Why Does Ice Crack Open? Something That Flows and Shatters at Once

Ice under sustained pressure deforms slowly, with crystals sliding past each other so that a mass of ice behaves as an extremely viscous fluid over long periods, which is why a glacier flows downhill. Under rapid stress the same material is brittle and fractures. Which behaviour dominates depends on the rate, so ice deep within a glacier, where pressure is high and deformation is slow, flows, while ice near the surface, where it is stretched faster than it can deform, cracks. The transition occurs at a depth of roughly thirty metres in most glaciers, which is why crevasses have a characteristic maximum depth and why the ice beneath that depth closes any crack by flowing into it.

Crevasse patterns follow the stresses, which follow the terrain:

• •Transverse crevasses across the flow where the bed steepens and the ice is stretched lengthwise
• •Marginal crevasses at an angle near the valley sides, where friction slows the edges relative to the centre
• •Longitudinal crevasses along the flow where the valley widens and the ice spreads sideways
• •Icefalls, where a steep section shatters the ice into a chaotic mass of blocks and cracks
• •A bergschrund at the head, where moving glacier separates from ice frozen to the rock above
• •Radial patterns where a glacier spreads out on leaving a valley

The hazard comes from concealment rather than from the cracks themselves. Snow drifting across a crevasse bridges it, and a bridge may be strong enough to walk on or may not, with no reliable way to tell from above, and bridges weaken through the day as the sun warms them and through the season. A concealed crevasse is invisible on a uniform snow surface, particularly in flat light where the eye cannot read the terrain at all. Falling into one means dropping into a narrowing slot where a person can become wedged, and rescue requires equipment and companions. The standard response is travelling roped together so that anybody who falls is held, with the party spaced far enough apart that only one is on any given bridge.

The techniques for travelling on crevassed ground are specific and are the reason mountaineering parties look as they do. Roping up with several metres between people means only one is on any given bridge, and the rope is kept reasonably taut so a fall is arrested early rather than after a long drop. Each person carries the equipment to set an anchor and to climb a rope, since a rescue requires both. Probing suspicious ground with a pole or an axe finds bridges before a foot does. Routes cross crevasse fields at right angles to the cracks where possible, since travelling along them means walking parallel to a hidden slot. And early morning travel takes advantage of overnight freezing, which strengthens bridges considerably compared with the afternoon.

The pattern is informative because it maps stresses directly. Crevasse orientation indicates the direction in which the ice is being stretched, since a crack opens at right angles to the tension, so mapping them reconstructs the flow field without any instruments. Their distribution reveals where the bed steepens or the valley changes shape, which is otherwise hidden beneath hundreds of metres of ice. Changes in the pattern over time track changes in flow speed, which is monitored by satellite on large ice sheets and which has revealed accelerations in several outlet glaciers. Crevasses also matter physically, since meltwater draining into them reaches the bed and lubricates it, which accelerates flow, and since deep fractures contribute to the breaking up of floating ice shelves.

Ice flows under slow sustained stress and fractures under rapid stress, so the surface layer cracks while the ice below flows and closes cracks, which sets a characteristic depth of around thirty metres. Patterns follow the terrain, with cracks across the flow where the bed steepens and at an angle near the valley walls. The hazard is snow bridges concealing them, which is why parties rope together.