A crevasse is a deep fissure or crack that forms in a glacier or ice sheet. These features result directly from the continuous, slow flow of glacial ice over underlying terrain. Glacial movement generates immense internal stresses that exceed the ice’s ability to deform without fracturing. Crevasses are an inherent feature of all ice masses thick enough to flow.
The Forces That Cause Crevasses
The initiation of a crevasse occurs when different parts of a glacier move at different speeds, a process called differential movement. This uneven flow subjects the upper, colder layer of the ice to forces that cause it to break in a brittle manner.
One primary mechanism is tensile stress, or stretching, which happens where the glacier flows over a convex slope or where the ice accelerates. This stretching causes the ice to pull apart, resulting in transverse crevasses that run perpendicular to the direction of flow.
Another element is shear stress, which is concentrated along the glacier’s edges or near obstacles. Since the ice at the center of a valley glacier moves faster than the ice dragging against the valley walls, this friction creates lateral stresses. These stresses generate marginal crevasses that often appear in a herringbone pattern angled upstream from the margins.
Why Crevasse Depth Is Limited
Crevasses are not bottomless; their depth is constrained by the physical properties of ice under immense pressure. Most open, dry crevasses reach a maximum depth between 30 and 50 meters (100 to 165 feet). This limitation is due to the increasing weight of the overlying ice, known as overburden pressure, which increases with depth.
At this depth, the ice structure transitions from a brittle state, susceptible to fracturing, to a ductile or plastic state. This boundary is known as the Brittle-Ductile Transition Zone. Below this point, the pressure is so high that the ice deforms and flows like a highly viscous liquid, rather than cracking. This plastic flow squeezes any fissure shut before it can propagate further downward.
Temperature also influences this limit, as colder ice is stiffer and more brittle, potentially allowing cracks to extend deeper. Conversely, meltwater within a crevasse can exert hydrostatic pressure on the crack walls. This water pressure can counteract the overburden pressure, allowing a water-filled crevasse to propagate deeper, sometimes reaching nearly the glacier bed.
Hidden Dangers and Crevasse Bridges
The danger presented by crevasses is compounded when they are obscured from view, creating a hazard known as a hidden crevasse. This concealment often occurs in the accumulation zone of a glacier or during seasons with heavy snowfall. Wind action can deposit snow across the opening of a crevasse, forming an arch of snow called a snow bridge.
These snow bridges appear to be a solid, continuous surface, making them indistinguishable from the surrounding snowpack. However, they are structurally weak and their stability is highly variable, depending on temperature, age, and snow density. A fall is most likely to occur some distance from the crevasse’s edge, where the snow bridge is thinnest and weakest. Recognizing crevasse patterns and traveling roped together are common precautions for mitigating the risk posed by these deceptive openings.