M5 Fractures and Rapid Events

Topic

The fracture and rapid-event mode encompasses cryospheric processes occurring on timescales ranging from mere seconds to hours, all sharing a common trait: they involve the mechanical failure of ice or snow.

Snow avalanches are the most frequent phenomenon within this mode; they are triggered when the shear stress acting on a snow layer exceeds the shear strength of an underlying weak layer—the precise plane where failure occurs. Slab avalanches are the most dangerous type, as they involve the sudden fracture of a cohesive snow slab that slides as a single, intact block. In contrast, powder snow avalanches reach speeds of 100 to 300 meters per second, generating an airborne cloud of snow particles capable of traveling independently of the underlying terrain's topography.

Crevasses—fissures in glacial ice—form when the tensile stress acting on the ice exceeds the fracture strength of the polycrystalline ice itself. On a glacier's surface, crevasses appear where ice flow velocities diverge, creating an extensional zone: at the exit of a bend in the glacier's path, at the head of an icefall, or where the glacier accelerates while passing over a rough section of its bed.

The maximum depth a crevasse reaches in dry ice is limited because the pressure tending to close the crevasse increases with depth; however, when surface water is available, it can propagate the crevasse down to the glacier's base through hydrofracture—a mechanism that drives the largest calving events. Jökulhlaups—an Icelandic term for glacial floods—are sudden discharges of glacial or subglacial lakes capable of releasing enormous volumes of water in just a few hours. This sudden drainage is triggered when water pressure overcomes the resistance of the ice dam containing it—whether because the ice dam fails structurally (like an arch) or the ice begins to float—or when the lake pressure exceeds the weight of the overlying ice, forcing a path for the water through the subglacial system itself.

The flow of water released during a jökulhlaup through subglacial channels follows the hydraulics of a pressurized tunnel, and this flow accelerates via a positive feedback loop: warm water flowing through the channel melts the ice walls and widens the passage; the widened channel allows an even greater volume of water to pass; and this increased flow generates yet more heat, completing a self-reinforcing cycle that—alongside snow avalanches and crevasses—constitutes the set of fracturing processes and rapid events mentioned at the beginning of this description.

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