A glacier is a large body of dense ice that forms on land and moves slowly under the influence of its own weight and gravity. This movement causes the ice to flow, which generates internal stresses. A crevasse is a deep, open crack that develops in the upper surface of the glacier ice. These fissures result directly from differential movement and the mechanical stress placed upon the ice mass as it flows across underlying terrain.
The Role of Ice Movement and Stress
The formation of a crevasse requires differential flow. Glacial ice moves like a highly viscous fluid, but its speed is not uniform across the mass. Ice near the surface and the center generally flows faster than ice near the bed or valley walls, which are slowed by friction. This variation in speed creates internal stress within the ice body.
The specific stress responsible for opening a crevasse is tensional stress, which acts to pull the ice apart. When the ice stretches too quickly, it fractures because its viscous flow cannot keep pace with the rate of extension. This fracturing behavior contrasts sharply with compressional stress, which occurs where the ice is pushed together, causing existing cracks to close. Tensional stress causes the ice to break in a brittle manner, initiating the vertical fissure that characterizes a crevasse.
The Physical Limits of Crevasse Depth
A crevasse cannot extend indefinitely downward; its maximum depth is controlled by a fundamental change in the mechanical behavior of the ice. Near the surface, the ice is under low pressure and behaves rigidly, meaning it fractures easily when pulled apart. This upper layer, where the ice is prone to breaking, is known as the brittle zone.
As depth increases, the immense weight of the overlying ice leads to a corresponding increase in confining pressure. This pressure dramatically alters the ice’s physical properties, causing it to transition from rigid, brittle behavior to plastic, ductile flow. Below this transition point, the ice deforms and flows like a very slow, highly viscous substance rather than fracturing. The maximum depth of a crevasse is defined by this brittle-ductile transition zone, where the pressure is sufficient to squeeze any potential crack shut.
The depth of this transition is typically between 30 and 50 meters, though this range can vary based on the glacier’s temperature and the local strain rate. Colder ice is stiffer and may allow crevasses to reach slightly greater depths before the ice begins to flow plastically. If a crevasse fills with meltwater, the hydrostatic pressure can counteract the confining pressure of the ice. This process, known as hydrofracture, can temporarily allow a water-filled crevasse to penetrate deeper than the standard transition depth.
Crevasse Types Defined by Glacial Stress Fields
The specific pattern and orientation of a crevasse on the glacier surface directly reflects the direction and type of stress field acting on that part of the ice. Glaciologists categorize crevasses based on how their orientation relates to the direction of ice flow. These patterns provide scientists with a visual map of the internal stresses within the glacier.
Transverse crevasses form perpendicular to the direction of flow, appearing as cracks that stretch across the glacier’s width. They are found in areas of longitudinal extension, where the ice is being stretched out along the direction of its movement. This often occurs when the glacier accelerates, such as when it flows over a convex slope or a sudden steepening of the underlying bedrock.
Longitudinal crevasses, in contrast, form parallel to the direction of the ice flow. These cracks develop in areas where the glacier spreads laterally, such as where a valley widens significantly. The spreading motion creates a tensile stress that acts perpendicular to the flow direction, pulling the ice apart sideways and resulting in fissures aligned with the glacier’s path.
Marginal crevasses, sometimes called shear or splay crevasses, form along the edges of the glacier near the valley walls. These features are caused by shear stress, which arises from the friction between the faster-moving central ice and the slower, stagnant ice pressed against the rock walls. This stress creates a characteristic oblique pattern, with the crevasses angled back toward the center of the glacier, often pointing at an angle of approximately 45 degrees up-glacier.