Glaciers are large, enduring bodies of dense ice that form on land. These impressive masses of ice are not static but rather dynamic features of Earth’s landscape, constantly moving and reshaping their environments. Understanding their movement is central to grasping their role in geological processes and the broader climate system. This article explores the fundamental force driving glacial motion and its various mechanisms.
Gravity: The Driving Force
Gravity is the primary force compelling glaciers to move downhill. The immense mass of a glacier, accumulated from compacted snowfall, is pulled downwards by Earth’s gravitational force. This constant downward pull converts the glacier’s potential energy, stored in its elevated position, into kinetic energy, enabling its slow but continuous movement.
Gravity acts on the entire body of the glacier, exerting stress on the ice even on gentle slopes. This stress, known as gravitational driving stress, is proportional to the ice’s density, thickness, and surface slope. Even when a glacier appears stationary, the ice within it still flows downslope under gravity’s influence. The sheer weight of the ice deforms it, allowing the glacier to flow.
How Glaciers Flow Downhill
The force of gravity translates into glacial movement through distinct mechanisms. Glaciers move by a combination of ice deformation and motion at their base.
One mechanism is internal deformation, also known as creep. Under immense pressure from its own weight and overlying ice, glacial ice behaves like a plastic material. Individual ice crystals within the glacier deform and slide past each other, allowing the entire ice mass to flow. This process is more pronounced in thicker and warmer ice.
Another process is basal sliding, where the glacier slides over the underlying bedrock or sediment. This occurs when meltwater at the base acts as a lubricant, significantly reducing friction between the ice and the ground. This meltwater can be generated by the pressure of the overlying ice, which lowers the melting point of ice, or from water seeping through cracks from the surface. Basal sliding can account for a substantial portion of a glacier’s movement, particularly in warmer, temperate glaciers.
If the glacier rests on soft, saturated sediment, known as subglacial till, the ice’s weight can deform this layer. This process, called subglacial till deformation, allows the glacier to move on top of or within this deforming till. The saturated sediments become weaker, enabling the glacier to ride on these deforming layers.
Key Influences on Glacial Movement
Several factors modulate the rate and characteristics of glacial movement. The angle of the slope plays a significant role; steeper slopes generally result in faster glacial movement because the gravitational pull in the direction of flow is increased. Even a few degrees of incline can be considered “steep” in glacial dynamics.
The thickness and overall mass of the ice also exert a considerable influence. Thicker glaciers possess greater mass, leading to higher internal pressures and increased gravitational force, which can accelerate their movement. This added weight enhances the ice’s ability to deform and slide.
Ice temperature is another important factor, dictating which flow mechanisms are dominant. “Warm” or temperate glaciers, which have meltwater present throughout their mass or at their base, are more prone to basal sliding and tend to move faster. In contrast, “cold” or polar glaciers, where the ice is frozen to its bed, move primarily through internal deformation, often at much slower rates.
Finally, the basal conditions beneath the glacier significantly impact its movement. The presence of meltwater is a key lubricator. The type of underlying material, whether it is smooth bedrock or deformable sediment, also affects friction and the efficiency of basal movement. Rougher beds can increase melting due to friction, facilitating sliding, while soft, saturated sediments allow for subglacial till deformation.