Net accumulation and net ablation together make up the glacial budget. Net accumulation occurs at higher altitudes and therefore lower temperatures, meaning less melting of ice and a larger build up of snow. This area is called the zone of net accumulation. In the winter snow builds up to become nevcompacted deeply frozen ice which over time becomes part of the glacier, therefore making the glacier bigger
Net ablation is essentially the melting of glacier ice, which takes place in the zone of net ablation. This may take place through meltwater running down fluvio-glacial rivers, the calving of icebergs on coastal glaciers, which is happening to the greenland glacier at the moment, and sublimation where the ice directly evaporates without going through the transitional water stage. The zone of net ablation is usually at lower altitudes where the temperature is higher.
If net accumulation in a year is higher than net ablation then the glacier will appear to grow and retreat forwards as the amount of ice added to the glacier is greater than that which has melted. However if net ablation is higher than net accumulation in a year the glacier will appear to shrink, since the glacial ice is melting and evaporating faster than snow is falling. This is usually true for the snout of the glacier where the temperature is warmer.
When there is equilibrium between the two there will be no change to the glacier.
Examine the field evidence that could be used to distinguish between landforms of glacial and fluvioglacial deposition
Glaciation and fluvioglaciation are both processes which cause depositional features. These features, although formed very differently can be confused due to their similarity in appearance. However in the field there are many differentiations which can distinguish between features formed by different processes.
Two features which are commonly mixed up are kames and drumlins. Kames are formed by fluvioglaciation and occur when melt water running off the glacier flows over the edge depositing material. This happens because when water drops from one place to lower ground the velocity of the flow slows down and so material is deposited. Eventually the material deposited will become a mound. This mound will have layers of moraine as the flow of melt water varies on a daily and seasonal basis, different types of material will be carried by varied volumes of water.
Drumlins, however, are formed by the glacier itself. They occur when a glacier begins to retreat, meaning the velocity of the ice drops and so material begins to be dragged along rather than moving with the glacier. Drumlins are mounds of glacial moraine which the glacier deposits whilst it is still moving. The glacier shapes the moraine leaving a steep sloping stoss end and a steeper sloping leeward end. This leaves a hill which is similar in shape to an upturned spoon.
Kames look like circular mounds which are missing the tapered end of a drumlin. They are 10-20m high and can be up to 50m long. They occur at the edge of ice sheets where the glacier has melted away to leave the kame. Kames are not usually found close to each other. Drumlins however are usually found in clusters or ‘baskets of eggs’. They are found at the base of glacial valleys as they are usually formed by valley glaciers. Kames and drumlins can also be differentiated by digging into the feature. If you were to dig into a drumlin you would find similar material all the way through. However by digging into a Kame you would find sedimentary layers from temporal differences. Examples of Kames can be found in Norfolk as a result of the ice age before last. Drumlins can be found in areas such the upper Ribblesdale Valley in the Yorkshire Dales which is a valley floor.
Two other features which are commonly confused are Kame Terraces and Lateral Moraines. Both are found on the side of a valley however kame terraces are formed by fluvioglaciation deposition and lateral moraines by glacial deposition.
Kame terraces are formed when a glacial stream becomes trapped between the side of a valley and the glacier itself. This allows for deposition to occur as the stream will flow quite slowly. When the ice melts the material which has been deposited is left on the side of the valley forming a bench-like feature. These features can be as long as the valley and are often pitted with kettles. Due to their ridge shape appearance it is not hard to mistake them for lateral moraines. These form at the side of a valley as the glacier retreats, they are made from eroded material and debris which has fallen from the valley sides. Kame terraces can be found in the cairngorms, which also feature lateral moraines. It can be hard to see the difference in the two however lateral moraines tend to form in ridges down the side of the valley whereas there would only be one kame terrace.
Outwash plains and ground moraine are another two features which can be hard to distinguish between in the field. Both are similar since both are features often not recognisable to the untrained eye due to their scale. Outwash plains are usually many square kilometres in area. They are formed when streams of melt water from a glacier deposit the material they are carrying onto the ground on a flat area. This moraine and till builds up making the area very fertile. Outwash plains are found in the Svalbard area of Norway.
Ground moraine is very similar but is formed by moraine deposited by ice sheets rather than melt water streams. Areas of ground moraine are usually made of till and also create fertile land which is now popular for farming. Ground moraine is deposited over regions of land, such as East Anglia.
In the field outwash plains may have streams crossing them and are generally flat. Ground moraines however create a gently rolling topography and may have features such as drumlins on the surface.