So, firstly, a bit of glacial terminology........
- ICE :- (I thought that frozen water would be enough, but apparently not) Ice is snow which has been compacted so that the air passages between the individual crystals of snow become sealed, thereby increasing the density. The density has to be greater than 0.85 for it to be classed as ice.
- GLACIER :- A moving mass of ice on a land suface
- ICE SHEET:- An ice sheet covering a plateau region, over an extensive area. Movement around the edges may be faster because of steeper gradients.
- CIRQUE GLACIER:- After snow has continued to accumulate in a hollow on a mountain side, ice forms and thickens with time. Ice fills the cirque (also known as a corrie) hollow, which is deepened. the glacier may grow to the point where it spills out of its semi-circular hollow into the valley below.
- VALLEY GLACIER:- A glacier following the line of a pre-existing river valley, fed by cirque glaciers or ice caps. These can be extremely deep and the moving tongue of ice can extend from some distance, down the valley, until it reaches the lowlands or the sea.
- PERIGLACIAL:- Applied to land areas which lie around the margins of ice sheets. They are cold areas where permafrost exists either continuously (everywhere) or discontinuously (in patches only).
- PERMAFROST:- Permanently frozen ground of which only the surface layer thaws breifly during summer. This layer is known as the active layer.
- 2 million years ago = the onset of the Pleistocene Ice Age
- There have been four major ice advances known as glacials
- These were seperated by warmer periods , called interglacials, where the ice retreated
- Anglian is the name given to the first glacial advance in Britain and it occured around 500,000 years ago
- The maximum ice advance was during the third glacial advance, known as the Wolstonian glaciation in Britain, which ended about 125,000 years ago
- The Ipswichian (also known as Eemian) interglacial period was particularly warm and so provoke great ice retreat
- The fourth and last ice advance ended around 25,000 years ago and was called the Devensian glaciation
Freshly fallen snow has a low density (under 0.5) due to the large amount of air trapped between the individual crystal. However, as more snow falls and the snow patch enlarges, the old snow becomes compacted. More air is then expelled by the successive melting and refreezing and if the snow manages to last a year it turns into higher density firn or neve (which has a density of around 0.7). After further annual snowfall the firn becomes more and more compacted due to the sealing of further air spaces. The crystals start to grow larger, again increasing density, until the density grows to above 0.85 - the point at which snow is considered to be ice. This transition from snow to ice can take up to 200 years in areas like Antarctica as the continually low temperature prevents melting and refreezing, which speed up the process, or only around 5 to 10 years in slightly warmer martime environments such as Alaska.
Glaciers are examples of open systems. Their main input is snow, either from direct snowfall or from avalanches. This snow accumulates in the upper part of a glacier and is stored, in the system, as glacier ice which is carried downslope by the glaciers movement. The main output of this system is water (no surprise there!) and, although some water does evaporate directly from the surface, most water loss results from the ice melt as lower altitudes, or latitudes with higher temperatures, are reached. Further melting occurs at the sides, where the glacier meets rock with a much lower albedo than the glacier ice itself, and as internal or basal melting. On top of this you also get calving where icebergs break off into streams, lakes and the sea.
Of great significance to the speed and effectiveness with which glacial processes operate is the size of a glaciers budget. Calculating the budget involves measuring the inputs in the zone of accumulation and outputs in the zone of ablation, as well as any change in the volume of glacial ice in store (sounds a bit similar to the water balance equation we learnt in the rivers module to me). Glaciers in mountainous areas on the western sides of continents feel the full force of the prevailing westerly winds and depressions, like in Norway, and many have snow inputs well above 2000mm (in water equivalent). Lying on the southern edge of the polar region much melting also occurs. A high budget such as this, accompanied by the effects of gravity from the steep relief, encourages faster glacial flow than in continental ice sheets. In interior Greenland and Antarctica less than 50mm of new snow may be recieved during a year, although the intense cold means that little is lost from the system, which means that the ice budget is low.
Ice budget is one of the factors used in the classification of ice masses into either ice sheets and valley glaciers and it also supports the basic thermal division between cold glaciers and warm glaciers. Continental ice sheets are cold glaciers, so called because the temperature throughout the glacier ice remains below freezing point all year round. The ice is therefore frozen to the bedrock below, which slows down the glacier movement to, often, as little as a few centimetres per year. Despite having greater thickness and weight, the lack of movement limits the erosive ability of the glacier and so the amount of glacial erosion that occurs. In contrast, valley glaciers are warm glaciers and many are located in temperate latitudes where meltwater is plentiful in summer - although during winter the some surface ice will melt. This means that water is making its way down through the cravasses in the ice to the base of glacier on many days of the year. Water can also be present at the base of the glacier even when the temperature is falling below freezing point as a result of the weight of the ice above and the friction of movement against the bedrock causing localized melting a the base. The presence of so much water is significant in easing movement and encouraging erosion.
So, thats kind of the beginnings of the the stuff about glaciers that I am trying to understand - sorry I realise that it is a bit long and wordy but I am still banned from talking about anything Geography related at home and so I am kind of just blurting it out on here instead. Next, I think I will move on to the processes and lanforms in valley glaciers and then the glacial processes that exist - with a lot more pictures and diagrams, I promise!