Glaciers carry their devastating forces down mountain valleys, eroding rocks and soil as they go. This erosion creates landforms such as striations lines or moraine deposits – creating unique forms in the landscape that come alive through glacial action.
On a larger scale, glacial erosion sculpts cirques and rock basins into troughs and fjords through glacial movement. These landforms become sources of slope instability that must be managed for geohazard management purposes.
Landforms
Glaciers’ immense volumes and abrasive surfaces come into direct contact with land, eroding both rocks and soil in their path and leaving behind dramatic landforms such as eskers (winding ridges of sand and gravel) or drumlins (asymmetrical hills made up of sediments that point in the direction of movement of the glacier).
Glaciers can significantly alter existing landscapes by altering the shapes of existing features. For instance, glacial erosion can convert V-shaped river valleys to U-shaped glacial valleys by widening both sides and deepening both bottom of each valley.
The type and size of rock that lies under a glacier both influence how much erosion takes place; warm-based glaciers tend to move over bedrock faster than cold-based ones, for instance. Rain or snowfall on glaciers also has an effect, as its waters seep into its base, providing it with extra lubrication so it slides faster over bedrock – potentially increasing erosion rates significantly.
Abrasion
As glaciers move over rocks, they erode them through scraping and wearing. Their movement leaves behind long scratch marks called glacial striations which indicate where their path lies. Glacial erosion shapes landforms such as cirques, horns and moraine.
Abrasion levels depend on several variables. First and foremost is that rocks being crushed or scraped must be large enough to have an appreciable impact.
One factor is the composition of the rock being eroded; typically, ice tends to erode harder rocks more rapidly than soft and fine-grained materials.
Finally, weather also plays a part in the amount of abrasion. Rainwater that percolates through glaciers can lubricate their interface between rock and ice and speed up sliding and erosion; snowfall has similar effects, though its cause might be harder to determine.
Plucking
Glycolysis – the crushing and rubbing together of rock particles by glacial movement – results in distinctive landforms such as scree slopes and blockfields, known as freeze-thaw weathering.
This process works best on rocks with multiple cracks and joints (lithologies), rather than smoother rocks with fewer fractures. Fragments play an important role here; larger fragments can be ploughed along existing fractures before being carried off by glaciers.
Plucking is an important form of erosion in glaciated areas and contributes to the creation of features such as drumlins, flutes and ribbed moraine. Plucking may also play a significant role in cavity development beneath glaciers. Such cavities tend to appear where there is an imbalance between pressure differences on either side of a glacier (known as “pressure differentials”) as well as fluctuations in basal water pressure – usually found on warm-based glaciers.
Melt Water
As glacial erosion progresses, melt water brings large quantities of fine-grained rock particles to the surface and down into stream channels, producing streams with high suspended sediment concentrations that give them their characteristic dilute-milk whitish hue. Furthermore, their powerful impact forces allow them to remove joint-bound fragments from hard bedrock with ease.
Mechanical glacifluvial erosion produces an array of landforms, from trench-like spillway systems and trenches, streamlined residual hills, longitudinal grooves and subglacial channels ranging in size from tiny rills to channels several meters wide and deep.
Subglacial erosion rates are generally thought of as being very slow compared to fluvial erosion rates; however, several instances of retreating cold-based glaciers revealing unaltered bedrock surfaces suggests otherwise and suggests the slow erosion rates observed in glaciated basins may be related more closely to glacier characteristics than climate or landscape influences; similarly it implies subglacial processes fundamentally linked with basal sliding, mass balance regimes and thermal regimes1.