A glacier is a massive, perennial body of ice that forms on land and moves slowly under the influence of its own weight and gravity. This movement, often described as “flow,” is a fundamental geological process that carves valleys and shapes landscapes. The rate at which this ice mass travels is not consistent throughout its structure, as velocity can vary from less than 10 meters to over 500 meters per year. Understanding the internal physics of this movement helps explain why certain sections of a glacier move significantly faster than others.
The Mechanics of Glacier Movement
Glacier movement is driven primarily by gravity and involves two distinct processes: internal deformation and basal slip. Internal deformation, often called creep, occurs when the pressure from the overlying ice mass causes the ice crystals to slide and shear past one another. This process allows the ice to behave like a viscous fluid, especially in thicker regions. The deformation rate increases exponentially with the applied stress.
Basal slip, or sliding, is a mechanism where the entire glacier mass moves over the bedrock below. This movement is enhanced by the presence of liquid water at the ice-bed interface, which acts as a lubricant, reducing friction. Meltwater is commonly generated by pressure-induced melting, where the weight of the ice lowers the freezing point of water.
This lubrication effect makes basal slip the dominant mechanism in warm-based glaciers, where temperatures at the base are at the pressure melting point. Conversely, cold-based glaciers, which are frozen to the bedrock, rely almost entirely on internal deformation. The combination of these two mechanisms dictates the overall speed of the ice mass.
Velocity Profile: Identifying the Fastest Zone
The fastest-moving part of a glacier is the area that experiences the least resistance to flow. This location is consistently found at the center of the glacier’s surface. The velocity profile is a direct result of frictional forces acting on the ice mass.
The most significant slowing force is basal friction, which occurs where the ice meets the underlying bedrock. The ice layers closest to the bed are dragged backward, causing the velocity to be lowest at the base. In a vertical cross-section, the speed increases progressively from the bed to its maximum at the surface.
A similar resistive force, lateral friction, slows the ice along the glacier’s sides where it scrapes against the valley walls. This lateral drag causes the ice speed to decrease from the centerline outward toward the edges. Consequently, the combined effect of friction at the bottom and the sides creates a U-shaped or parabolic velocity profile across the glacier’s width. The point farthest from both the valley walls and the bedrock, the center of the surface, experiences the maximum velocity.
External Conditions Influencing Flow Rate
External factors determine the overall magnitude of the flow rate. One primary influence is the surface slope, or gradient, of the land beneath the ice. Steeper slopes increase the gravitational driving stress on the ice, which accelerates its flow.
Ice thickness is another determinant because thicker ice generates greater pressure. This increased pressure enhances the rate of internal deformation and facilitates basal slip by increasing the likelihood of pressure melting at the base. Glaciers tend to move faster when they are thicker.
The availability of meltwater is a short-term variable that causes seasonal fluctuations in speed. Increased surface melting sends water to the glacier bed through vertical channels called moulins. This influx increases basal lubrication, which temporarily drives acceleration in ice velocity.