- Created by: Laura Godfrey
- Created on: 18-05-10 11:47
The Glacier as a system
Just as a river could be seen as a system of inputs, outputs, transfers and storage, so can a glacier. Whether a glacier grows or retreats is directly affected by the comparison between the inputs and outputs.
Glaciers originate from heavy snowfalls over a prolonged period of time. The snow initially has many air or pore spaces between the flakes. Over time the weight of new snow above it compacts it all, squeezing the air out of the pore spaces, similar to what you might do when making a snowball. This compaction causes some of the snow to become freezing water that binds the compacted snow together even more, creating ice. As the sir is squeezed out of the ice it will turn a slight shade of blue.
Ablation - The melting of the ice, mainly during summer months, and usually at the snout end of the glacier.
Accumulation - The build up of the glacier due to snow being compacted into ice.
Calving - The splitting of the end of the glacier into smaller sections. These could become icebergs, if the glacier snout ended in the sea. .
Glaciation - The effect of large masses of ice on the landscape. Compressed snow accumulates to eventually form ice and create a glacier.
Ice Sheets - These are large masses of ice which cover an entire land surface. Antarctica is the best example as the ice sheet covers the entire continent.
Snout - the lower end of the glacier.
Valley Glaciers - The most common of the two types of glacier. These are confined by the valley sides that have already been carved out by a river. Valley glaciers can be found in all the main mountain ranges of the world, such as the Franz Josef Glacier in the Southern Alps of New Zealand, and the Rhone Glacier in Switzerland.
The Ice Budget
A glacier will grow and retreat depending upon how much precipitation is inputted into the glacier system. This growth can be seen on two time scales,either every year or over a much longer period.
In general glaciers around the world are retreating at the moment as the climate slowly warms up and they melt slightly every year. The evidence for this can be seen in some of the large Antarctic Ice Sheets either retreating or breaking off from the continent all together. Valley glaciers are also being measured to see how quickly they are retreating.
The Ice Budget
During a single year a valley glacier may well both grow and retreat, depending on the rate of accumulation compared to the rate of ablation. This is called the ice budget or glacial budget.
When the rate of accumulation is greater than the rate of ablation,the glacier will grow. This is called a positive regime.
When the rate of accumulation is less than the rate of ablation, the glacier will retreat. This is called a negative regime.
Abrasion - Glaciers carry a large amount of material with them. Some of these sharp boulders are embedded in the bottom of the glacier and act as erosive agents for the glacier. These rocks mean that the glacier acts like sandpaper, scouring along the valley floor. The effects of abrasion are that the rock surface of the valley will be polished and may have deep grooves cut in them. These grooves are called striations.
Plucking - This is the main erosive process of a glacier. As the glacier moves along the valley the ice melts slightly around large boulders, before re-freezing around them. As it then moves on the boulders are literally ripped out of the ground and will often become agents of abrasion.
Freeze-Thaw - Water enters cracks in the rock during the day. Overnight the temperature drops and the water freezes. As it freezes, it expands. The expanded ice places pressure on the rocks around it. Over time this constant pressuring of the rock causes it to crack and split. This process will break of rocks ready to be plucked by the glacier.
Glaciers carry a huge amount of material. These rocks are called moraine and can be carried great distances by a glacier.
The rocks are mainly carried once they have been plucked away from the valley surface by the glacier. Most of the material is carried nearer to the base of the glacier. The glacier also carries however frost shattered material from the valley sides once it has fallen onto the ice surface. This material is called lateral moraine, and is carried at the sides of the glacier. When two valley glaciers meet and merge the two lateral moraines will form a medial moraine, running down the centre of the new, larger glacier.
Glaciers will always reach a point when they will start to melt, mainly due to the rise in temperature as they descend in height down the valley. As the ice melts it cannot carry as much material and so this is deposited. The main depositional feature of a glacier is its terminal moraine, but it will also create recessional moraines and eventually also leave behind lateral, medial and ground moraines.
Arêtes - Where two corries formed back to back they both eroded backwards until they created a narrow knife-edge ridge between them This is called an Arête.
Corries - Also known as cirques, they are the starting point of a glacier.
At the beginning of the last age snow began to accumulate in hollows on hillsides, slowly accumulating enough to turn into ice. This ice slowly gouged out a steep back wall through the processes of freeze-thaw and plucking. Large crevasses on the top of the ice, called bergschrund's allowed water to flow into the ice, where it froze to create more ice.
The bottom of the corrie was eroded by abrasion as the ice moved forward in a rotational way. Where the rate of erosion was less, at the front of the ice, a rock lip was left. As more and more ice accumulated it flowed over the lip and into the valley below, creating a glacier.
Once the ice melted the corrie was often filled by melt water to form a lake. The rock lip and moraine acted as a natural dam. These lakes are known in Britain as Tarns.
Glacial Trough - The other name for a U-shaped valley cut by a glacier.
Hanging Valleys - see u-shaped valleys
Pyramidal Peaks - Formed in exactly the same as an Arête only this time three corries back onto each other. Eventually the backwards erosion leaves a sharp pyramid peak. An example of this is the Matterhorn in the Alps.
U-shaped Valley - Just as a river produces a distinctive V-shaped valley, so a glacier produces a U-shaped one. Usually a glacier will follow the general direction of a river valley, however it doesn't flow around areas of hard rock it cuts straight through them. This means that when the glacier melts it leaves behind a valley with very steep sides.
Glacial valleys that flowed into the main valley have been chopped off and the rivers in them now flow over huge waterfalls from hanging valleys. The interlocking spurs of the river valley have also been cut through to leave truncated spurs behind. On the valley floor a long, thin ribbon lake may well form, with the terminal or recessional moraine forming a dam to hold the water in.
Boulder Clay/Till - This is the mixed angular material found on the valley floor, which has been deposited by the glacier as it melts. It is made up of sand, stones and clays. The stones are angular, as they have not yet been rounded by water action. This is also called ground moraine.
Erratics - These are large boulders that have been carried by the glacier and then deposited in an area of differing rock type, so that they look completely out of place. Erratics can have originated from hundreds of miles away.
Drumlins - These are smooth mounds of deposited material that are formed parallel to the direction of the movement of the glacier. They are quite large, usually between 30 and 40 metres high and up to 500 metres long.
Drumlins are formed by moraine being deposited due to an obstruction causing an increase in friction or a slowing of the glacier. Most of the material is deposited at one end of the drumlin, with the rest tapering off towards its thin end. The flow of ice over this deposited material then shapes it into the characteristic form of a drumlin.
Moraines - as already mentioned there is a range of moraines. Moraine is the term used for any material carried or deposited by a glacier. The five main types of moraine are:
Terminal Moraine: This is the material deposited at the snout of the glacier at the furthest extent of its growth. The terminal moraine may act as a dam for a ribbon lake.
Lateral Moraine: Runs parallel to the glacier and is the material that has been eroded from the valley sides by the actions of freeze-thaw and the glacier itself. Once the glacier melts this will be left on the valley floor.
Medial Moraine: Where two glaciers meet, their lateral moraines meet to form a medial moraine, which runs down the centre of the glacial surface. Once the glacier melts this will be left on the valley floor.
Ground Moraine: otherwise known as boulder clay or glacial till, this is the material deposited on the valley floor by the glacier. It is usually the result of plucking and abrasion.
Recessional Moraine: Similar to a terminal moraine they were created when the glacier retreated and then stopped, allowing it to build up a pile of deposits. A recessional moraine could act as a dam to create a ribbon lake.
Upland glaciated areas are not particularly conducive to farming,with their steep slopes, high precipitation, low temperatures and relatively thin, poor soils. In the Alps some sheep farming occurs, and used to use the methods of transhumance, which meant grazing the sheep on the high mountain pastures during the warmer summer months, before bringing them down to lower valley areas in the winter.
In Great Britain the glacial valley floor is very valuable farming land, asit is sheltered, flat and well irrigated. The soils are variable but can bevery fertile in areas of clay deposits. The farming is still mainly pastoral,although some areas can be used for arable farming.
Tourism & Recreation
The scenery of a glaciated area is spectacular, with it's high mountain pastures, wide valley floor, crashing waterfalls and sharp arête's. Many people love walking in the Swiss Alps purely to see this scenery.
For the more adventurous traveller, glacial valleys provide some fantastic rock climbing opportunities, as well as being perfect for things like hang-gliding.
Glaciers also can be very useful to the winter skiing industry, by guaranteeing that the resort will have some skiing, even if the winter snowfall is very poor. Some glaciers in North America offer all-year-round skiing.
The ribbon lakes are perfect for recreational activities such as water-skiing, sailing and fishing, as well as many other water-based activities. The prime example of this is the many ribbon lakes of the Lake District.
As seen in many parts of the Alps, glaciated valleys provide a perfect opportunity for the production of hydroelectric power. Their steep sides, high precipitation and low population density make them ideal places for dams to be built and reservoirs created (often by just increasing the area already filled by a ribbon lake).
Management is needed in glacial areas to cope with many demands and pressures:
Glacial valleys are very attractive to tourists, and so methods have to be put in place to protect the environment from damage.
This includes trying to prevent soil erosion by introducing artificial paths or by diverting popular routes to allow the old paths time to recover.
Some places have become 'honeypot' sites, which attract a huge number of tourists every year. By promoting other similar areas, the local authorities can try to alleviate the pressure on certain very popular places.
Conflicts can also occur between local farmers or residents and tourists. The increased traffic, footpath erosion and the problems of family dogs worrying sheep all have led to conflicts between the local people and the visitors. The main way to alleviate these conflicts is to educate people into the correct way to treat the countryside when they visit.
Speed limits for boats have been introduced in some of the most popular lakes, such as Windermere. These are aimed at protecting local wildlife and preventing too much disruption to those enjoying the tranquil nature of the area.
Tourist facilities, such as visitor centres and parking areas have been introduced to try to cater for the influx of visitors, without taking over local villages and towns. These also provide a good base from which to try to educate people on how to treat the countryside when the visit.