To human perception, ice appears as an unyielding, brittle solid that shatters under sudden impact. Yet glaciers—enormous bodies of perennial ice formed from compacted snow—move relentlessly downhill across landscapes, carving U-shaped valleys and gouging bedrock. In physical glaciology, glacial motion is explained by the fact that under the immense overburden weight of its own mass, solid ice behaves mechanically as a viscoelastic material. Over short durations, ice fractures brittly; over sustained periods under continuous gravitational stress, ice deforms plastically and flows like an ultra-viscous fluid. Glacial ice moves down slopes through two primary physical processes: Internal Plastic Deformation (creep) and Basal Sliding.
Internal Plastic Deformation occurs within the deeper portions of the glacier, typically at depths greater than thirty to fifty meters below the surface. At these depths, the tremendous hydrostatic confining pressure exerted by overlying ice prevents brittle cracking. Instead, individual ice crystals deform through a crystallographic mechanism called Dislocation Glide, wherein microscopic line defects in the hexagonal ice crystal lattice slip along parallel basal planes. In 1955, British physicist J.W. Glen formulated Glen’s Flow Law (ε˙=Aτn, where n≈3), which proves that ice deformation is non-linear: doubling the shear stress increases the strain rate eightfold (23=8), meaning thicker, steeper glaciers flow exponentially faster. The upper thirty to fifty meters, lacking confining pressure, forms the "Brittle Zone," which fractures into deep chasms called Crevasses as it is carried along atop the flowing plastic ice below.
The second major mechanism is Basal Sliding, which occurs when a glacier slides physically across its underlying bedrock bed, lubricated by a thin layer of pressurized meltwater. This process is governed by Pressure Melting Point Depression: high pressure lowers the melting point of ice below 0°C, enabling liquid water to exist beneath thick ice even in sub-freezing temperatures. Glaciers are thermally divided into "Warm-Based" (Temperate) glaciers, where the basal ice reaches the pressure melting point and basal sliding accounts for up to ninety percent of total movement, and "Cold-Based" (Polar) glaciers, where basal ice is frozen solid to bedrock, restricting movement entirely to slow internal creep.