2.3.1

Mass Balance

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Inputs and Outputs of Glacial Systems

Glaciers operate as a system with inputs (accumulation) and outputs (ablation) and both influence the mass balance of a glacier.

Illustrative background for Inputs (accumulation)Illustrative background for Inputs (accumulation) ?? "content

Inputs (accumulation)

  • Direct snowfall and other forms of precipitation.
  • Meltwater from melting snow and ice that re-freezes.
  • Rock fragments that have fallen on to the glacier or been eroded by it.
  • Avalanches of snow and/or rock, and wind-blown deposits.
  • Solar energy. But, snow and ice have a high albedo meaning much radiation is reflected (up to 90%).
Illustrative background for Outputs (ablation)Illustrative background for Outputs (ablation) ?? "content

Outputs (ablation)

  • Meltwater during the spring and summer months.
  • Sublimation and evaporation can take place all year round, but mostly occur in summer.
  • Meltwater streams also transport eroded material. They are often cloudy as they contain rock flour.
  • Calving into lakes and the sea.
  • Solar energy reflected in the albedo effect.

Mass Balance

The mass balance of a glacier is influenced by inputs and outputs of glacial systems and can change over time, both long-term and short-term.

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Budget

  • The mass balance (budget) is the relationship between accumulation and ablation.
    • If accumulation is greater than ablation, the glacier gains mass and the glacier snout advances.
    • If ablation is greater than accumulation, the glacier loses mass and the glacier snout retreats.
    • If accumulation is equal to ablation, the snout of the glacier remains in the same place - it is stationary.
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Equilibrium lines

  • An equilibrium line can be shown on a glacier where at any one time the amount of accumulation equals that of ablation.
    • This is also known as the firn line.
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Spatial variations

  • There are spatial variations in a glacial budget.
  • Most of the accumulation occurs at the upper end of a glacier because at higher altitudes it is much colder so this exceeds losses due to ablation.
    • There is, therefore, net accumulation and this part of a glacier can be called the accumulation area.
  • At the lower end of a glacier, the reverse is the case and ablation at warmer, lower altitudes exceeds accumulation.
    • So, there is net ablation and this part of a glacier is called the ablation area.

Temporal Variations and Feedback Mechanisms

The mass balance of a glacier can change over time. Feedback mechanisms also play a significant role.

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Temporal variations

  • There are temporal variations in a glacial budget.
  • During winter months, there is a net accumulation in the glacial budget and so the glacier expands (positive balance).
  • During summer months, there is a net ablation in the glacial budget and so the glacier reduces in size (negative balance).
Illustrative background for Feedback mechanismsIllustrative background for Feedback mechanisms ?? "content

Feedback mechanisms

  • Positive feedback - a small increase in snowfall caused by a lowering of temperature would increase the albedo of the ice surface, contributing to further cooling and more accumulation.
  • Negative feedback - higher temperatures would increase evaporation and therefore precipitation (snowfall) so more snow accumulation balances the losses from ablation at higher temperatures.

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1Tectonic Processes & Hazards

2Option 2A: Glaciated Landscapes & Change

3Option 2B: Coastal Landscapes & Change

4Globalisation

5Option 4A: Regenerating Places

6Option 4B: Diverse Places

7The Water Cycle & Water Insecurity (A2 only)

8The Carbon Cycle & Energy Security (A2 only)

9Superpowers (A2 only)

10Option 8A: Health & Human Rights (A2 only)

11Option 8B: Migration & Identity (A2 only)

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