Crevasse Types and How They Form
Crevasses are the most visually dramatic features of a glacier surface and also among the most informative. They are fractures in glacier ice, opened by tensile stress — the pulling apart of ice as different parts of the glacier move at different speeds or in different directions. Because the pattern of crevasses on a glacier's surface reflects the stresses within the ice at the moment of fracture, an experienced observer can read a glacier's flow dynamics, bed topography, and internal stress regime from the crevasse pattern alone. Understanding why different crevasse types form where they do transforms crevasses from obstacles into a map of the glacier's hidden interior.
Glacier ice behaves as a viscous fluid over long timescales — given enough time, it flows and deforms without fracturing. But it behaves as a brittle solid over short timescales. When tensile stresses build faster than the ice can accommodate by deformation, it fractures. Crevasses are typically tens of metres deep; below approximately 30 to 60 metres, the weight of the overlying ice closes fractures before they can propagate further. The surface expression is dramatic, but crevasses do not penetrate to the glacier bed in most cases.
Transverse Crevasses
Transverse crevasses form perpendicular to the direction of glacier flow and are among the most common crevasse types on valley glaciers. They indicate longitudinal extension — the glacier is being stretched in the direction of flow. This stretching occurs where the glacier accelerates, typically where the bed slope steepens, where the valley narrows and the glacier accelerates through a constriction, or where the glacier flows over a bedrock step.
A classic setting for transverse crevasses is the icefall — a steep section of a glacier where the ice is in near-continuous crevassing and the fractured blocks, called seracs, are constantly overturning and collapsing. The Khumbu Icefall on Everest, through which every expedition using the South Col route must pass, is a dramatic example. The icefall is caused by the glacier flowing rapidly down a steep bedrock step from the Western Cwm above to the Khumbu Glacier below, and the continuous fracturing produces an unstable chaos of ice towers. Seracs in an active icefall are unpredictable: they can collapse without warning at any time of day, making passage through them one of the most objectively dangerous sections of any major Himalayan climb.
Marginal Crevasses
Marginal crevasses form at the edges of a glacier, oriented at roughly 45 degrees to the flow direction, and reflect the shear stress between the faster-moving central part of the glacier and the slower-moving margins, where friction against the valley walls retards flow. The ice at the margins is sheared as the centre overtakes it, and the resulting fractures angle upstream toward the centre.
Marginal crevasses are a reliable indicator of flow direction: the V of the angle points upstream. They can be deep and persistent, remaining open through multiple seasons, and are particularly hazardous when covered by snow bridges that make the crevasse invisible from above. Snow bridges form when windblown snow and precipitation build across the open crevasse; the bridge may appear solid but can fail under the weight of a single person. On glacier traverses, staying away from the glacier margins reduces marginal crevasse exposure.
Longitudinal Crevasses
Longitudinal crevasses run parallel to the flow direction and form in zones of lateral extension — where the glacier is widening. This occurs where a glacier flows out of a narrow valley into a wider basin, or where a tributary glacier merges with a main glacier and the combined flow spreads outward. Longitudinal crevasses can be accompanied by transverse crevasses in the same zone, creating a grid of intersecting fractures and isolated ice pinnacles called seracs.
Radial Crevasses
When a glacier flows from a confined valley onto a broader plain or into a piedmont lobe — a fan-shaped expansion at the foot of a valley — the ice spreads radially outward in all directions. The fractures that form reflect this radial spreading and appear in a fan pattern centred on the point where the glacier exits its confinement. Piedmont glaciers such as the Malaspina Glacier in Alaska display radial crevasse patterns on a massive scale, visible from satellite imagery.
Bergschrund and Rimaye
The bergschrund is a specific crevasse type that forms at the upper edge of a cirque glacier, between the glacier and the headwall above. In summer, as the glacier melts and sags downslope, it pulls away from the rock face, opening a gap — sometimes a single crevasse, sometimes a complex fracture zone — through which rock debris falls onto the glacier surface and meltwater percolates. The bergschrund is an important erosional zone: meltwater freezing in the headwall cracks contributes to frost-shattering of the rock, which is a major mechanism by which cirques deepen and steepen.
The rimaye (or bergshrund in Alpine terminology) is distinct from the bergschrund in being a crevasse that opens between the glacier surface and a rock wall mid-glacier, not necessarily at the headwall. Both features are significant hazards for mountaineers traversing toward high routes.
Reading Crevasse Patterns in the Field
The practical application of crevasse type knowledge is safety and route-finding. A crevasse field with regular parallel transverse crevasses crossing the full width of the glacier usually indicates an icefall or a steepening section ahead. Marginal crevasses indicate where the margins are most hazardous. A zone of intersecting longitudinal and transverse crevasses indicates a zone of complex stress — potentially where ice from different flow streams is merging.
Routes through crevassed terrain follow the paths of least stress concentration and maximum snow bridge stability. This is why experienced glacier guides walk specific lines through apparently featureless snowfields — they are reading the terrain below and following lines that minimise crevasse exposure even when the crevasses themselves are invisible.
The glacier map provides an overview of the world's major glacier systems, many of which are characterised by the crevasse features described here. For travellers planning trips to glaciated terrain, understanding crevasse types and their implications for route safety is part of the preparation that transforms a glacier visit from a vague adventure into an informed and manageable experience.