P5 Flashcards
Glacier movement
- The fundamental cause of ice movement is gravity.
- Ice moves downslope from higher altitudes to lower areas either on land or at sea level.
- As the ice mass builds up over time in the accumulation zone, the weight of the snow and ice exerts an increasing downslope force due to gravity (known as shear stress).
- Shear stress increases as the slope angle increases and, once the shear stress is great enough to overcome the resisting forces of ice strength and friction, the glacier ice pulls away and moves downward away from the zone of accumulation.
- The momentum of the ice’s movement towards the ablation zone prevents further build-up, thereby maintaining the glacier at a state of dynamic equilibrium with the slope angle.
- This forward movement of glacial ice towards the margins/snout, occurs regardless of whether the glacier as a whole is advancing or retreating.
- Thus the speed of glacier movement forward depends on the degree of imbalance, or the gradient, between the zone of accumulation and the zone of ablation.
movement in warm-based and cold- based glaciers
- Warm, wet-based glaciers in temperate maritime climates experience greater snowfall in winter and more rapid ablation in summer; therefore the imbalance between accumulation and ablation zones is greater, so the glacier ice must move downslope more rapidly to maintain the equilibrium with the slope angle.
- In cold-based, polar glaciers the slower rates of accumulation, and especially ablation, result in a smaller gradient of equilibrium and slow ice movement.
- Further contrasts in movement rates occur because of the contrasts in the nature of the substrate (base) on which the glacier rests and its own base.
- This determines the relative importance of the three processes which facilitate glacier movement: basal sliding, internal deformation and subglacial bed deformation.
Basal sliding
- This relates to the presence of meltwater beneath a glacier.
- This type of ice movement applies to warm-based glaciers; it cannot occur where a glacier is frozen to its bed.
- The meltwater acts as a lubricant reducing friction with both the entrained debris and with the underlying bedrock (this is known as slippage).
- It can account for up to 75 per cent of glacier movement in warm-based glaciers.
Two specific processes enable glaciers to slide over their beds:
- enhanced basal creep
- regelation creep
- enhanced basal creep
basal ice deforms around irregularities on the underlying bedrock surface
regulation creep
sometimes known as slip, which occurs as basal ice deforms under pressure when encountering obstructions such as rock steps.
* As the glacier moves over the obstruction the pressure on the basal ice will increase up glacier, leading it to reforming in a plastic state as a result of melting under this pressure.
* Once the glacier has flowed over the obstruction the pressure is lowered and the meltwater refreezes
Internal deformation
Cold-based, polar glaciers are unable to move by basal sliding as their basal temperature is below the pressure melting point. They therefore move by internal deformation, which has two main elements:
• intergranular flow, when individual ice crystals deform and move in relation to each other
• laminar flow, when there is movement of individual layers within the glacier.
The deformation of ice in response to stress is known as
- ice creep
- and is a result of the increased ice thickness and/or the surface slope angle.
- Where ice creep cannot respond quickly enough to the stress, ice faulting occurs, which manifests itself in a variety of crevasse types at the surface.
- When the slope gradient is increased, there is acceleration of ice and extensional flow.
- Such conditions can occur in the zone of accumulation and can result in an ice fall.
- Near the ablation zone, where there is usually a reduction of slope angle, the ice decelerates and there is compressional flow, which leads to a whole series of thrust faults in the ice, with closed-up crevasses.
Transverse crevasses
- cut across the glacier at approximately right angles to the direction of glacier flow.
- These can be very deep and wide, and result from ice faulting at depth within the ice mass.
- Changes in the width of the valley can also lead to ice fracturing, for example forming longitudinal crevasses that are orientated parallel to the flow direction of the ice, as the ice masses spread out laterally in a less-constrained environment.
Radial crevasses
can form in a splayed pattern at the snout of the glacier, where ice spreads out in a broad lobe.
Marginal crevasses
form near the sides of a glacier as a result of differential movement within the glacier as friction on the sides of the valley slows ice movement relative to ice near the middle of a glacier.
Subglacial bed deformation
- This occurs locally when a glacier moves over relatively weak or unconsolidated rock, and the sediment itself can deform under the weight of the glacier, moving the ice ‘on top’ of it along with it.
- Locally this process can account for up to 90 per cent of the forward motion of glacier ice, often in polythermal outlet glaciers as in Iceland.
Velocity of glacier ice
- The overall velocity of a glacier comes from a combination of the processes described above.
- Warm-based glaciers have a greater overall velocity of ice movement than cold-based glaciers because of the addition of basal sliding to internal deformation and flow, which affect both types.
- Even greater velocities are reached when a warm-based glacier moves over deformable sediment.
- Observations of glaciers across the world have shown that great variations in the total velocity of glacier ice occur, with most glaciers having velocities between 3 m and 300 m per year.
A number of factors have an impact on the rate of movement:
- altitude, which affects the temperatures and precipitation inputs
- slope, which can be directly related to flow - steeper slopes lead to faster speeds
- lithology, which can affect basal processes and the possibility of subglacial bed deformation
- size, which can affect the rapidity of response
- mass balance, which affects the equilibrium of the glacier and also whether it is advancing or retreating.
surges
- The highest velocities of all occur during a glacier surge.
- glaciers collapse when the mass and slope angle of the ice builds up to a critical level within the accumulation zone.
- At the time of a surge - a rare event as only four per cent of all the world’s glaciers are prone to surging — the ice races forward at velocities between 10 and 100 times the normal velocity.
