Topic 2: EQ2 Flashcards
How is glacier ice formed?
Glacier ice is formed primarily from compacted snow, with smaller contributions from other forms of precipitation, e.g hail and sleet, which freeze directly on top of or inside a glacier. The first stage of formation is an accumulation of permanent snow either at high altitude or latitude areas. The lower layers or granular snow becomes increasingly compressed to form firn. As more layers are added, even more pressure is applied on the firn causing it to transform into very dense (0.9 g/cm^3) glacier ice. The glacier the deforms from further pressure and moves away from the centre, flowing outward.
What is the timescale for the process of the formation of glacier ice?
-The transformation from snowflake to firn can be quick in temperate areas (a few days), but slower in polar areas (over a decade).
-The final stage from firn to glacial ice can take as little as 25 years but may be up to 150years in polar areas.
-The overall rate of transformation from snow to ice can be as low as 100 years in temperate areas, but as high as 4000 years in Antarctica.
What are the inputs into a glacial system?
The main input into the glacial system is precipitation, the majority of which falls as snow. However, it isn’t the only input as there are other factors such as avalanches and wind input which also add snow to the top layer of the glacier.
What are the outputs of a glacial system?
The main output is meltwater where where ice melts as flows down past the snout of a glacier. As with inputs though, there as also many other factors involved with glacial outputs. This involved evaporation and sublimation, as well as calving, which is when large ice shelves which hang onto the edge of the land detach leaving an ice berg as a result.
How does energy affect glaciers?
Mass of a glacier at the top will hold a lot of gravitational potential energy. Some of this is changed to kinetic energy as the glacier gradually moves down slope, however lots of energy is also dissipated as heat energy, which causes some melting to occur in the ablation zone.
What is the mass balance of a glacier?
This is defined as the gains and the losses of the ice store in the glacier system. There is the accumulation zone which comes from inputs and the ablation zone where the outputs generally take place.
If accumulation exceeds ablation, then there is a positive mass balance and this causes the glacier to grow and therefore advance at the snout.
However in summer, when the rate of ablation is likely to exceed the rate of accumulation, there is a negative mass balance and as a result glacier will thin/shrink and then the glacier will begin to retreat as a result.
What is the equilibrium point of a glacier?
This is where losses from ablation are balanced by the gains which come from accumulation.
However, glaciers are dynamic. This means that the ratio of inputs and outputs vary continuously over both short and long term scales.
How can a longer term mass balance situation of a glacier be complex?
The longer term trends of a glacier can be determined by the ‘health’ of a glacier. Currently, about 75% of the worlds ice masses are retreating because of the recent short term climate change. In the last decade, temps have risen by roughly 0.6°C and as a result are thinning, melting and retreating.
What are the main techniques used to observe the changing rates of glacial mass balance?
Altimetry - repeated measurements of ice sheet surface elevation, this can been used to determine if an ice sheet is thickening of thinning.
Gravimetry - Gravitational attraction of the ice sheet from which changed in the ice sheet mass can be inferred.
Mass budget method - Compares the amount of snow accumulation on the ice sheet, and also the amount of meltwater or iceberg calving density leaving the ice sheet.
What are the processes and transfers which take place in a glacier?
- Ice movement
- Erosion
- Transport
- Deposition.
What is the causes of glacier movement?
The fundamental cause of this is gravity. The ice moves downslope and as the ice mass builds up the extra weight causes an increased force pushing the glacier downslope. This is known as sheer stress. If the force of the sheer stress is great enough to exceed the forced for friction, then the glacier ice pulls away and moves downward away from the zone of accumulation. The steady momentum helps prevents a build up in the accumulation zone, and therefore the glacier maintains a state of dynamic equilibrium with the slope angle. The movement occurs regardless of weather the glacier is advancing or retreating.
What factors affect the speed of glacier movement?
The warm, wet-based glaciers in temperate climates experience greater snowfall in winter and more rapid ablation in summer. As a result the imbalance between accumulation zones and ablation zones is greater. This means the glacier ice must move downhill 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.
There are also further contrasts which are caused due to the nature of the base which the glacier sits on. This determines the relative importance of the three processes which facilitate glacier movement: basal sliding, internal deformation and sub glacial bed deformation.
What is 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 a lubricant reducing friction with both the entrained debris and with the underlying bedrock. In warm based glaciers, it can account for up to 75% of glacier movement.
What are the three specific processes that enable glaciers to slide over their beds?
-Enhanced basal creep
-Regulation creep
-Surges (less common)
What is enhanced basal creep?
This is where basal ice deforms around irregularities on the underlying bedrock surface. This is the method used for smaller obstacles.
What is regelation creep?
Also known as slip, and occurs on obstacles over 1m in size. This occurs when encountering obstructions such as rock steps. As the glacier moves over the obstruction the pressure on the basal ice will increase up a glacier. This will lead it to reform in a plastic state as a result as a result of melting under this pressure localised pressure. Once the glacier has flowed over the obstruction the localised pressure is removed and the meltwater refreezes.
What is 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. Internal deformation has two main elements: interganular and laminar flow.
What is intergranular flow?
This is when individual ice crystals deform and move in relation to each other.
What is laminar flow?
When there is movement of individual layers within the glacier.
What is the PROCESS of internal deformation?
The deformation of ice in response to stress is known as ice creep and is a result of increased ice thickness and/or the surface slope angle. Where ice creep cannot respond quickly enough to the stress, ice faulting occurs, causing crevasses at the surface.
When the slope gradient is increased, there is an acceleration of ice and extensional flow. This occurs in the accumulation zone and results in ice fall.
Near the ablation zone, there is usually a reduction in slope angle, and the ice decelerates and there is compressional ice flow, which leads to a whole series of thrust faults in the ice, with closed up crevasses.
