Mechanism of Glacial Collapse

Meltwater can destabilise a steep glacier by reducing its grip on the bed and enlarging fractures within the ice. If water input overwhelms drainage, rising subglacial water pressure promotes rapid basal sliding, allowing part of the glacier to detach and accelerate downslope.

Water entry and pressure build-up

Surface meltwater descends through crevasses and vertical shafts called moulins, reaching the glacier bed. When water arrives faster than the subglacial drainage system can remove it, water pressure increases beneath the ice.

• High water pressure lowers effective pressure, which is the difference between the ice overburden pressure and water pressure at the bed.
• Lower effective pressure reduces friction between the glacier and its bed, while water-saturated sediments may also lose shear strength.

Fracturing and loss of stability

Water entering cracks exerts pressure against their walls and can deepen them through hydrofracture. Simultaneously, enhanced basal motion stretches the glacier, opens additional crevasses and weakens the remaining zones that connect the unstable ice mass to the slope.

• Acceleration can create a positive feedback: faster motion causes more fracturing, which admits more water and further reduces resistance.
• On a steep or overhanging bed, loss of basal traction and internal cohesion can produce slab detachment along the ice-bed interface or through fractured ice.

Why failure becomes catastrophic

Once resisting forces fall below the downslope gravitational force, the detached mass rapidly converts gravitational potential energy into motion. It may become an ice avalanche, entrain rock and sediment, displace water or temporarily block a river, creating cascading hazards.

• Failure is favoured by steep slopes, fractured ice, weak beds, intense melting and an inefficient drainage system.
• Meltwater is therefore usually a trigger acting upon pre-existing instability, rather than the sole cause of collapse.
• Efficient drainage can lower water pressure, so increased melting does not invariably produce detachment.