A glacier is a persistent body of dense ice, driven by the force of gravity. These frozen masses form where the accumulation of snow exceeds the rate of melting over many years. As the snow compacts into ice, the immense pressure causes the entire mass to behave like a very slow-moving fluid. Glacial movement occurs through a combination of mechanisms. The two primary ways this large-scale motion occurs are through internal deformation and basal slip.
Internal Deformation
Internal deformation involves the ice changing its shape internally due to the weight and pressure of the overlying ice mass. The sheer stress from gravity pulls the glacier downslope, causing the ice crystals to rearrange and slide past one another. This process is often referred to as plastic flow or creep, as the solid ice deforms.
The movement is accomplished at the molecular level through a process called intracrystalline gliding, where layers within the ice crystals shear parallel to each other. Deformation is greatest in the deeper parts of the glacier where the ice is under the highest pressure. Because the surface ice is less constrained, it moves faster than the ice near the bed, which is slowed by friction with the ground. This internal restructuring means that all glaciers, even those frozen to their beds, will move slowly downhill.
Basal Slip
Basal slip occurs when the entire glacier slides over the underlying bedrock or sediment. This movement is possible when a thin layer of water is present at the interface between the ice and the ground, acting as a lubricant to reduce friction. This water layer can be created through pressure melting. The immense pressure from the overlying ice lowers the melting point at the glacier’s base, causing it to turn into liquid even if the temperature is slightly below the standard freezing point.
Friction generated by the glacier’s movement against the bedrock also contributes heat, which can induce additional melting at the base. This meltwater allows the glacier to slide forward, a process that is most effective in temperate, or “warm,” glaciers where the ice temperature is already near the melting point. Basal slip represents a movement of the ice mass relative to the ground, which differentiates it from internal deformation. The presence of water-saturated sediment beneath the ice can also contribute to this sliding motion.
Factors Influencing Glacial Speed
The speed at which a glacier moves is influenced by several external and internal factors. A primary influence is the steepness of the ground slope, as gravity is the driving force. Glaciers resting on a steeper incline flow faster than those on a gentle slope.
The thickness of the ice mass also determines velocity. Thicker ice exerts greater pressure on the underlying layers, which enhances both internal deformation and the production of meltwater for basal slip. The thermal regime of the glacier, whether it is polar (cold-based) or temperate (warm-based), dictates the availability of meltwater. Warm-based glaciers, which have liquid water at their bed, utilize basal slip and move much faster than cold-based glaciers, which are frozen firmly to the ground and rely on internal deformation. Seasonal variations in temperature also create fluctuations in speed, with increased surface meltwater in the summer often reaching the base, reducing friction, and causing a temporary acceleration of the flow.