Glacier Flow and Ice Deformation Explained
Ice appears solid and rigid, yet glaciers move. Over days, weeks, and years, a mass of ice that seems as immovable as rock flows downhill, rounds corners, grinds its bed, and carries rock debris from mountain heights to valley floors. This motion is not like the flow of liquid water — it operates by fundamentally different physical processes operating at the scale of ice crystals and at the interface between ice and the underlying substrate. Understanding glacier flow is foundational to glaciology, linking the physics of ice deformation to questions of ice sheet stability, sea level rise, and the erosional history of glaciated landscapes.
Glacier flow results from three main processes that operate simultaneously in varying proportions: internal deformation of the ice by creep, basal sliding at the ice-bed interface, and the deformation of soft sediments beneath the glacier. The relative contribution of each process varies enormously between different glacier types, different thermal regimes, and different seasons.
Internal Deformation: Creep and Crystal Mechanics
Ice is a crystalline material, and like many crystalline solids under sustained stress, it deforms by creep — the slow, permanent reorientation of the crystal structure without fracture. The primary creep mechanism in glacier ice is dislocation creep: defects within individual ice crystals move through the crystal lattice in response to applied shear stress, allowing the crystal to change shape without breaking. At the same time, crystal grain boundaries migrate and individual crystals recrystallise, a process that allows ice to deform at stresses far lower than those that would cause fracture in brittle ice.
The relationship between shear stress and strain rate in ice is described by Glen's flow law, derived from laboratory experiments by John Glen at the University of Cambridge in the 1950s. The flow law is non-linear: strain rate is proportional to the cube of applied stress (approximately), which means that ice deforms disproportionately faster as stress increases. This non-linearity has important consequences for ice sheet dynamics — small increases in driving stress, for example at the margins of ice sheets where ice is channelled into outlet glaciers, produce large increases in flow velocity.
The temperature of ice strongly affects its deformation rate. Cold ice — far below freezing — is stiff and deforms slowly. Ice near the pressure melting point (the temperature at which ice melts given the overlying pressure) is much more deformable. Alpine glaciers, whose ice is commonly at or near the pressure melting point throughout, are called temperate glaciers and flow primarily by both internal deformation and basal sliding. Cold-based glaciers, found in polar regions where temperatures are far below freezing even at depth, deform primarily by internal creep and often do not slide over their bed at all.
Basal Sliding
In temperate glaciers and in the warm-based sectors of polythermal ice masses, a thin film of water exists at the glacier-bed interface, maintained by the pressure melting effect and by geothermal and frictional heat. This water film dramatically reduces friction between the ice and the bedrock, allowing the glacier to slide as a more or less rigid slab over its substrate.
Two mechanisms contribute to basal sliding. Enhanced creep around bedrock bumps: ice impinging on an obstacle deforms more rapidly due to the increased stress concentration, allowing the ice to flow around the obstacle. Regelation: ice melts on the high-pressure upstream side of a small bump, the meltwater flows around the obstacle, and refreezes on the low-pressure downstream side, releasing latent heat that helps maintain the melt cycle. Regelation is efficient for small obstacles; enhanced creep is more important for large bumps. The two mechanisms interact to produce the net sliding velocity.
Basal sliding velocities in temperate glaciers can be a substantial or even dominant fraction of total surface velocity. Studies using borehole inclinometry — instruments lowered into boreholes drilled through a glacier to measure the tilt of the borehole over time — have shown that in some Alpine glaciers, basal sliding accounts for more than half of the total surface motion.
Subglacial Sediment Deformation
In many glacier systems, particularly those overlying thick sequences of glacial till and unconsolidated sediment, a third flow mechanism operates: the deformation of subglacial sediment itself. When water pressure in the subglacial sediment is high enough to reduce the effective stress (the total stress minus pore water pressure) to near zero, the sediment behaves as a viscoplastic material and deforms under the shear stress imposed by the overlying ice.
This mechanism was controversially proposed in the 1980s by Barclay Kamb and colleagues based on borehole observations beneath Ice Stream B (now Whillans Ice Stream) in Antarctica, where they found a layer of soft, water-saturated till deforming at the glacier bed. Subglacial sediment deformation has since been confirmed at numerous other sites using seismic surveys, borehole instruments, and measurements of till mechanical properties. In ice streams — fast-flowing corridors within ice sheets — sediment deformation can account for the majority of the exceptionally high flow velocities observed.
Velocity Profiles and Measurements
The velocity of a glacier is not uniform through its cross-section. At the surface, where ice accumulates and flow is unimpeded from above, velocity is highest. With depth, internal deformation and the drag of the bed reduce velocity in a characteristic profile that depends on ice temperature, thickness, and basal conditions. At the very base, on a hard rock bed with no sliding, velocity goes to zero; on a soft sediment bed or where basal sliding is active, a finite basal velocity is maintained.
Laterally, velocity is highest at the glacier centreline and decreases toward the valley walls, where lateral drag from the ice-rock interface slows the flow. This lateral velocity gradient, when integrated over long periods, is responsible for differential transport of debris and for the characteristic curvature of medial moraines from their origin at tributary glacier junctions toward the glacier snout.
Modern velocity measurements use differential GPS positioning of stakes drilled into the glacier surface, measured repeatedly over days to weeks. For large ice sheets, satellite radar interferometry (InSAR) provides continuous velocity fields across entire ice sheets, resolving features from individual outlet glaciers to the broad slow-flowing interior. Velocity data from Greenland and Antarctica derived from missions including the European Space Agency's Sentinel-1 have transformed our understanding of ice sheet dynamics.
Seasonal and Diurnal Velocity Variations
Glacier flow velocity is not constant throughout the year. In temperate mountain glaciers, a seasonal speed-up occurs in spring and early summer when surface meltwater drains to the bed and raises subglacial water pressure, reducing friction and increasing sliding velocity. As the drainage system becomes more efficient through the summer — developing channelised Röthlisberger channels that efficiently evacuate meltwater — basal water pressure drops and velocity decreases. In autumn, as meltwater input declines, the distributed drainage system partially collapses and conditions for the next spring speed-up are set.
Diurnal velocity cycles have been measured on some glaciers, where afternoon melt produces a pulse of water reaching the bed that causes a detectable increase in sliding velocity relative to cooler overnight periods. These short-term fluctuations, measured by continuous GPS stations on glacier surfaces, provide detailed information about subglacial hydrology that is otherwise inaccessible.
Explore the locations of fast-flowing outlet glaciers, ice streams, and temperate mountain glaciers with the map, placing these dynamic systems in their geographic and topographic context.