2.1 How can glaciated landscapes be viewed as systems? Flashcards
2.1 How can glaciated landscapes be viewed as systems?
Key idea ⮕ Glaciated landscapes can be viewed as systems.
The components of open systems
-Inputs
-Outputs
-Throughputs
Inputs (The components of open systems)
Including kinetic energy from wind and moving glaciers, thermal energy from the heat of the Sun and potential energy from the position of material on slopes; material from deposition, weathering and mass movement from slopes and ice from accumulated snowfall.
Outputs (The components of open systems)
Including glacial and wind erosion from rock surfaces; evaporation, sublimation and meltwater.
Throughputs (The components of open systems)
Which consist of stores, including ice, water and debris accumulations; and flows (transfers), including the movement of ice, water and debris downslope under gravity.
Glacier mass balance
The glacier mass balance, or budget, is the difference between the amount of snow and ice accumulation and the amount of ablation occurring in a glacier over a one year time period.
The majority of inputs occur towards the upper reaches of the glacier and this area, where accumulation exceeds ablation, is called the accumulation zone.
Most of the outputs occur at lower levels where ablation exceeds accumulation, in the ablation zone. The two zones are notionally divided by the equilibrium line where there is a balance between accumulation and ablation.
Ablation / Ablation zone
Refers to the low-altitude area of a glacier or ice sheet with a net loss in ice mass due to melting, sublimation, evaporation, ice calving, aeolian processes like blowing snow, avalanche, and any other ablation.
Accumulation / Accumulation zone
Accumulation / Accumulation zone
Sublimation
The transition of a substance directly from the solid to the gas state, without passing through the liquid state.
Climate and glaciers
Wind is a moving force and as such is able to carry out erosion, transportation and deposition. These aeolian processes contribute to the shaping of glaciated landscapes, particularly acting upon fine material previously deposited by ice or meltwater.
Precipitation is a key factor in determining the mass balance of a glacier, as it provides the main input of snow, sleet and rain. (See page 41)
Geology and glaciers
The two key aspects of geology that influence glaciated landscape systems are lithology and structure.
Lithology (Geology and glaciers)
Describes the physical and chemical composition of rocks.
Some rock types, such as clay, have a weak lithology, with little resistance to erosion, weathering and mass movements, as the bonds between the particles that make up the rock are quite weak.
Others, such as basalt, made of dense interlocking crystals, are highly resistant and are more likely to form prominent glacial landforms such as arêtes and pyramidal peaks.
Others, such as limestone, are predominantly composed of calcium carbonate. This is soluble in weak acids and so is vulnerable to decay by the chemical weathering process of carbonation, especially at low temperatures.
Structure (Geology and glaciers)
Concerns the properties of individual rock types such as jointing, bedding and faulting. It also includes the permeability of rocks.
In porous rocks, such as chalk, tiny air spaces (pores) separate the mineral particles. These pores can absorb and store water - a property known as primary permeability. Carboniferous limestone is also permeable, but for a different reason. Water seeps into limestone because of its many joints. This is known as secondary permeability. The joints are easily enlarged by solution.
Structure also includes the angle of dip of rocks and can have a strong influence on valley side profiles. Horizontally bedded strata support steep cliffs with near vertical profiles. Where strata incline, profiles tend to follow the angle of dip of the bedding planes.
Relief, aspect and glaciers
Latitude and altitude are the major controls on climate, however, relief and aspect have an impact on microclimate and the movement of glaciers.
The steeper the relief of the landscape, the greater the resultant force of gravity and the more energy a glacier will have to move downslope. Where air temperature is close to 0ºC, it can have a significant influence on the melting of snow and ice and the behaviour of glacier systems.
The formation of glacier ice
Forms when temperatures are low enough for snow that falls in one year to remain frozen throughout the year. Fresh snow falls on top of the previous year’s snow. Fresh snow consists of flakes with an open, feathery structure and a low density of about 0.05 g/cm³ (grams per cubic centimetre).
Each new fall of snow compresses and compacts the layer beneath, causing the air to be expelled and converting low density snow into higher density ice.
Valley glaciers
Valley glaciers are confined by valley sides. They may be outlet glaciers from ice sheets or fed by snow and ice from one or more corrie glaciers. They follow the course of existing river valleys or corridors of lower ground. They are typically between 10 and 30 km in length, although in the Karakoram Mountains of Pakistan they are as long as 60 km.
Ice sheets
Glaciers are large, slow-moving masses of ice. Ice sheets are the largest accumulations of ice, defined as extending for more than 50,000 km². There are currently only two: Antarctica and Greenland.
Today these possess 96% of the world’s ice. During the last glacial period (LGP or Pleistocene) huge ice sheets also covered much of Europe. The Antarctic ice sheet covers 13.3 million km² and has a volume of about 30 million km³. At its thickest, in eastern Antarctica, it is over 4,700 m deep.
Warm-based (temperate) glaciers are characterised by
-High altitude locations
-Steep relief
-Basal temperatures at or above pressure melting point
-Rapid rates of movement, typically 20-200 m per year
Cold-based (polar) glaciers are characterised by
-High latitude locations
-Low relief
-Basal temperatures below pressure melting point and so frozen to the bedrock
-Very slow rates of movement, often only a few metres per year
Pressure melting point
The pressure melting point is the temperature at which ice melts at a given pressure.
The pressure melting point is nearly a constant 0 °C at pressures above the triple point at 611.7 Pa (Pascals), where water can exist in only the solid or liquid phases, through atmospheric pressure (100 kPa) until about 10 MPa.
Triple point
The temperature and pressure conditions at which the solid, liquid, and gaseous phases of a substance coexist at equilibrium.
Factors that influence the movement of glaciers
-Gravity - the fundamental causes of the movement of an ice mass
-Gradient - the steeper the gradient of the ground surface, the faster the ice will move if other factors are excluded
-The thickness of the ice - as this influences basal temperature and the pressure melting point
-The internal temperatures of the ice - as this can allow movements of one area of ice relative to another
-The glacial budget - a positive budget (net accumulation) causes the glacier to advance
Two zones in a glacier where different movements occur
-An upper zone where the ice is brittle and breaks
-A lower zone where under pressure the ice deforms
Basal sliding
Glaciers move differently depending on the temperature of the ice at their base.
Warm-based (temperate) glaciers mainly move by basal sliding. If the basal temperature is at or above pressure melting point a thin film of meltwater exists between the ice and the valley floor and so friction is reduced.
Basal sliding actually consists of a combination of different mechanisms: Slippage, Creep or regelation, Bed deformation.
Slippage (Basal sliding)
Where the ice slides over the valley floor as the meltwater has reduced friction between the base of the glacier (and any debris embedded in it) and the valley floor. Friction itself between the moving ice/debris and the valley floor can also lead to the creation of meltwater.
Creep or regelation (Basal sliding)
When ice deforms under pressure due to obstructions on the valley floor. This enables it to spread around and over the obstruction, rather as a plastic, before re-freezing again when the pressure is reduced.
Bed deformation (Basal sliding)
When the ice is carried by saturated bed sediments moving beneath it on gentle gradients. The water is under high pressure. This movement has been likened to the ice being carried on roller skates.
Internal deformation
Cold-based (polar) glaciers are unable to move by basal sliding as the basal temperature is below pressure melting point. Instead they move mainly by internal deformation, although ice at 0ºC deforms 100 times faster than ice at -20ºC.
Internal deformation has two elements: Intergranular flow, Laminar flow. Both of these movements occur when the glacier is on a slope; they do not occur on level surfaces where the ice remains intact.
Intergranular flow (Internal deformation)
When individual ice crystals re-orientate and move in relation to each other.
Laminar flow (Internal deformation)
When there is movement of individual layers within the glacier - often layers of annual accumulation.
Example of Internal deformation
The Meserve glacier in Antarctica moves only 3-4 m per year at its equilibrium line and 100% of this movement is by internal deformation.
When ice moves over a steep slope it is unable to deform quickly enough and so it fractures, forming crevasses. The leading ice pulls away from the ice behind it, which has yet to reach the steeper slope.
This is extending flow.
