Glacier Mass Balance

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.

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.

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.

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.

Accumulation Processes: 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.

Accumulation Processes: 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⁾

Fig. 3 - Avalanche on Mount Everest in 2006

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.

Now let's look at sublimation processes.

Ablation 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.

Ablation Processes: 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.

Fig. 4 - An ice field in Patagonia

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.

Ablation Processes: 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.

Fig. 5 - a meltwater stream

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.