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Glacier Mass Balance

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Glacier mass balance is a geographical concept that underlies all theories relating to glacial flow and behaviour. It refers to the state of mass gain (accumulation) and mass loss (ablation) of the glacier.

The particular glacial landscapes we are discussing should be viewed as an open system of states and processes. Processes are the various ways in which a glacier may move, or the inputs and outputs entering and exiting the system.

Mass balance geography definition

If the glacier has a positive mass balance this means it is gaining more mass than it is losing, and if the glacier has a negative mass balance then this means it loses more mass than it is gaining. If glaciers are gaining and losing approximately the same amount of mass, then they are in equilibrium.

What are the input and output processes in relation to mass balance?

Input/output processes are responsible for the glacier gaining or losing mass respectively. Each input/output process has its respective input/output of matter (the thing that the process either adds or takes away to/from the system), e.g. snow is an input of the input process of precipitation.

glacier mass balance, StudySmarterAccumulation and ablation zones in a glacier, Wikimedia

Inputs processes include:

  • Precipitation inputs: snow, hail, and or rain.
  • Wind inputs: snow.
  • Avalanching inputs: snow and other material that falls in.

Outputs processes include:

  • Meltwater runoff output: meltwater.
  • Ice calving output: icebergs and other landforms.
  • Sublimation output: water vapour.

Mass balance measurement

To calculate mass balance the following equation is needed:

glacier mass balance, mass balance measurement, StudySmarter

What is a glacial budget?

Whether a glacier is growing or shrinking depends on the balance between accumulation and ablation. This is also known as the glacial budget. The year-to-year change in these two processes is another way of looking at the mass balance and is calculated by dividing the glacier into two zones, the accumulation zone, and the ablation zone:

  • The accumulation zone is where there is a net gain of ice over the course of the year, and here the inputs exceed the outputs.
  • The ablation zone is where there is a net loss of ice during the course of the year, and here the outputs exceed the inputs.
  • The point between the accumulation and ablation zone where the net accumulation and ablation are about equal is known as the equilibrium point or equilibrium line.

Glaciers advance and retreat in response to long-term trends in mass balance:

  • if accumulation exceeds ablation, then the glacier has a positive mass balance and advances.
  • The opposite is true if ablation exceeds accumulation, then the glacier has a negative mass balance and it retreats.

What is the glacier equilibrium line?

The equilibrium line is the boundary between the accumulation and ablation zone. Recall the two different states of positive/negative mass balance – if the inputs are equal to the outputs, then the equilibrium line will stay in place. However, if the inputs and outputs are not equal (as is often the case), then the equilibrium line will move. An increase in either of the processes will increase the zone which they are affecting, i.e., greater accumulation means a larger accumulation zone, etc.

If accumulation > ablation, the equilibrium line moves towards the ablation zone to represent the expansion of the ablation zone.

If accumulation < ablation, the equilibrium line moves towards the accumulation zone to represent the expansion of the accumulation zone.

Since the equilibrium line is not usually stationary and is subject to change, the equilibrium of the glacier is said to be dynamic (fluctuating). This also implies that the movement of the glacier is also dynamic as it doesn’t solely move in one direction nor remain stationary; this is sometimes referred to as the potentiality of the glacier.

Why are accumulation and ablation relevant to glacier mass balance and its movement?

Remember that glacier mass balance is the state of glacier mass gain and mass loss. A positive mass balance means that the glacier is gaining more mass than it is losing whereas a glacier with a negative mass balance is losing more mass than it is gaining.

  • The glacial processes that lead to a glacier gaining mass are known as accumulation processes.
  • Glacial processes that lead to a glacier losing mass are known as ablation processes.

An increase in glacial mass will cause the glacier to advance as it pushes the equilibrium line towards the glacial snout (see glacial movement), conversely, a decrease in glacial mass causes glacial retreat as well as the pullback of the equilibrium line towards the source of the glacier.

As the processes of accumulation and ablation cause direct changes in glacier mass balance and hence its movement, they are the precursor to the majority of glacial processes and are therefore fundamental to our understanding of glaciers as a whole.

What are the specific accumulation processes and what are their effects?

There are several accumulation processes.

Precipitation

Precipitation is generally what causes glaciation to start in the first place (see Glaciated Landscapes) and makes up the majority of glacial accumulation.

The input of physical matter via precipitation increases the mass balance of the glacier. Greater mass leads to higher gravitational potential energy and hence more downslope movement of the glacier.

Glacier Mass Balance Precipitation StudySmarterPrecipitation as an accumulation process, Wikimedia

Precipitation can take the form of either rain, hail, or snow and will usually occur at the glacier’s highest elevation. Rainfall will turn to ice due to the low temperatures of the glacier and snow will likely form into glacial ice due to the pressure exerted on it by the constant layering of snow, turning it into firn (firnification).

It is important to note that unless rainfall freezes it is usually considered to be 'lost to the system' and won’t contribute to glacial accumulation as it is more likely to percolate to subglacial regions or simply fall off the glacier.

Wind

The role of wind is important to glacial accumulation. If the wind blows in the direction of the glacial source, then it is likely that any snow, ice, or other matter that is carried will be deposited in the glacier’s accumulation zone.

While this does not make up a large percentage of total accumulation, it is nonetheless a contributing factor that could be consequential if the difference between accumulation and ablation is very small.

Avalanching

Inputs of snow and ice can enter the glacial system as a result of avalanches from high altitudes.

In some cases, these can be very minor, and it is generally difficult to determine the contribution of avalanches to a glacial mass balance. However, one study concluded that according to simplified models of three Himalayan glaciers, 'avalanche-accumulation contribution dominates the accumulation and exerts overwhelming control over the dynamics of a significant number of Himalayan glaciers'. (1)

glacier mass balance, avalanche, StudySmarterAvalanche on Mount Everest in 2006, Chagai/Wikimedia

However, the study also states that their findings 'demand further work on more refined theoretical tools and more importantly, direct field measurement techniques to quantify the avalanche activity and their long-term trends in the region'.

So while it is likely that avalanches contribute a significant amount to glacial accumulation in some glaciers, this may not occur in all cases and further studies are necessary to determine this more conclusively.

What are the specific ablation processes and what are their effects?

Now let's look at sublimation processes.

Sublimation

Sublimation of surface ice is one of the most significant processes in glacial ablation. Sublimation is the change in state from a solid to a gas, and in the context of glaciation, this change is from ice to water vapour. When glacial surface ice goes through sublimation the water vapour will rise and escape the glacial system causing mass loss.

Sublimation also consumes a lot of energy compared to melting. As a result, high levels of sublimation have the effect of reducing overall ablation. In some cases (most notably in high altitudes and other very cold environments), sublimation can account for all the surface ice loss of a glacier. One notable example of this is the Taylor Glacier in the Transantarctic Mountains.

Calving

Ice calving is the process of large bodies of ice detaching from a glacier and being lost either to a ridge or a large body of water. In the cases where ice is calved into a large body of water, marine-based ice masses such as icebergs or ice fields may form.

Glacier Mass Balance Ice Field Example StudySmarterExample of an ice field, Wikimedia

Generally, when ice is calved, it is a very large mass of ice that is removed. Blocks up to 60 metres high can break loose and fall into the water. A significant example of this occurrence is the Johns Hopkins Glacier in Alaska which sometimes causes immense waves. Due to this, boats are forbidden from entering within 3km of the glacier. It is also a great tourist attraction in locations such as Alaska.

Where ice is calved is often the terminus of the glacier (its endpoint). These locations are often characterised by large numbers of icebergs. The calving of glaciers in Greenland may produce between 12,000 and 15,000 icebergs alone.

Run-off

In land-terminating glaciers (glaciers that terminate on dry land rather than water), the main process of ablation is surface melt due to the fact that as the glacier travels downslope, their altitudes decrease and temperatures increase. This causes the surface ice to melt and run off the glacier. This forms rivers which contribute to draining the glacier.

Glacier Mass Balance Meltwater Stream StudySmarterExample of meltwater stream, Wikimedia

The image above shows a supraglacial meltwater stream (stream on top of a glacier). It is possible for the glacial stream to percolate through the cracks in glacial ice and make its way to the glacial bed. The water (now in the glacial bed) will make its way to the glacial margin and exit as a meltwater stream.

Understanding the delicate equilibrium of glacier mass balance is crucial. It may help us to understand our role in the upkeep of glaciers and their importance to the global environment.

Glacier Mass Balance - Key takeaways

  • Glacial mass balance is the basis for our understanding of the glacial system and glacial processes
  • The equilibrium point of a glacial system is dynamic
  • The variation of accumulation and ablation determines the point of equilibrium within a glacial system
  • Accumulation and Ablation determine glacial mass balance and as a result glacial movement
  • The terminus of a glacier is often determined by whether a glacier encounters a body of water
  • The exact extent of the contribution of some processes is not 100% understood e.g. avalanching
  • Processes of Accumulation increase glacial mass balance
  • Processes of Ablation decrease glacial mass balance
  1. Sourav LAHA, R. K. (2017). Evaluating the contribution of avalanching to the mass balance of Himalayan glaciers. Cambridge University Press

Glacier Mass Balance

 In general terms, Mass Balance (b) = Ablation (a) + Accumulation (c). Mass Balance in usually given in metres water equivalent (m.w.e)

More accurately, the link is from climate to mass balance as it is the climate that primarily influences mass balance and not the other way around. As our climate is increasingly warming, there will be increased ablation within a glacial system (such as sublimation) causing a decrease in the mass balance.

Understanding mass balance allows us to understand glacial systems as a whole, including glacial processes and most importantly glacial movement i.e. increased mass balance, is related to glacial advance. We explore this in-depth in further chapters,

Mass balance in geography refers to the difference or relationship between accumulation and ablation within a glacial system, these dictate either the increase or decrease in glacial mass respectively.

How to calculate glacier mass balance:

To calculate glacier mass balance you need to use the following equation


Mass Balance (b) = Ablation (a) + Accumulation (c)

Final Glacier Mass Balance Quiz

Question

What input processes can cause the accumulation of ice mass?

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Answer

Snow, precipitation, and avalanching

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Question

Why do we call the glacial equilibrium dynamic?

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Answer

Because the equilibrium line shifts according to the ratio of accumulation and ablation

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Question

What are the states of a glacial system?


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Answer

Positive and Negative Mass Balances.

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What happens if accumulation exceeds ablation?


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Answer

Glacier advances

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What happens if ablation exceeds accumulation?


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Answer

Glacier retreats.

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Question

What is the equilibrium line?

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Answer

The point at which the accumulation and ablation is more or less equal

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Question

What is ablation?


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Answer

The processes by which mass exits the glacial system.

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What is accumulation?


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Answer

The processes by which mass enters the glacial system.

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Question

Name 2 ablation processes?


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Answer

Ice calving, Run-off

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Question

Name 2 accumulation processes?


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Answer

Precipitation, Wind

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Question

Give an example of significant ice calving

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Answer

Greenland where ice calving can be responsible for up to 15,000 glaciers a year

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Question

What is generally the primary input source for glacial mass?


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Answer

Precipitation

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What determines the type of glacial terminus?


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Answer

Whether the glacier terminates due to a body of water or due to reaching a low enough altitude on dry land.

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Question

What does supraglacial mean?


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Answer

On top of a glacier.

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Question

What is the upper bound of sublimation as a percentage of total ablation?


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Answer

100%

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Question

Name a glacier where sublimation accounts for most ablation


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Answer

The Taylor Glacier

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Question

Name a glacier where sublimation accounts for most ablation

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Answer

The Taylor Glacier

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Question

Is avalanching a significant accumulation process?


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Answer

In some instances, it is suggested that it may be the dominant process, however, this is not the case in all glaciers and further research is needed to confirm the findings which suggest that avalanching may make up the majority of the accumulative processes on particular glaciers.

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Question

What is finification?


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Answer

The process of constant layering of snow that eventually causes the formation of firn.

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What is ice calving?


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Answer

When a significant mass of ice is detached from its glacier or ice sheet.

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Is wind considered a significant contributor to glacial accumulation?


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Answer

It is not. 

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Question

 What is sublimation in the context of glacial accumulation/ablation? 


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Answer

The state change from a solid to a gas, in the context of a glacial system this contributes to ablation.

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