Useful terms
Creep - slow and continuous movement of ice mass under gravitational stress.
Plastic - material (e.g. ice) is capable of continuous and permanent deformation without fracturing.
Equilibrium line - the boundary on a glacier where accumulation in the upper reaches changes to ablation in the lower reaches. The line can move depending on seasonal conditions.
Pressure melting - even though temperatures are below zero, ice at the base of a glacier is able to melt due to the overburden pressure of the mass above it.
Creep - slow and continuous movement of ice mass under gravitational stress.
Plastic - material (e.g. ice) is capable of continuous and permanent deformation without fracturing.
Equilibrium line - the boundary on a glacier where accumulation in the upper reaches changes to ablation in the lower reaches. The line can move depending on seasonal conditions.
Pressure melting - even though temperatures are below zero, ice at the base of a glacier is able to melt due to the overburden pressure of the mass above it.
How glaciers are formed
Glaciers are formed in areas where there is year round snow cover, which is then compacted by further layers of snow falling on top of it. This cycle continues for a number of years with each year's snow further compressing the previous years. So much so that it forces the snow to recrystallise into larger and larger crystals with fewer air pockets between them, which eventually turns into glacial ice. Once a mass is reached that is large enough for gravity to pull it downslope the glacier starts to creep forward. Glaciers can move distances ranging from a few centimetres a year up to a few kilometres. The whole formation process can take up to 100 years.
Glaciers are formed in areas where there is year round snow cover, which is then compacted by further layers of snow falling on top of it. This cycle continues for a number of years with each year's snow further compressing the previous years. So much so that it forces the snow to recrystallise into larger and larger crystals with fewer air pockets between them, which eventually turns into glacial ice. Once a mass is reached that is large enough for gravity to pull it downslope the glacier starts to creep forward. Glaciers can move distances ranging from a few centimetres a year up to a few kilometres. The whole formation process can take up to 100 years.
Iceberg calving at terminus/snout of arctic glacier.
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Glacier mass balanceMass balance is the difference between a glaciers annual accumulation (mass gain) and ablation (mass loss). Glaciers accumulate mass through precipitation, wind drifted snow and avalanches (inputs), while ablation occurs through melting, avalanches and iceberg calving (outputs). These two zones meet at the equilibrium line which can move depending on where loss and gains occur. Initial glacier formation occurs whenever accumulation exceeds ablation. Accumulation always occurs in a glacier's colder, higher, upper portion, from which point it travels down hill away from, into the ablation zone where mass loss occurs.
A glacier that is that is in a steady state transfers just enough ice from its accumulation zone through the equilibrium line to match losses it makes in the ablation zone and subsequently will not change in steepness or size. A glacier with a positive mass balance, where accumulation exceeds ablation, can thicken and extend its terminus. While one with a negative mass balance, where accumulation is less than ablation, will thin and retreat. Its worth noting that a glacier in recession is still moving, it just isn't making enough mass gains in its upper reaches to match the losses further down. |
How glaciers move
Glaciers move downslope at very slow speeds due to their own mass and the forces of gravity. All glaciers flow through the processes of internal deformation and basal sliding. The speed a glacier moves can be affected by the qualities of the underlying bedrock, geometry of the valley in which it is flowing through and also the temperature and shape of the ice, along with atmospheric conditions.
Glaciers move downslope at very slow speeds due to their own mass and the forces of gravity. All glaciers flow through the processes of internal deformation and basal sliding. The speed a glacier moves can be affected by the qualities of the underlying bedrock, geometry of the valley in which it is flowing through and also the temperature and shape of the ice, along with atmospheric conditions.
Basal sliding
This is where the whole mass of ice slides over the underlying bedrock on a thin film of water which acts as a lubricant. This allows glaciers to move at a faster speed than if their movement is primarily dependent on internal deformation. The main source of this water is from pressure melting of the glacier ice. When ice is held under high pressures it's melting point reduces meaning that ice at the bottom of the glacier is able to change into water, even though atmospheric temperatures at the surface of the glacier are at or below 0℃. To a lesser extent surface water (rain) or melt may reach the base of a thin glacier and also geothermic fluctuations might contribute to some melting at the base. |
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Internal deformation
This is where ice crystals, held under huge pressure within the glacier, arrange themselves into parallel layers which are then able to glide over each other. The ice that is closest to bedrock experiences the largest amount of pressure placed upon it and subsequently this creates a driving force that is larger than the internal strength of the glacier. This means that the largest driving force is created at the base of the glacier, so the lowest layer of ice is able to move the largest distance, even though friction against the bed does reduce this somewhat. With the movement of the glacier starting at the bottom, those layers above benefit from the cumulative effect of the movement of the layers below, with it appearing that the layers towards the surface move more than those below. In fact the amount of motion reduces the further you move towards the surface of the glacier (as shown in fig. 5), up until you reach the rigid zone, which has not had enough pressure placed upon it to form the layers of crystals that slide over each other. This zone of ice moves as large chunks due to the movement of layers below, so crevasses can occur in the surface from movement stresses. |
Ice flow patterns
Rotational flow
This is where ice moving downhill pivots around a particular point causing a rotational movement. It is responsible for creating bowl like erosional features like cirques. |
Compressional flow
This occurs when the gradient of the glacier bed declines, therefore reducing the momentum of the glacier. At this point the ice thickens and it's erosional strength increases. |
Extensional flow
This occurs when the gradient of the glacier bed increases, therefore increasing the speed of the glacier. At this point the ice thins and the glacier's ability to erode the bed is reduced. |
Temperate alpine glaciers
Found at locations where atmospheric temperatures are high enough for glacial ice to be at or near it's melting point, such as the Alps and Rockies mountain ranges. These glaciers primarily move by basal sliding as atmospheric temperatures allow for large amounts of melt water to exist between the glacier and the underlying bedrock. They also experience internal deformation but this has a much minor role in the movement of this type of glacier. As the glacier ice is already quite close to it's melting point less pressure is required to achieve melting. Temperate glaciers tend to move at faster speeds than polar glacier due to the comparative ease of basal sliding over internal deformation. They also cause more erosion to the bed than polar glaciers. |
Cold polar glaciers
Found at locations that experience perennially low atmospheric temperatures that keep glacier ice well below it's melting point, such as Antarctica. These glaciers primarily move through the process of internal deformation, as they remain frozen solid to the underlying bedrock for the majority of the year and across most of their length. They also experience basal sliding, where geothermal temperatures are high enough to maintain a film of meltwater underneath the glacier or where the stress placed on the ice when moving round large bedrock obstacles causes pressure melting, but this is a far less potent force in this type of glaciers movement than internal deformation. |