Glacial Systems

Glacial systems are the balance between inputs, stores, transfers and outputs in relation to glaciers. Glaciers form when temperatures are low enough for the snow that falls in one year to remain frozen throughout the year. This means that the following year, fresh snow falls on top of the previous year's snow. Fresh snow consists of flakes with an open, feathery structure and a low density. Each new fall of snow compresses and compacts the layer beneath, causing the air to be removed and converting low-density snow into higher-density ice. Snow that survives a whole summer is known as firn or neve and has a density of around 0.4g cm-3. This firn or neve has increased pressure between its individual grains which causes pressure melting to eventually loosen snow into a dull, white structure-less mass with less pore space which is therefore more impermeable. With further compaction by following years of snow fall, the snow becomes glacier ice with a density of between 0.83g cm-3 and 0.91g cm-3. This process is known as diagenesis. It may take between 30 years and 1000 years for this to happen. True glacier ice is not encountered until a depth of about 100m and is characterised by a bluish colour rather than the white of fresher snow. The white colour is due to the presence of air.

The majority of inputs occur towards the top of the glacier and this area, where accumulation exceeds ablation; this is called the accumulation zone. Accumulation is the addition of ice by processes such a snowfall and avalanches. Accumulation is normally at its highest during the winter months as snow falls at a greater rate. Also, ablation is the loss of ice to processes such as melting, evaporation, calving and rivers. Ablation tends to increase further down the glacier from the firn line. The glacier itself is water in storage and also rock debris that has either been picked up by ice or has fallen from near higher ground. The transfer occurring is the moving of the ice. Most of the outputs occur lower down where ablation exceeds accumulation, in the ablation zone.

The two zones are notionally divided by the firn or equilibrium line, where there is a balance between accumulation and ablation. The nature of the balance between annual ablation and the glacier's forward motion is vital. If ablation is greater, then the glacier front will retreat, and if it is less, then it will advance. In the glacial system, the glacier is in a state of dynamic equilibrium determined by the ever changing relationship between inputs and outputs. If they balance exactly, then the glacier will be in a steady state, meaning it will neither advance nor retreat. Below I have shown a diagram which illustrates the inputs, outputs, stores and transfers of the glacial system.

The mass balance or glacier budget is the difference between the total accumulation and the total ablation for one year. A positive figure shows us that a net gain of ice through the year, which means that there is net accumulation and so the glacier will advance or grow. The firn line will then also move down the valley. If the glacial budget figure is negative then that shows that there is a net loss of ice through the year, meaning there is net ablation and there will be a retreat or contraction of the glacier and the firn line will move up the valley. In the aftermath of the Little Ice Age, around 1850, the glaciers of the Earth have retreated substantially. Glacier retreat has increased since the 1980s, the coldest decade since 1900. The rate of glacial movement can be slow or glaciers may move several hundreds of feet in a season resulting in a glacial surge. If the amount of accumulation equals the amount of ablation, the glacier is in equilibrium and will remain in position and stable.

Locations such as the Alps and the Rockies experience high rates of accumulation in the winter and, due to significantly warmer, above zero temperatures, high rates of ablation in the summer. This makes the glaciers in these locations very active, with large volumes of ice being transferred across the firn line, and significant seasonal advance and retreat. Not only do the rapid movements cause erosion and produce erosional landforms, but the ablation also produces lots of meltwater. The movement of ice depends upon whether it is warm or cold, which in turn depends on the pressure melting point (PMP). The pressure melting point is the temperature at which ice is on the verge of melting. A small increase of pressure can therefore increase melting. At the surface the PMP is at 0 degrees, but within an ice mass it is fractionally lowered by increasing pressure. Ice at its pressure melting point deforms more easily than ice at a temperature below it pressure melting point.

There are also a number of other factors which influence the movement of glaciers. The main fundamental cause of glacial movement is gravity. Also the gradient the glacier is situated can also have affect. The steeper the gradient of the surface, the faster the ice will move, if other factors are excluded. The internal temperature of the ice can also affect movement as it can allow movement of one area of ice relative to another. The thickness of the ice can affect the pressure melting point and also the basal temperature (temperature at the base of the glacier. This can affect movement as the basal temperature affects the rate of basal sliding. Temperate glaciers mainly move in this way. If the basal temperature is at or above the pressure melting point, a thin film of meltwater exists between the ice and the valley floor and so friction is reduced.