A glacier is a large, persistent body of dense ice that forms on land from accumulated snow that compacts and recrystallizes over time. Glaciers move under the influence of their own weight and gravity. They are often called “rivers of ice” because, despite their solid appearance, they are constantly flowing outwards and downwards in a slow, continuous process.
How Ice Moves Internally and Externally
Glacial movement is achieved through a combination of two primary physical mechanisms: internal deformation and basal slip. Internal deformation, also known as creep, involves the ice crystals within the glacier changing shape and sliding past one another. The immense pressure from the overlying ice mass causes the ice to behave plastically, allowing it to flow without fracturing. This internal flow is generally concentrated in the lower layers of the glacier.
The second major mechanism is basal slip, which is the movement of the entire glacier over the ground beneath it. This sliding is facilitated by the presence of a thin layer of meltwater at the ice-bed interface. The meltwater acts as a lubricant, reducing friction and allowing the ice mass to glide over the bedrock or underlying sediment. Basal slip is especially important in “warm-based” glaciers where the ice at the bottom is at the pressure melting point.
The ice flow is not uniform from top to bottom. The upper layers, which are under less pressure and are more brittle, ride along on the more plastic lower layers. This means the surface of the glacier moves faster than the ice closer to the bed, which is slowed by friction. Cold-based glaciers are frozen to the bedrock and move almost exclusively through slow internal deformation. In contrast, warm-based glaciers rely heavily on basal slip, which accounts for a large percentage of their total movement.
What Controls the Speed of Glacial Movement
The speed at which a glacier moves is not constant. The most direct factor is the slope of the underlying terrain, as a steeper slope increases the gravitational force pulling the ice mass downhill. Glaciers on steeper inclines flow faster than those on more gentle gradients.
The thermal regime of the glacier—its internal temperature profile—also plays a large part in its velocity. Warmer, or “wet-based,” glaciers move faster because the presence of meltwater at the base allows for effective basal sliding. Conversely, colder, or “cold-based,” glaciers are frozen to the ground, which restricts basal slip and limits movement to internal deformation.
Meltwater acts as a lubricant, and its availability significantly increases the rate of basal sliding. This water can come from surface melt, friction from movement, or geothermal heat. When the volume of meltwater is high, the water pressure beneath the ice increases, which further reduces friction and allows for faster flow. The thickness of the ice is also a factor, as a thicker glacier generates more pressure, enhancing both internal deformation and the potential for pressure melting at the base.
Surging Glaciers and Differential Flow
Glaciers do not flow at the same speed across their width. Due to friction against the valley walls, the ice along the center line of a valley glacier moves faster than the ice along the margins. This variation in speed creates shear stress, often leading to the formation of crevasses near the edges where the ice is stretched and pulled apart.
A glacier surge is a short-lived event where the ice flow velocity increases dramatically. During a surge, a glacier can move up to 100 times faster than its normal rate, advancing several meters per day for months or years. These surges are often cyclical, involving long quiescent phases where ice builds up, followed by a rapid discharge phase.
The cause of a surge is often tied to a sudden change in the subglacial plumbing system, particularly a rapid increase in water pressure beneath the ice. This sudden influx of high-pressure water effectively decouples the ice from its bed, eliminating basal friction and triggering fast sliding.