- Created by: kearney2015
- Created on: 18-05-15 21:29
A glacier is a mass of ice that moves very slowly downhill. They are found at high altitude across the globe, even on high mountains close to the equator, and at lower altitude in high latitudes close to the North and South Poles. The formation of glaciers and the process by which they shape the landscape around them is called glaciation.
Glaciation in the last ice age
18,000 years ago ice covered about 30 per cent of the land in the world. In Britain, ice covered land as far as the Bristol Channel. During the last ice age the temperature remained below 0°C which allowed the ice to remain on the land all year.
Glaciers and weathering
Glaciers once covered large areas of the Earth and shaped the landscape around them. The legacy of ancient glaciers lives on - for example in areas such as the Lake District.
There are three processes by which glaciation affects the landscape - erosion,transportation, and deposition.
The predominant process is freeze-thaw weathering. Freeze-thaw describes the action of glacial meltwater on joints, cracks and hollows in rock. When the temperature reaches freezing point, the water inside cracks freezes, expands and causes the cracks to widen. When the temperature rises, the water thaws and contracts. This eventually causes rocks to break up. For freeze-thaw to take effect, the air temperature needs to fluctuate around freezing point.
Freeze-thaw weathering produces angular rock fragments.
Abrasion and plucking
Abrasion and plucking.
Plucking occurs when rocks and stones become frozen to the base or sides of the glacier and are plucked from the ground or rock face as the glacier moves. It leaves behind a jagged landscape.
Abrasion occurs when rocks and stones become embedded in the base and sides of the glacier. These are then rubbed against the bedrock (at the bottom of the glacier) and rock faces (at the sides of the glacier) as the glacier moves. This causes the wearing away of the landscape as the glacier behaves like sandpaper. It leaves behind smooth polished surfaces which may have scratches in them called striations. Striations are carved out by angular debris embedded in the base of the glacier.
Corries, also known as cwms or cirques, are often the starting point of a glacier. The diagram below shows the formation of a corrie, cwm or cirque.
Snowflakes collect in a hollow. As more snow falls, the snow is compressed and the air is squeezed out to become firn or neve. With the pressure of more layers of snow, the firn will, over thousands of years, become glacier ice. Erosion and weathering by abrasion, plucking and freeze-thaw action will gradually make the hollow bigger.
Even though the ice is trapped in a hollow and unable to move down hill, gravity will still encourage it to move. This circular motion is known as rotational slipand can cause the ice to pull away from the back wall creating a crevasse or bergschrund. Plucked debris from the back wall causes further erosion through abrasion which deepens the corrie. Some of this debris is deposited at the edge of the corrie, building up the lip.
These processes create a characteristic rounded, armchair shaped hollow with a steep back wall.
When ice in a corrie melts, a circular lake is often formed at the bottom of the hollow. This is known as a tarn, eg Red Tarn on the eastern flank of Helvellyn.
Glaciers cut distinctive U-shaped valleys with a flat floor and steep sides. The glacier widens, steepens, deepens and smoothes V-shaped river valleys, eg Great Langdale Valley in the Lake District. The images below show the difference between a U-shaped valley and a V-shaped valley. Just like rivers, glaciers have tributaries. As the main glacier erodes deeper into the valley, the tributary is left higher up the steep sides of the glacier. U-shaped valleys ending with a waterfall at the cliff-face are called hanging valleys.
When a river erodes the landscape, ridges of land form in its upper course which jut into the river. These are called interlocking spurs. A glacier cuts through these ridges leaving behind truncated spurs.
Arêtes and pyramidal peaks
An arête is a knife-edge ridge. It is formed when two neighbouring corries run back to back. As each glacier erodes either side of the ridge, the edge becomes steeper and the ridge becomes narrower, eg Striding Edge found on Helvellyn in the Lake District.
A pyramidal peak is formed where three or more corries and arêtes meet. The glaciers have carved away at the top of a mountain, creating a sharply pointed summit, eg Mont Blanc, The Matterhorn and Mount Everest.
Valley floor landforms Erosional
As a glacier flows over the land, it flows over hard rock and softer rock. Softer rock is less resistant, so a glacier will carve a deeper trough. When the glacier has retreated, (melted) water will collect in the deeper area and create a long, thin lake called a ribbon lake. Many of the lakes in the English Lake District are ribbon lakes, eg Windermere. The areas of harder rock left behind are called rock steps.
Valley Floor Landforms 2 Erosional
If the glacier hits a particularly resistant outcrop of rock it will flow over and around it. This leaves a rock mount smoothed by abrasion from the glacier. These come in two types:
Roches moutonnée often have steep, jagged faces created by plucking on the lee (far) side and a gradual incline which is smoothed and polished by abrasion on the other (stoss) end. It may have striations on it indicating the direction of glacier movement.
Valley Floor Landforms 3 Erosional
Crag and tail tends to be larger than a roche moutonnée. Crag and tail is the opposite of the roche moutonnée as the ice hits the steep resistant rock outcrop first. This protects the lee (far) side of the obstacle from erosion. Edinburgh castle is built on crag and tail.
Deposits laid down by ice are called till or boulder clay which is an assortment of material of various shapes and sizes. Boulder clay can be found in areas of the UK such as Lincolnshire and East Anglia. It is easily eroded, and is one reason why the Holderness coast is so vulnerable to wave attack.
Drumlins and erratics
Unlike river deposits that are often sorted into different sizes, all glacial deposits are angular and mixed up (unsorted). The extreme of this can be seen in erratics. These are large rocks or boulders that are often found on their own, rather than in piles. They are unusual shapes, unusually large and of a rock type uncommon to the area they have been dumped, eg the Norber erratic.
Deposited landforms 2
Drumlins are elongated hills of glacial deposits. They can be 1 km long and 500 metres wide, often occurring in groups. A group of drumlins is called a drumlin swarm or a basket of eggs, eg Vale of Eden. These would have been part of the debris that was carried along and then accumulated under the ancient glacier. The long axis of the drumlin indicates the direction in which the glacier was moving. The drumlin would have been deposited when the glacier became overloaded with sediment. However glaciologists still disagree as to exactly how they were formed.
When glacial ice melts, different types of rock are laid down that have been carried along by the glacier. Piles of these deposits are called moraines.
Different types of moraine
- Terminal moraines are found at the terminus or the furthest (end) point reached by a glacier.
- Lateral moraines are found deposited along the sides of the glacier.
- Medial moraines are found at the junction between two glaciers.
- Ground moraines are disorganised piles of rocks of various shapes, sizes and of differing rock types.
Upper glacial valleys: farming and forestry
Highland glacial valleys, provide opportunities for different activities. These activities all have advantages and disadvantages for communities and the environment.
Highland farming: Farming - especially sheep farming - has been a way of life in highland areas for centuries.
- It has shaped the landscape, helped to build local communities and is an important part of local economies.
- Farming may harm the environment.
- Trees and other vegetation need to be cleared to provide grazing. This vegetation is important for protecting the landscape from erosion and for providing habitats for wildlife.
- The introduction of too many animals leads to overgrazing, which causes soil erosion which may squeeze out wildlife and pollute water sources. For many farmers this way of life is becoming less economically viable.
Conifer forests have been planted in many highland areas.
- Forestry creates local jobs and provides timber.
- New conifer forests can help prevent soil erosion and can help counter the impact of deforestation elsewhere.
- Forestry may push out other vegetation and force other activities - such as farming - into decline.
- For some wildlife, the forest will provide a habitat, but other animals will lose their habitat.
- Some people argue that conifer forests debase the landscape - though others think they enhance it.
Tourism in the highlands
- The lakes and mountains of glacial highland areas attract British and international tourists.
- They visit the highlands for activities such as skiing, climbing, mountain biking, hiking and hang gliding.
- This gives people who live in towns an opportunity to enjoy the countryside, and brings wealth to the local people who provide them with accommodation and other services.
- Not everyone in local communities welcomes tourists. Some fear interference with their livelihoods (eg farmers), or congestion and pollution from cars and litter.
- Tourist developments like building ski lifts can spoil the landscape.
- Too much recreational activity may damage fragile environments (eg soil erosion can interfere with flora andfauna).
Highland water works
Glacial valleys, with their steep sides and high rainfall, are ideal for damming to create reservoirs for drinking water and hydroelectric power.
This creates local jobs and new opportunities for sports and businesses, such as fish farming.
- It also provides people in other parts of the country with water and renewable electricity.
- Damming has a major impact on local environments.
- Flooding valleys and altering the course of rivers prevents the landscape being used by farmers, tourists and wildlife and affects the water cycle.
- Dams and electricity pylons are considered by some to be blots on the landscape.
Lower glacial landscapes
The lowland areas across Britain, where ancient glaciers have created plains and rolling landscapes and deposited fertile material transported from highland areas, provide an opportunity for different activities.
Settlement in the lowlands
- Lowland areas provide much more suitable locations than harsh highland areas for building villages, towns and cities. Edinburgh, for example, is situated on the site of an ancient glacier.
- Towns displace vegetation and wildlife and transform landscapes. They also create problems such as pollution and water and power consumption.
Farming in the lowlands
- The more fertile lowlands offer much better opportunities for arable and pastoral farming than the highlands.
- This creates jobs and helps boost the local economy.
- Intensive farming displaces vegetation and wildlife. It also damages the environment through the use of fertilisers, pesticides and weed killers, and may spread disease to animals and humans.
Tourism in the lowlands
- Lowland areas attract tourists who do recreational activities such as horse riding, walking, fishing and boating.
- Fertile areas are also good for creating golf courses.
- Badly-managed tourism can cause damage to the environment through soil erosion, pollution and over-fishing.
- It can also interfere with local livelihoods (eg those of farmers).
- Golf courses take land out of agricultural use and remove variety from the landscape.
Causes of avalanches
An avalanche is a sudden downhill movement of snow. It is a significant hazard to people living in, or visiting, glacial areas. A slab avalanche is the most dangerous form of movement. It can be caused by:
- heavy snowfall
- deforestation (for example because of new ski runs) making the slope less stable
- steep slopes, as this helps to increase the speed of movement
- vibrations (for example from an earthquake, noise or off-piste skiers)
- layering of snow - for instance where snow is already on the mountain and has turned into ice, and then fresh snow falls on top which can easily slide down
- the wind direction piling snow which can overhang a mountain
Effects of avalanches
An avalanche is able to obstruct anything in its path. Roads and railways can be blocked. Power supplies can be cut off.
A powerful avalanche can even destroy buildings.
People can also be killed.
90 per cent of people who die in avalanches trigger them themselves.
People usually die from a lack of oxygen when buried in snow, rather than from getting too cold.
Management of avalanches
There are several ways in which avalanches can be managed:The areas in which avalanches occur may also be used for human activities, such as skiing. Villages and towns are also often located in the valleys. It is important for the people, economy, and the environment that avalanches are managed.
Prediction- People try to predict when avalanches are going to occur. The Alps has an 'avalanche season' between January and March when most avalanches happen. Where avalanches are going to occur is hard to predict. Historical data, weather information and information about the actual snow on the mountainside is collected together to try and forecast the likelihood of an avalanche.
Explosions- Avalanches can be started deliberately in order to prevent the snow building up. This is one of the most important ways of preventing avalanches.
Communication- Signs of the risk of avalanches can be displayed in villages and also by the ski lifts. In the Alps the risk is assessed on a five-point scale. Areas can be sealed off which are considered too dangerous to ski on. Early warning systems are also used.
Land can be grouped into red, yellow and green areas. The red areas are considered too dangerous to be built on. The orange areas can be built on with restrictions, such as reinforcing buildings. Roads and railways can be protected by tunnels over them in the areas where an avalanche path is likely to travel.
Snow fences and barriers
These can be used to divert and break up the path of the avalanche.
Trees can be planted, increasing stability of the slope and helping to reduce the damage further down the valley.