Glacial Erosion

AIM: to be able to understand the different glacial erosion processes

Weathering? Erosion? Huh?

Before we begin delving into the different processes causing glacial erosion, we must first take a moment to differentiate between the term weathering and the term erosion. 

Weathering – is the wearing away or breaking-apart of rocks in a particular place and occurs in situ (without movement). Weathering doesn’t just happen in glacial environments but it happens everywhere. Take a look at the roads and pavements out the window; see the cracks in the concrete? Those cracks and other associated damages are due to their exposure to the great British weather conditions as well as other types of damage. Can you think of any other stresses placed upon pavements and roads?

 Figure 1: Image showing the processes effects on roads.

The processes associated with weathering are:

 

• The freezing of rainwater in small cracks. Once frozen the water expands by 10% and gradually the ice widens the cracks.

• Seasonal temperature changes causes certain elements in the concrete and tarmac to expand and contract as it is heated and cooled, which can cause damage and bending of the structure.

• Some materials contain minerals which can be dissolved by mild acids within rainwater. Persistent rain over long periods of time can cause damage.

• The roots of trees can break apart tarmac when rising to the surface. This is usually known as biological weathering.

• Humans can also damage materials with their persistent trampling and overuse of pavements and the environment.

      Figure 2: Image showing the effects of roots (Source:Reference)

Erosion – this is a much wider term in regards to both the breaking down of rocks and their removal. Take a look at the photographs below of various glacial environments and see all the gaps and the valley shape – have a think how a glacier/water/ice could have created these shapes?

Figure 3: Image of  Snowdonia post glacial environment (Source: Reference)

 Figure 4: Image illustrating the effects of glacial erosion (Source: Reference)

 

Glacial erosion tends to occur in upland regions and is carried out by two main processes:

• Abrasion – this occurs where the material that the glacier is carrying (eg stones) collides and rubs away at the valley floor and sides. Imagine sandpaper or a giant nail file eroding away. The coarser material may leave scratches on the rock. This is known as striations; the finer the debris the smoother the rock surface becomes. The debris involved in this process is often worn down to a very fine material known as rock flour.

 

• Plucking- Imagine plucking your eyebrows. Well in glacier terms it means the action when the glacier creeps and freezes into and onto rock outcrops. As the ice advances it pulls away the rock with it and this normally happens at the base of the glacier. This is due to the pressure and friction here results in pressure melting. Plucking leaves jagged landscapes behind.

 Figure 5: A diagram indicating the process of plucking (Source: Reference)

The two other processes associated with glacial action involve weathering. These processes produce the debris that glaciers use in their erosive action.

• Freeze-thaw action – this occurs in areas where the temperature drops considerably over night and where rainwater freezes in small cracks. As ice occupies 10% more space than water the cracks widen with its expansion and rock breaks off once pressure is released. On steep slopes this material that has broken off will collect at the bottom of the cliff – this is named scree.

• Nivation – this is a series of processes that occur under a layer of snow which leads to the disintegration of the rock surface.

Rates of erosion are greatest where.....

In regards to Abrasion:

• Basal sliding can occur
• There is a lot of rock to act as cutting agents in the process
• The rock debris is more resistant than the bedrock
• The glacier is travelling at a faster rate than bedrock
• Thick ice which enables the embedded debris to create extra pressure on the bedrock

In regards to Plucking:

• The bedrock is well jointed
• Thick ice to create greater frictional drag
• Lots of meltwater to enable entrainment during refreezing

Glacial Erosion – Key issues

• Highly dependent on glacial movement
• Dependant on geology
• More potent than fluvial erosion
• Access to the glacial bed is difficult so understanding of the process has evolved slowly

IN PRACTICE: To get the students involved in a better understanding of the key processes a visual lesson will be provided with the use of play dough and sandpaper to help copy the processes on a glacier. Further investigation into the affects the processes have upon the earth will be undertaken for homework in preparation for the erosional landscape lesson to follow.