6b) Assess the relative importance of the factors influencing rates of glacial erosion.
There are several factors affecting the rates of glacial erosion including: presence of basal debris, debris size and shape, relative hardness of particles and bedrock, ice thickness and the velocity of ice.
Though a very important factor in erosion not all glaciers have a lot of basal debris. A temperate glacier, like the glacier d’Argentière, will have lots of basal debris from processes such as frost shattering and plucking high up in the profile of the glacier where as the ice caps in Greenland will not have as much debris. This means there is larger erosion from basal debris in the temperate glaciers than in the polar glaciers. But the amount of debris also varies along the profile of the glacier. As at the top of the glacier in the zone of accumulation there will be alot of debris because of the temperature fluctuations at altitude, though not all of the debris is at the base of the glacier. Further down the the glacier the rocks will have had time to make their way to the base of the glacier by melting down to the base or falling down crevasses. Along the middle and lower part of the profile is where most of the basal erosion will occur.
The size of the debris is also a major factor though not as much as the amount because the amount depicts the surface area where as size may affect the surface area of exposed rock but also the volume. Lots of small rocks would mean there is a large surface area and hence a large amount of erosion, but a lot of large rocks means that little of the rock is grinding along the bed and more of the rock is held in the ice. Higher in the profile of the glacier is when there is likely to be large rocks and lower down the profile of the glacier are the small rocks. In a temperate glacier there is likely to be smaller particles because of frost shattering high up in the glacier but in polar glaciers there are larger particles because plucking is the major erosion process. Shape is another factor but not a major one. More angular rocks will scratch the bed rock and a smooth rock will polish the surface.
A third major factor in erosion is the relative hardness of the bed rock and particles. If the particles are harder than the bed rock then there will be more erosion than if the particles were made of softer rock.
The thickness of the ice is another major factor but only when it is coupled with how fast the ice is moving. The thicker the ice the greater the pressure and the more water melts because of the pressure melting point. This means the ice moves more and there is more abrasion and a better chance of plucking as well. This happens in the temperate climate, such as the glacier d’Argentiène which moves at 250m yr-1 and is 100m thick abraded a marble plate up to 36 mm yr-1. A polar glacier however, Breidamerkurjökull in Iceland, abraded a marble plate at only 3 mm yr-1 when it was moving at 9.6 m yr-1, however it is only 40m thick. If Breidamerkurjökull were to accelerate to 15.4 m yr-1 then the rate of erosion increased up to 3.75 mm yr-1. This means that velocity plays a bigger part than the thickness of the ice.
In conclusion the all aspects are important to the rates of glacial erosion, but the most important are the velocity of the ice and the hardness of the particles in relation to the bed rock. The amount of the debris is not as important and the size and shape are the least important factors in the rate of erosion.
