Alexei Volkov
Glaciers are in a constant state of motion, moving from areas of accumulation to areas of ablation. This movement is facilitated by basal sliding, which occurs when a layer of water is generated between the glacier and the underlying rock surface, helping the glacier move downslope. In addition to basal sliding, glaciers also move through plastic flow, which occurs when the pressure from the weight of the glacier causes the ice to behave like a slow-moving plastic stream. This plastic deformation is an important concept in the study of glacier flow, as it helps explain how glaciers can move around obstacles and how they maintain a state of equilibrium.
| Characteristics | Values |
|---|---|
| Definition | Plastic flow in glaciers refers to the ability of glacial ice to deform and flow over time, like a slow, viscous fluid. |
| Mechanism | Basal sliding at the interface between the bottom of the glacier and the underlying terrain. |
| Layers | Two layers, Layer A and Layer B, with Layer A exhibiting plastic deformation and Layer B exhibiting shearing or creep. |
| Temperature Dependence | Ice deforms more rapidly at higher temperatures, behaving like a pliable material. |
| Thickness Requirement | A thickness of at least 22 meters is required for plastic flow in temperate glaciers. |
| Velocity Factors | Roughness of rock surface, meltwater presence, weight of glacier, and underlying water interaction. |
| Climate Influence | Climate dictates the thermal regime of glaciers and the presence of water, influencing basal melting and sliding. |
| Role in Landscape Formation | Plays a pivotal role in shaping the planet's landscape. |
| Monitoring | InSAR technology can monitor glacier behaviour and assess climate change development. |
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What You'll Learn
Ice sheet movement
Ice sheets move downslope in multiple directions from a central area of high altitude. As ice sheets are not restricted to a channel or valley, they must expand due to the constant accumulation of ice and snow. Ice sheets move predominantly through plastic flow, which occurs when material, especially ice, moves under intense pressure, behaving like a very viscous fluid. This process is irreversible, and the material does not revert to its original shape when the pressure is removed.
Plastic flow in glaciers is caused by pressure at depth, with a minimum thickness of 22 metres required for plastic flow in temperate glaciers. The central and upper portions of a glacier flow faster than the bottom and sides, where friction between the ice and valley walls slows the flow. The rigid upper zone, known as the zone of fracture, is carried along by the underlying plastic flow. When the zone of fracture moves faster than the underlying flow, the surface cracks, forming deep fissures called crevasses.
The movement of ice sheets and glaciers is influenced by various factors, including the roughness of the rock surface, the amount of meltwater, and the weight of the glacier. Basal sliding, another mechanism contributing to glacier movement, occurs when the pressure from the glacier's weight generates a layer of water, enabling the glacier to move downslope. The temperature at the base of a glacier also plays a role, as a warm base allows the ice to slide, while a cold base freezes the ice to the underlying material, restricting sliding movement.
The concept of balance velocity explains how glacier flow helps maintain equilibrium by balancing inputs and outputs in the system. The rate of glacier movement adjusts to compensate for mass loss in the ablation zone, ensuring a stable state. Additionally, the deformation of glaciers can be understood through the processes of creep and shearing. Creep deformation, similar to plastic flow, contributes significantly to the overall deformation of rock glaciers, particularly in small or cold glaciers. Shearing, on the other hand, results from continuous failure within the microstructure.
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Basal sliding
The concept of basal sliding is closely related to the idea of glacier movement and equilibrium. Glaciers transfer ice from the accumulation zone, where mass gain is dominant, to the ablation zone, where mass loss occurs. The rate of glacier movement, or velocity, is influenced by the balance between these two zones. When the mass loss in the ablation zone is higher, the glacier requires higher velocities to transfer mass gained in the accumulation zone, thus maintaining a state of balance.
The interaction between basal sliding and plastic flow can result in the formation of crevasses. The zone of fracture, which is the upper rigid layer carried along by the underlying plastic flow, sometimes moves faster than the underlying flow itself. This speed difference, especially on steep slopes, can cause the surface to break into deep fissures known as crevasses. Additionally, the curvature of a valley glacier can contribute to crevasse formation, as the ice flows faster toward the outside of the curve than the inside.
Overall, basal sliding is a crucial aspect of glacier movement, particularly for thin glaciers on steep slopes. It involves the downslope movement of the entire glacier, facilitated by the lubrication of meltwater. Understanding basal sliding and its interplay with plastic flow provides insights into the dynamics and equilibrium of glaciers, helping to explain their movement patterns and adaptations to varying conditions.
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Plastic deformation
In the context of glaciers, plastic deformation occurs in ice crystals when they undergo intracrystalline gliding. This process involves the layers within the ice crystal shearing parallel to each other without disrupting the crystal lattice's continuity. The rate of plastic deformation under constant shear stress initially peaks and then stabilizes. This stabilization, known as the shear-strain rate, can be plotted against the stress for various values of applied stress, resulting in a curved graph.
The plastic flow of glaciers is influenced by several factors, including the thickness of the ice, the slope of the underlying terrain, and the temperature at the base. Ice sheets, for example, move downslope from a central area of high altitude and must expand due to the accumulation of ice and snow. The weight of the ice generates a layer of water that aids in the downslope movement, known as basal sliding. However, plastic flow becomes a dominant mechanism for ice deeper than approximately 50 meters, causing the ice to move like a slow-moving, plastic stream.
The central and upper portions of a glacier tend to flow faster than those near the bottom and sides, where friction between the ice and valley walls slows the flow. This contrast in flow rates can lead to the development of crevasses, particularly in steep terrain where the glacier speeds up, and the rigid surface ice cracks to accommodate the change in velocity.
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Intracrystalline gliding
Plastic flow is a process that causes glacial ice buried underneath more than about 50 meters to move like a slow-moving, plastic stream. The central and upper portions of a glacier flow more quickly than those near the bottom and sides, where friction between the ice and valley walls slows down the flow. This process is different from that of rock glaciers, which advance in a conveyor belt-type mode.
Plastic deformation, or creep, is of great importance to the study of glacier flow. It involves two processes: intracrystalline gliding and recrystallization.
The orientation of the basal plane changes as deformation progresses, with the c-axis rotating toward the compressional axis and away from the tensile axis. The rate of deformation for a given stress depends on the temperature of the ice. The warmer the ice, the easier it is to deform. Consequently, the depth variation in ice temperature needs to be included when modelling glacier flow.
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Creep deformation
Creep is a significant factor in the deformation of rock glaciers, contributing to up to 50% of the total deformation in some cases. It is also observed in the deformation of boreholes in the Schafberg rock glacier in Switzerland. The transition from creep-dominated to shear-dominated rock glacier deformation is a complex process that depends on local conditions.
The study of creep deformation in glaciers is important for understanding the dynamics of glacier movement and the factors that influence it, such as the weight of the glacier, the roughness of the rock surface, and the amount of meltwater. By combining measurements of borehole inclinometry and surface velocity with three-dimensional full-Stokes ice flow modeling, scientists can better understand the ice rheologies and sliding velocities of glaciers.
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Frequently asked questions
Plastic flow is a type of deformation that occurs in glaciers. It involves the movement of glacial ice, causing it to behave like a slow-moving, highly viscous fluid. This movement is irreversible and occurs without fracturing the ice.
Plastic flow in glaciers is caused by the pressure and weight of the ice itself, as well as the underlying rock surface. The ice deforms and flows when the stress levels exceed a certain threshold, typically around 100 kilopascals.
Basal sliding occurs when a layer of water is generated between the glacier and the underlying rock, allowing the entire glacier to move as a single mass. Plastic flow, on the other hand, refers to the movement of the ice itself, where the ice behaves like a viscous fluid and flows around obstacles.
Plastic flow is a major contributor to the overall movement of glaciers. It allows the glacier to move around irregularities in the underlying terrain and navigate curves in the valley. The faster flow of the central and upper portions of the glacier, compared to the bottom and sides, also influences the formation of crevasses on the glacier surface.
