A glacier is a persistent, large body of dense ice that forms on land and moves continually under the influence of its own weight and gravity. This massive structure is in constant motion, slowly deforming and flowing downhill from an area of accumulation to a zone of melting or discharge. The science of glacial movement, known as glaciology, reveals that this process is a complex interaction of physics, pressure, and temperature. Understanding how ice flows requires examining the transformation of the source material and the two fundamental mechanisms that drive the motion.
The Transformation: From Snow to Flowing Ice
Glacial movement begins with the creation of the unique material that makes up the ice body. Fresh snow, which is approximately 90% air, must first undergo densification to become solid, flowing ice. As successive layers accumulate, the weight of the overlying snow compresses the layers below. This pressure causes the snowflakes to recrystallize into smaller, rounded ice granules, forming an intermediate material called firn.
The transformation to glacial ice occurs as this granular material is buried deeper, squeezing out the remaining air pockets. When the density reaches approximately 830 kilograms per cubic meter, the air passages seal off, trapping the air in bubbles. This dense, crystalline ice structure possesses a unique property: under sustained pressure, it behaves like a highly viscous, plastic fluid, allowing it to deform and flow.
Movement Mechanism I: Internal Deformation
Internal deformation, also known as ice creep or plastic flow, is the mechanism where the ice mass flows without external lubrication. This process occurs because ice crystals deform when subjected to the constant pressure from the glacier’s own weight. The gravitational force acting on the downward slope creates shear stress within the ice mass, which is the primary driver of this internal movement.
Under this sustained shear stress, individual ice crystals within the glacier shift and slide past one another along their internal molecular planes. This mechanism ensures all glaciers move, even if their base is frozen to the ground. The rate of internal deformation is not uniform; it is slowest at the base and sides where friction with the bedrock resists motion. Conversely, movement is fastest near the glacier’s surface and center, where resistive forces are minimized.
Movement Mechanism II: Basal Sliding and Lubrication
Basal sliding is the second primary mechanism of movement, involving the entire glacier slipping over the land or sediment beneath it. This process is highly dependent on the presence of a thin layer of liquid water at the ice-bed interface, which acts as a lubricant to reduce friction. The water film allows the massive ice body to decouple from the ground and slide forward, often contributing significantly to the overall speed of the glacier.
Meltwater can form at the base through pressure melting, where the immense weight of the overlying ice lowers the melting point of the ice. Even if the temperature is slightly below 0° Celsius, the high pressure can cause the basal ice to melt, creating the necessary layer of lubrication. Another related process, regelation, allows the glacier to flow around small obstacles on the bedrock. Ice melts on the upstream side of a bump due to increased pressure and then refreezes on the downstream side where the pressure drops, allowing the ice to move past the impediment.
The efficiency of basal sliding is also influenced by the hydraulic pressure of the subglacial water network. If large amounts of meltwater accumulate, the high water pressure can temporarily lift the glacier slightly off its bed, further reducing friction and leading to faster movement. Basal sliding is the dominant movement mechanism in glaciers with abundant subglacial water, which are often referred to as warm-based or temperate glaciers.
Controlling Factors: Speed, Slope, and Thermal Regime
The actual speed of a glacier is determined by how the two movement mechanisms interact with various external and internal conditions. Gravity is the fundamental driving force, meaning the angle of the underlying slope plays a direct role in the magnitude of the shear stress applied to the ice mass. A steeper slope will generate greater gravitational force, leading to faster flow rates.
Ice thickness is another significant factor because the pressure exerted by the ice is directly proportional to its depth. Thicker glaciers experience greater internal deformation and are more likely to reach the pressure melting point at the base, enhancing basal sliding. The thermal regime, or the temperature structure of the ice mass, is a determining factor for which movement mechanism dominates.
Warm-based glaciers, common in temperate regions, have a basal temperature at or near the pressure melting point, which allows for substantial basal sliding and faster movement. Cold-based glaciers, typical of polar regions, remain frozen to the bedrock, meaning they move almost entirely through the slower process of internal deformation. Glacial movement is therefore a complex, combined result of the ice’s inherent plasticity, the presence of lubricating water, and the environmental factors of slope and thermal state.