Mechanisms of Glacier Flow

A glacier flows when gravity acting on its mass produces stress along the sloping ice surface. This stress is accommodated through internal deformation of ice, movement at the glacier bed, or both, transferring ice from higher accumulation zones towards lower ablation zones.

Gravity and the force balance

The downslope component of a glacier's weight creates driving stress, which increases broadly with ice thickness and surface slope. Flow occurs as this force overcomes resistance from the glacier bed, valley walls and internal friction.

• Ice generally moves fastest near the surface and centre because basal and lateral drag slow the ice adjoining the bed and valley sides.
• A glacier can continue flowing even when its terminus is retreating; retreat means that ice loss at the front exceeds the supply reaching it.

Internal deformation of ice

Under sustained stress, glacier ice behaves as a viscous, deformable solid. Ice crystals change shape and move relative to one another through creep, allowing the glacier to deform continuously without the whole mass sliding.

• Deformation is usually greater where ice is thicker because overlying ice produces higher stress.
• Near the surface, where pressure is lower, ice may respond brittly to stretching and form crevasses.

Movement at the glacier bed

Where basal ice reaches its pressure-melting temperature, meltwater can reduce effective contact with the bed and facilitate basal sliding. Ice also moves around bedrock obstacles through pressure-related melting and refreezing, combined with enhanced creep.

• Some glacier motion occurs through deformation of water-rich, unconsolidated subglacial sediment beneath the ice.
• Cold-based glaciers remain frozen to their beds, so their motion is dominated by internal deformation and is generally more strongly resisted.
• Basal temperature, meltwater pressure, bed roughness and sediment character determine the relative importance of basal motion.