1
Q
Glacier movement
A
- 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.
2
Q
movement in warm-based and cold- based glaciers
A
- 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.
3
Q
Basal sliding
A
- 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.
4
Q
Two specific processes enable glaciers to slide over their beds:
A
- enhanced basal creep
- regelation creep
5
Q
- enhanced basal creep
A
basal ice deforms around irregularities on the underlying bedrock surface
6
Q
regulation creep
A
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
7
Q
Internal deformation
A
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.
8
Q
The deformation of ice in response to stress is known as
A
- 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.
9
Q
Transverse crevasses
A
- 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.
10
Q
Radial crevasses
A
can form in a splayed pattern at the snout of the glacier, where ice spreads out in a broad lobe.
11
Q
Marginal crevasses
A
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.
12
Q
Subglacial bed deformation
A
- 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.
13
Q
Velocity of glacier ice
A
- 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.
14
Q
A number of factors have an impact on the rate of movement:
A
- 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.
15
Q
surges
A
- 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.
16
Q
The glacier landform system
A
- The movement of the ice allows the ice sheet or glacier (ice masses) to pick up debris and erode at its base and sides, as well as to transport and modify the materials it is carrying.
- The more rapid this movement is, the more likely the glacier is to transform the landscape.
- Conversely, stagnant ice, a frequent ‘state’ of lowland ice sheets, is more likely to ‘protect the landscape’ and only reshape it by dumping huge amounts of debris.
- A combination of both direct ice action and indirect impacts - such as the formation of fluvio glacial features by meltwater, disturbance of pre-existing drainage systems, and complex ice induced sea level changes - shapes glacial landscapes.
17
Q
Glacial processes
A
Glacial erosion is the removal of material by ice and meltwater and involves a combination of several processes:
Abrasion
Plucking
Fracture and traction
Dilation
Meltwater erosion
18
Q
Abrasion
A
- by individual clasts (stones), which leads to micro-features such as striations and chatter marks.
- Additionally rock flour (grade sizes under 0.1 mm in diameter) polishes the underlying rocks by ‘sand paper’ action.
19
Q
Plucking
A
- is often referred to as glacial quarrying.
- Quarrying is a two-stage process with the initial widening of the joints by fracture and the subsequent entrainment of any loosened material.
- The importance of plucking as a process is clearly very dependent on rock type and the incidence of preexisting joints.
20
Q
Fracture and traction
A
occur as a result of the crushing effect of the weight of moving ice passing over the rock and variations in pressures lead to freezing and thawing of the meltwater (basal melting), which aids the plucking process.
21
Q
Dilation
A
occurs as overlying material is moved, causing fractures in the rock parallel to erosion surfaces as the bedrock adjusts to the unloading.
22
Q
Meltwater erosion
A
can be both mechanical (similar to fluvial erosion, except that the water is under hydrostatic pressure) and chemical, whereby glacial meltwater can dissolve minerals and carry away the solutes, especially in limestone rocks.
23
Q
Glacial debris entrainment
A
Entrainment is the incorporation of debris on to or into the glacier from supraglacial or subglacial sources.
24
Q
Supraglacial:
A
Debris transported on the surface of the glacier.
sources include material falling from hillsides being washed or blown on to the glacier from the surrounding land, plus atmospheric fall-out such as volcanic ash (a common feature on Icelandic glaciers).
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Subglacial:
Debris transported beneath the glacier.
sources include material eroded from the glacier bed and valley walls, material frozen to the base from subglacial streams,
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Englacial:
Debris transported inside the glacier.
material that has worked its way down through the glacier or ice sheet.
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Glacial sediment transportation
- For ice sheets, most debris is transported subglacially via the basal layer.
- For valley glaciers there is more transport by englacial and supraglacial debris as a result of more frequent ice contact at their lateral margins.
- As pebbles (clasts) are transported they come into contact with each other and are ground down (comminuted) by a process similar to attrition in rivers.
- Glacial sediment transport therefore occurs horizontally and vertically through glaciers, by the movement of the ice itself, meltwater transporting sediment through the glacier drainage system or by glacial deformation of subglacial and proglacial sediments.
28
Glacial deposition
- Glacial deposition occurs when material is released from the ice at the margin or the base of a glacier.
- Deposition may occur directly on the ground (ice contact) or sediments may be released into meltwater.
- Deposition mechanisms include: release of debris by melting or sublimation of the surrounding ice, lodgement of debris by friction against the bed, deposition of material from meltwater, and disturbance and remodelling of previously deposited sediments.
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Macro-scale
- Ice sheet eroded knock and lochan landscapes
- cirques
- artes and pyramidal peaks
- glacial troughs
- ribbon lakes
- till plains
- terminal moraines
- sandurs
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Meso-scale
- Crag and tail
- roches moutonnées,
- drumlins
- kames
- eskers
- kame terraces
- kettle holes
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Micro-scale
-striations
- glacial grooves
- chatter marks
- erratics
32
Key concept: Glacier process environments
- Commonly identified glacial process environments include:
* subglacial geomorphology (beneath the glacier)
* glacier margin geomorphology, either lateral (at the sides of the glacier) or terminal (at the end of the glacier)
* proglacial geomorphology and meltwater landscape geomorphology
* paraglacial landscapes where, after glacial retreat, surface features have to rapidly adjust to their new post-glacial environments. Deglaciation causes instability (leading to massive landslipping) and rapid erosion lasting until a new equilibrium is established between any surface features and the post-glacial process environment
- periglacial process geomorphology in permafrost areas adjacent to ice cover.
Again, the rapid melting of permafrost can lead to a transitional paraglacial stage before an equilibrium occurs and post-glacial modification takes place.
While characteristic assemblages of features can be recognised, a further dimension of difference is added by contrasts between the ice mass types, especially between contrasting thermal regimes.
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Becoming an ice detective part 1
- The distinctive assemblages of landforms left behind after glaciation are very useful when trying to reconstruct the exact position and extent of the ice cover - this is known as inversion modelling.
- It is not an exact science and morphological mapping of areas of past glaciation has to be combined with an analysis of any deposits (known as fabric analysis).
- Hutton's principle of uniformitarianism is the key to understanding and identifying the features found in relict landscapes resulting from past glaciation.
- The principle states that 'the present is the key to the past'.
- By looking at present-day environments such as Svalbard, Greenland or northern Canada, glaciologists can see features actually in the process of formation.
- For example, ice-cored round hills (pingos) in the Mackenzie Delta in northern Canada were finally linked to some mysterious rounded hills with collapsed craters in the North York Moors, now also identified as possible former pingos.
- It is also worth remembering the principle of equifinality, which states that a particular feature can be formed in a number of ways, when trying to explain the formation of drumlins
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Becoming an ice detective part 2
- A further complication is that most glaciated regions are polycyclic/polygenetic; that is, they are the product of several episodes of glaciation and may have been modified under periglacial, paraglacial, interglacial or post-glacial conditions.
- Glacial landscapes have been considerably modified by subaerial weathering, mass movement and fluvial action even since the Loch Lomond Stadial
- Figure 5.15 shows a student's map of the Nant-y-Llyn Valley in the Berwyn Mountains, Wales.
- It is comparatively easy to chart the direction of ice flow from the corrie southward, but harder to work out the extent and height to which the glacial trough was filled with ice.
- On some U-shaped valley sides it is possible to identify a trimline - below this line you can see evidence of glacial abrasion, such as striations and polished rock surfaces, whereas above it only block fields and scree deposits occur, both of which are evidence of periglaciation.
- Finding the extent of the ice cover during the last glaciation (the Loch Lomond Stadial) involved a trip down valley to look for a possible terminal moraine.
- It is always worth using a 'drift' geology map to help in the identification of superficial deposits such as moraine.
