Glaciers move mainly by two intertwined processes: internal deformation of the ice itself and motion at the base of the glacier. The combination of these mechanisms, along with the bed and meltwater conditions, determines how fast and in what pattern a glacier flows.

How it works

• Internal deformation (creep)
  • The ice behaves like a very slow-flowing solid. Under the weight of accumulated ice and gravity, layers slide past each other internally. This causes the glacier to flow downstream, even if the bed is frozen or rough. The rate of deformation increases with ice thickness and temperature.

• Basal sliding
  • When meltwater at the base or a warm bed reduces friction, the ice can slide over the bed rock or sediments. Basal sliding can dominate in temperate glaciers where liquid water is present at the bed, accelerating ice movement.

• Bed and subglacial conditions
  • The nature of the bed matters: hard, crystalline bedrock tends to impede sliding, while soft, deformable sediments can lubricate movement (soft-bed deformation). The presence of channels or streams at the base can also channel flow and create faster zones.

• Net flow direction
  • A glacier flows in the direction of the steepest downward slope on its surface. Even if deeper layers move differently, the surface slope largely governs overall flow direction and speed.

What controls speed

• Ice thickness: thicker glaciers experience higher gravitational driving stress, often moving faster.

• Temperature and meltwater: warmer ice and the presence of basal water promote sliding, increasing speed.

• Bed properties: rough, hard beds slow movement; soft beds and deformable sediments enable easier basal motion.

• Slope gradient: steeper terrains generally yield faster flow due to higher gravitational component along the slope.

Variations and special cases

• Ice streams and surges: zones with rapid flow within otherwise slower ice, often tied to basal hydrology and bed conditions.

• Cold glaciers: when the base is dry and frozen, movement is dominated by internal deformation rather than basal sliding.

If you’d like, I can tailor this to a specific glacier or region and discuss how these processes differ there.