapse instantly under a person’s weight. Travel on glaciated terrain often requires specialized techniques, such as roped travel and probing, to detect the hidden voids. Falling into a crevasse can lead to fatal injuries from the impact or hypothermia.