As I know little on glaciation and the processes involved, it has come to my attention that there are several areas of glaciation, the processes involved and the types of movement that I have failed to make reference too in my previous posts.
First of all I realised there are two types of glacier: temperate glaciers and polar glaciers.
Temperate (alpine) glaciers melt in the summer, releasing huge amounts of melt water. This acts as a lubricant, reducing friction. Temperate glaciers move by basal flow, extending/compressing flow, creep and surges. This type of glacier is more likely to erode, transport and deposit material.
Polar (continental) glaciers occur in areas where the temperature is permanently below 0°C (the Arctic and Antarctic regions), and therefore no melting occurs. Movement is slower than temperate glaciers as they are frozen to their beds and thus move mainly by internal flow. Much less erosion, transportation and deposition occurs.
Before now, I hadn't realised how complex ice movement can be and how it moves in different ways dependent whether it is on higher or lower ground. Ice has great ‘rigidity and strength’, but under steady pressure it behaves as a ‘plastic (mouldable) body’. However, when put under sudden compression or tension, it will break or shear apart. Thus there are two zones within the glacier:
The upper zone – where ice is brittle, breaking apart to form crevasses
The lower zone – which has steady pressure. It is here where meltwater (resulting from pressure and bedrock friction) allows a more rapid, plastic flow. At depth in the glacier the melting point of the ice is raised slightly by the increased pressure. Basal ice is therefore more likely to melt at temperatures close to 0°C, (the ‘pressure melting point’).
There are several types of ice movement, I believe the main types are:
Compressing flow – occurs when there is a reduction in the gradient of the valley floor leading to ice movement decreasing resulting in a thickening of the ice mass. Movement occurs by ice above pushing (compressing) the ice further down the valley. It is here that ice erosion is at its maximum.
Extending flow – occurs when the valley gradient becomes steeper. The ice accelerates and becomes thinner, leading to reduced erosion.
Basal flow (sliding/slippage) – as the glaicer moves over the bedrock, there is friction. The lower ice is also under a great deal of pressure and combined with the friction results in some melting. The meltwater assists the ice in moving downslope.
Surges – occur when an excessive build up of meltwater under the glacier leads to ice moving rapidly forward, sometimes as much as 250-300m in a day. These surges can be hazardous to the people living in the glacial valley below.
Creep – occurs when stress builds up within a glacier, allowing the ice to behave like ‘plastic’ and flow. This usually occurs when the ice meets an obstacle.
Rotational flow – occurs within the corrie (cirque). It is here that the ice moving downhill can ‘pivot about a point’, thus producing a rotational movement. This, combined with increased pressure within the rock hollow, leads to greater erosion and an over-deepening of the corrie floor.
I have found the websites listed below very useful when explaining what these types of movement are and how they move downhill. I have also found that within a glacier there are different rates of movement. The sides and base of the glaicer move at a slower rate than the centre surface ice. As a result, the ice cracks producing crevasses on the surface. These also occur where extending flow speeds up the flow of ice and where the valley widens or the glacier flows from a valley on to a plain.
I would teach this by the use of a powerpoint presentation, with a range of diagrams indicating the different types of movement. As a visual learner I am always keen to use pictures in my explanation so students can visualise what is being taught. A good video clip is: http://www.youtube.com/watch?v=-cBcMfaSxpk that covers everything that has been discussed in the previous posts to date (I would possibly suggest putting the music on mute as the repetitive beats become some what annoying). I would also suggest making notes whilst watching the video.
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