Glaciers are expansive, persistent bodies of dense ice that form over many years from accumulated snowfall. Despite their immense scale, these ice masses are not static features of the landscape. Glaciers are dynamic systems, continuously moving and reshaping the terrain beneath them.
The Role of Gravity
Gravity is the fundamental force propelling glacier movement. The sheer weight of accumulated ice, combined with the downward slope of the land, creates a powerful gravitational pull. This force causes the glacier to flow downhill, much like water in a river. The direction of flow is determined by the overall slope of the ice surface, even if the underlying terrain might be irregular.
Internal Deformation of Ice
One primary mechanism of glacier movement is internal deformation, often referred to as creep. Under the immense pressure of its own weight, glacial ice behaves like a highly viscous, slow-moving fluid. Within the glacier, individual ice crystals deform and slide past one another.
This process allows glaciers to move even when their base is frozen solid to the bedrock. The deformation rate is slowest at the base and sides of the glacier due to friction, with faster movement occurring towards the surface and center.
Sliding at the Glacier’s Base
Basal sliding is another significant mechanism, where the entire ice mass slides over the ground beneath it. This process is greatly facilitated by meltwater at the glacier’s base, which acts as a lubricant. This meltwater can originate from surface melting that seeps down through cracks, or from pressure melting. The immense pressure exerted by the overlying ice lowers the melting point of ice, allowing it to melt even at temperatures below 0°C.
Frictional heat generated by the glacier’s movement over the bedrock also contributes to this lubricating water layer. This thin film of water reduces friction between the ice and the ground, enabling the glacier to slide more easily. Basal sliding can account for a substantial portion of a glacier’s overall movement, particularly in warmer glaciers.
Factors Affecting Movement Rate
Several factors influence the speed at which glaciers move. The slope of the underlying terrain is important; steeper slopes generally result in faster glacier flow due to increased gravitational pull. Ice thickness also impacts movement, as thicker ice exerts greater pressure, leading to more internal deformation and potentially more basal melting.
The temperature of the ice is another factor. “Warm” or temperate glaciers, which have temperatures near their melting point and often contain meltwater throughout, rely heavily on basal sliding and can move relatively quickly. In contrast, “cold” or polar glaciers are frozen to their bed and primarily move through internal deformation, resulting in much slower speeds, sometimes only a few centimeters per day. The nature of the bed material and its roughness also affect basal sliding; a smooth, slippery bedrock allows for faster movement compared to rough or sticky ground, which increases friction.