Glacier Flow and Ice Dynamics

Introduction

Ice that flows like a very slow river: the physics of glacier motion

Glaciers are not static masses of ice — they flow, albeit slowly, under the influence of gravity and their own weight. The fundamental relationship governing ice deformation is Glen's flow law: strain rate (ε̇) is proportional to shear stress (τ) raised to the power n, where n ≈ 3 for ice (ε̇ = A × τⁿ). This power-law relationship means ice deformation is highly nonlinear: doubling the shear stress increases the strain rate approximately eight-fold. The rate factor A is strongly temperature-dependent — cold polar ice at −30°C is roughly 1,000 times more viscous than temperate ice near the melting point.

Ice movement occurs through two primary mechanisms: internal deformation (creep) and basal sliding. Internal deformation involves the slow creep of ice by dislocation movement within individual ice crystals and grain boundary sliding between adjacent crystals. Over time, crystals develop preferred orientations (crystal fabric), which enhances deformation in the direction of maximum shear — a process that progressively softens the ice in fast-flowing zones. Basal sliding occurs where the glacier sole is at the pressure melting point; a thin film of meltwater reduces friction at the ice-bed interface. Geothermal heat (~65 mW/m² on average) and frictional heating from sliding itself generate this meltwater. Where the bed is composed of soft, water-saturated sediment (till), deformation of the subglacial till can also contribute significantly to glacier motion.

Ice velocities span a remarkable range. Cold polar glaciers frozen to their beds move only a few metres per year entirely by internal creep. Temperate mountain glaciers typically move tens to hundreds of metres per year. Ice streams — narrow corridors of fast-moving ice that drain the interiors of large ice sheets — can move kilometres per year; Jakobshavn Isbrae in Greenland reaches ~40–50 m/day (131–164 ft/day). The velocity profile through the ice column is parabolic in creep-dominated flow: fastest at the surface, decreasing to near-zero at the bed. Where basal sliding dominates, the column translates at the sliding velocity plus a creep component.

Flow regime — extending versus compressive — controls glacier structure. In extending flow, ice accelerates (over steepening bed or through a narrowing), creating longitudinal tensile stress. When tensile stress exceeds ice tensile strength (~100–200 kPa), crevasses fracture the ice surface perpendicular to flow. Icefalls are spectacular manifestations of extreme extending flow, with seracs and a maze of crevasses. In compressive flow, ice decelerates (flattening bed, spreading terminus), creating thrust faults, pressure ridges, and folded ice. Understanding these flow regimes is essential for interpreting glacier hazards, predicting calving rates at marine-terminating glaciers, and projecting ice-sheet contributions to sea level rise.

Key Terms