A crevasse is a deep crack or fissure that forms within a glacier or ice sheet. These features are common in glacial environments worldwide, varying in size from millimeters to meters wide and tens of meters deep, sometimes extending hundreds of meters. Crevasses visibly demonstrate the significant stresses within these massive, moving ice bodies. Their presence indicates areas where the ice experiences substantial internal forces.
Glacial Movement as a Prerequisite
Glaciers are dynamic systems of ice that flow slowly under gravity. This movement is driven by two primary mechanisms: internal deformation and basal slip. Internal deformation, also known as plastic flow, involves the gradual deformation and sliding of individual ice crystals past each other under sustained pressure. This process allows the ice to behave plastically.
Basal slip occurs when the glacier slides over its underlying bedrock or sediment. A thin layer of meltwater at the glacier’s base often facilitates this sliding by acting as a lubricant. Both internal deformation and basal slip contribute to the differential movement within the glacier, creating the fundamental stress that ultimately leads to crevasse formation.
Stress and Ice Fracture
The movement of glaciers generates various types of stress within the ice, leading to its fracture and crevasse formation. Ice, despite its solid appearance, behaves as a brittle material when subjected to rapid or intense stress. When the applied stress exceeds the ice’s elastic limit, brittle failure occurs, resulting in cracks.
Tensile stress arises when the ice is stretched or pulled apart. This often happens when a glacier accelerates, such as when it flows over a convex slope or a steepening section of its bed. The pulling forces cause the ice to separate, forming an open fissure. Shear stress involves forces acting parallel to a surface, causing different parts of the ice to slide past each other. This type of stress is prevalent near glacier margins or where flow speeds vary laterally. While compressive stress generally closes existing cracks, it can indirectly contribute to crevasse formation by causing ice to buckle or by creating tensile stress in adjacent areas.
Crevasse Patterns and Their Origins
The specific types of stress within a glacier dictate the orientation and pattern of the crevasses that form. These patterns provide insights into the underlying ice dynamics. Transverse crevasses form perpendicular to the direction of ice flow. They typically arise in areas of longitudinal extension, where the glacier accelerates or flows over an upward-sloping bedrock feature, causing the ice to stretch.
Longitudinal crevasses run parallel to the direction of ice flow. These often develop in areas where the glacier spreads laterally, such as when it flows into a wider valley, or due to shear stress that causes the ice to pull apart sideways. Marginal crevasses, also known as splay or shear crevasses, form at an angle to the flow direction, often pointing obliquely up-glacier from the edges. They are the result of significant shear stress and friction between the moving ice and the stationary valley walls. A bergschrund is a specialized crevasse that forms at the head of a glacier, separating the moving glacier ice from the stagnant ice or rock at the back of the cirque. This separation is primarily due to tensile stress as the glacier pulls away from the mountain headwall.
Influencing Factors in Crevasse Formation
Beyond the fundamental stress mechanisms, several other factors influence the development and characteristics of crevasses. Ice thickness plays a role, as thicker ice can generate greater stress and result in deeper crevasses. Most crevasses are limited in depth, typically closing at about 30 to 50 meters (100-160 feet) due to the immense pressure of the overlying ice, which causes the ice to behave plastically at depth.
The underlying topography significantly impacts crevasse formation. Bumps, steps, or changes in the bedrock can induce localized stress concentrations, leading to crevasse development even in otherwise stable areas. Glacier velocity is another factor; faster-moving glaciers generally experience more intense and frequent crevasse fields because the increased speed leads to greater differential stresses. Additionally, the presence of meltwater can lubricate the glacier’s base, affecting its flow and potentially widening or deepening existing cracks through hydrofracturing. Ice temperature also influences its brittleness and ability to deform, affecting how and where fractures occur.