Glacial Characteristics (AQA A Level Geography)

• 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

Glacial budget showing the zones of ablation and accumulation. If the average annual mass balance remains the same, despite short-term seasonal variations, then the system is in dynamic equilibrium, however, balance can move over time

• 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

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

Warm and cold based glacial profile - meltwater is the important factor in movement