Similarly, there are many outputs of a glacier. As well as aiding the movement of a glacier, meltwater is an output of the system. Ice which has been stored in the glacier can melt; usually in warmer, summer months, and flow out of the glacier as meltwater and leave the system. Evaporation is another significant output, snow from the surface of the glacier can melt and evaporate. Most outputs occur from the surface of the glacier, ice and snow can sublimate or be blown away in strong winds. Finally, with glaciers which end at the sea, blocks of ice fall from the snout of the ice mass into the water to create icebergs, this is called calving. This can also occur when there’s a lake at the front of the glacier.
These inputs and outputs contribute to the size of a glacier. The glacial budget is the balance between a glacier’s inputs and outputs, the dominant process determines the size and movement of the glacier. Accumulation is the input of snow and ice into the glacial system; most accumulation is snow. Ablation is the output of water from the glacier. The glacial budget is the balance between accumulation and ablation over a year – it shows whether the volume of water in the glacial system has increased or decreased. This determines whether the glacier advances forwards or retreats back. The glacier is divided into two zones, the zone of accumulation at the top of the glacier, where there’s higher precipitation levels, and the zone of ablation at the bottom where melting and sublimation of glacier ice occurs. The place where accumulation and ablation are equal are equal is called the equilibrium point. If there’s more accumulation than ablation over a year, the glacier has a positive regime (or a positive mass balance). If this occurs then the glacier grows and advances forwards, eroding the valley as it progresses and meanwhile depositing moraine/lodgement till (material deposited by an advancing glacier). Despite an advancing glacier eroding and depositing, eroding is a more significant process and an advancing glacier tends not to deposit a lot in comparison. Glaciers erode the valley floor and sides by plucking and abrasion. Plucking is when ice in contact with rock surfaces thaw slightly and then refreeze around rocks protruding from the valley floor and sides. When the glacier moves forward, it plucks the rocks away from the valley. Abrasion is when debris which is carried along by the glacier scrapes away at the valley. Alternatively, if there’s less accumulation than ablation over a year, this is a negative regime (or a negative mass balance). The glacier shrinks and retreats. However a retreating glacier does not erode, but still deposits ablation till in the ablation zone. There are three main ways debris is transported. Supraglacial material is carried on top of the glacier’s surface. Englacial material is carried within the body of the glacier. Subglacial material is moved along at the base of the glacier. If there’s the same amount of accumulation and ablation over a year, the glacier stays the same size and the position of the snout does not change. The glacial budget can change throughout the year. More ablation occurs during the warmer times of the year, around April to October, as more ice melts when it’s warmer. During colder months, there’s more accumulation than ablation. Over the year this might balance out, the glacier advances in winter but retreats in summer, so overall the volume of water in the glacier remains the same. Although changes in global temperature can have a profound effect on the glacial budget. For example, temperatures in the 19th century were colder than in the 18th century, so in general there was more accumulation than ablation. This meant that glaciers advanced because they had a positive regime. Nowadays in the 21st century, due to global warming, glaciers have tended to have a negative regime and therefore are retreating. In the late 1960s/early 1970s, the ice fields slipping down Mont Blanc and the surrounding mountains of the European range covered some 375 sq km. By the late 2000s, this area had fallen to about 275 sq km.
The glacier also has stores. The main store obviously being ice, glaciers of the Antarctic have a volume of 30,109,800km3. Meltwater also makes up a small portion of the glacier, it’s considered a store when it’s actually in the glacier – found on the ice, in the ice or below the ice. When it is down the valley away from the glacier it’s not a store in the glacier any more. Finally, glaciers carry debris (rocks, gravel and sand) which are considered a store within the system.
There are many processes within a glacier, which either erode or deposit material, or create movement within the glacier, affecting its position and shape. Basal sliding occurs with an abundance of meltwater, so it’s a more common process in temperate regions, with warm based glaciers e.g. the Alps. It’s an external process by which meltwater lubricates the base and the glacier slides over the valley. Similarly, regelation occurs because of meltwater. At a rock outcrop on the valley floor, pressure melting occurs against the rock and meltwater is created, enabling basal sliding. When the water reaches the other side of the outcrop, there’s less pressure therefore lower temperature and thus the water refreezes, this melting and refreezing is called regelation. Laminar flow is an internal movement whereby layers of internal ice slide over one another, this contributes to a high percentage of movement in cold based glaciers since there’s less basal sliding. As well as layers of ice sliding over each other, ice crystals within the glacier do the same process, but with ice crystals the process is called intergranular flow. Furthermore when an ice crystal splits and the two halves slide over each other, this is called intragranular flow. All these processes cause the glacier to move and if the glacier is moving it’s changing shape and eroding the landscape.
