Glacial Characteristics (AQA A Level Geography)

Glacial Budgets

• Glaciers are open systems with direct inputs of snow and ice from precipitation, blown in on the wind or with avalanches

• Snow and ice settle and compact to form firn or névé 

• Each subsequent snowfall adds to these layers, and further compacts the firn into glacial ice

• Compaction squeezes air out of the firn, and the resulting glacial ice absorbs longwave light but scatters short-waved blue light, making the ice appear blue

• The formation of glacial ice takes approximately 30 years

• These inputs are known as accumulations 

  • The accumulation is transferred downhill by gravity

  • The accumulation loses mass through melting and evaporation called ablation, which is the output of the system

  • The balance between the accumulation and ablation over a year is called the glacial budget

  • It determines if the mass of the glacier has increased or decreased

• There are two zones:

  • Accumulation zone

    • Found in the upper part of the glacier

    • Inputs are usually more than the outputs

    • There is a net gain of ice during the year

    • Glacier front advances

  • Ablation zone

    • Found in the lower part of a glacier

    • Output exceeds inputs

    • Net loss of ice during the year

    • Glacier front retreats

• Where gains and losses balance on the glacier, the area is called the equilibrium line or point

• Over time, variations in the glacial budget will move the line up or down the glacier

• Linked to the advance and retreat of the glacier front

Mass balance

• More accumulation over a year and the glacier has a positive regime or positive mass balance

• The glacier will gain mass and advance in response to high accumulation in the upper zone

• A negative mass balance or regime is when there is less accumulation than ablation (usually during the summer months)

• The glacier will lose mass and retreat in response to low accumulation in the upper zone

• Dynamic equilibrium is when the overall amount of ablation and accumulation balances over a year

• The glacier remains the same size and the position of the glacier front does not change

Ice Advance & Retreat

• Natural global climate cycles have historically, advanced and retreated glaciers

• Long-term trends in the glacial budget are a good indicator of advance and retreat

  • if input exceeds output, then ice mass will increase and the glacier advances

  • If output exceeds input, then ice mass will decrease and the glacier retreats

• Glaciers respond to change either as a negative or positive feedback

  • E.g. if the size of the ice input decreases, a glacier may slow down so that less water and ice are output and the mass balance remains fairly constant  - negative feedback

  • Snow and ice have a high albedo rate that reflects a lot of the suns incoming radiation back out to the atmosphere

  • Glacial retreat reduces the available ice for reflection, decreasing the rate of albedo and increasing the absorption of solar radiation

  • Temperatures rise and glaciers retreat further - a positive feedback

Historic Patterns of Ice Advance and Retreat

The Mer de Glace, French Alps

• During the Little Ice Age (1550-1850) the Mer de Glace advanced by over 1km to the floor of the Valle de Chamonix

• With the increase in global temperatures the glacier has retreated by 2.3km and density has thinned

• During the 1970s and 1980s, global cooling allowed the glacier to advance by 110m

• Overall the trend shows glacial retreat, however, this is not continuous as some decades show glacial advance and the average annual budget shows a dynamic balance, although the depth of ice has thinned

Warm & Cold Based Glaciers

Pressure melting point (pmp)

• The temperature at which ice melts at a given pressure is the pressure melting point (pmp)

• The melting point of water depends on air pressure above the ice

• As air pressure increases, the temperature at which ice melts lowers

• At 1 atmospheric pressure, the melting point of ice is 0°C

• At 200 atmospheres, the melting point decreases to -1.85°C

Warm-based glaciers

• Occur in temperate regions such as southern Iceland and western Norway

• They are relatively small and range in width from hundreds of meters to a few kilometres

• Melting occurs during the summer months

• It is this meltwater that 'lubricates' the base and sides of the glacier, which assists movement (called basal sliding) and increases rates of erosion, transportation and deposition

• As such, all ice in these glaciers is at, or close to, the melting point of ice

• Temperatures at the base are therefore, at or just above the pressure melting point

Cold-based glaciers

• Occur in polar regions such as central Greenland and Antarctica

• They are large, vast sheets and caps of ice covering hundreds of km²

• Temperatures remain below melting point, with low rates of precipitation, resulting in low levels of accumulation 

• Basal temperatures remain below the pmp, therefore, basal sliding does not happen

• This results in little erosion, transportation and deposition

• Any movement is by internal deformation

  • The ice stays frozen to the bedrock and moves slowly at 1-2cm a day

  • Orientation of the ice crystals in the glacier, to the direction of movement, allows the crystals to slide over each other