Mass Balance

Geography: Edexcel A Level, topic 2.3.1

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.

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%).

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.

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.

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.

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.

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.

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

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).

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.

What is another word for the outputs of a glacial system?
How much solar radiation is reflected in the albedo effect?
If accumulation is equal to ablation in the mass balance of a glacier, what happens?
Which of the following are true of mass balances in glaciers?
During which season is there a net ablation in the glacial budget?