Erosion in Glacial Landscapes

• Glacial erosion occurs as glaciers advance and this mainly occurs in upland areas. The two main types if erosion by glaciers are plucking and abrasion.

Plucking

Plucking mainly happens when meltwater seeps into joints in the rocks of the valley floor/sides. This then freezes and becomes attached to the glacier. As the glacier advances it pulls pieces of rock away. A similar mechanism takes place when ice refreezes on the down-valley side of rock obstructions. Plucking is particularly effective at the base of the glacuer as the weight of the ice mass above may produce meltwater due to pressure melting. It will also be significant when the bedrock is highly jointed which allows meltwater to penetrate. Plucking is also known as quarrying

Abrasion

As a glacier moves across a surface, the debris embedded in its base/sides scours surface rocks, wearing them away. The process is often likened to the action of sandpapering. The coarse material will scrape, scratch and groove the rock.  The finer material will tend to smooth and polish the rock. The glacial debris itself is also worn down by this process, forming a fine rock flour that is responsible for the milky white appearance of glacial meltwater streams and rivers.

• Presence of basal debris- pure ice is unable to carry out abrasion of solid rock and so basal debris is an essential requirement. The rate of abrasion increases with the amount of basal debris up to a point where it produces great friction, which slows down rates of movement.
• Debris size and shape- particles embedded in ice exert a downward pressure proportional to their weight, and so larger debris is more effective in abrasion than fine material. Angular debris is also more effective as the pressure is concentrated onto a smaller area of debris-bedrock interface
• Relative hardness of particles and bedrock- abrasion is most effective when hard, resistant rock debris at the glacier base is moved across a weak, soft bedrock. If the bedrock is more resistant than the debris, then little abrasion will be accomplished.
• Ice thickness- the greater the thickness of overlaying ice, the greater the pressure exerted on the basal debrisand the greater the rate of abrasion. This is, however, only true up to a point. Beyond a certain thickness the pressure becomes too great and there is too much friction between the debris and the bedrock for much movement to occur. This is not a fixed thickness, as it depends upon ice density and the nature of the debris, but it is typically 100-200m

• Basal water pressure- the presence of a layer of meltwater at the base of a glacier is vital if sliding and therefore abrasion is to take place. However, if the water is under pressure, perhaps because it is confined, the glacier can be buoyed up, reducing pressure and erosion
• Sliding of basal ice- this is important as it determines whether abrasion can take place. Abrasion requires basal sliding to move the embedded debris across the rock surfaces. The greater the rate of sliding, the more potential there is to erode as more debris is passing acros the rock per unt of time
• Movement of debris to the base- abrasion does not only wear away the bedrock, it also wears away the basal debris. Debris needs to be replenished (by glacial erosion and weathering processes) if abrasion is to remain effective.
• Removal of fine debris- to maintain high rates of abrasion, rock flour (fine debris) needs to be removed so that the larger particles can abrade the bedrock. This is mainly done by meltwater

• Nivation- a process that is not easily classified as erosion or weathering. This complex process is thought to include a combination of freeze thaw action, solifluction, transport by running water and, possibly, chemical weathering. Nivation is thought to be responsible for the initial enlargement of hillside hollows and the incipient development of corries

• Snow falls and is protected in small depressions/hollows
• For example, snow can stay on the ground as it is protected fr4om the wind and if in the Northern Hemisphere snow in north facing hollows is protected from the sun so it is less likely to melt
• Snow slowly compacted by further snowfalls and melting into neve (granular snow that had been melted and refrozen) and in longer term into fern (snow compacted from previous years but not quite as dense as glacial ice).
• Weathering such as freeze thaw can weaken the soil and rocks underneath the snow patch and create slope failure at the back of the snow patch.
• Erosion from meltwater can wash sediment out of the base of the snow patch.
• The weather particles are moved downslope by soil creep, solifluction and rill wash (river erosion in tiny streams)

These processes combined are known as nivation and overtime this leads to the formation of nivation hollows which can develop into ciruq or corrie glaciers.