Glacial erosion creates classic landforms such as striations and grooves in bedrock. But which term best captures its rate of operation?
Abrasion refers to the process by which glaciers move across bedrock surfaces by dragging their surface across it, particularly warm-based glaciers with basal sliding or “glued” glaciers1. Abrasion can be especially destructive for warm-based glaciers with basal sliding, while it remains minimal in cold-based glaciers “glued” to their beds1.
1. Abrasion
As glaciers move over rocks, their movements cause abrasion – a type of erosion known as scratch marks known as striations which is unique to landscapes where glaciers once existed. This phenomenon creates scratch marks on rock surfaces which mark specific landscapes where glaciers once existed.
Glacial erosion has many ways of altering a landscape’s form and formation. One is its influence in shaping landforms – which include lakes, peaks and aretes – into distinctive features of glaciated terrain.
Glacial erosion sculpts cirques – bowl-shaped depressions created by glaciers – and U-shaped valleys into mountain sides by glaciers flowing over them, while deepening V-shaped valleys carved by rivers by eroding their sides and bottom. Additionally, glacial erosion may erode mountaintops to form pointed mountain peaks known as horns, as well as deposit moraine or debris far away from its source in an outwash plain.
2. Quarrying
Erosion can produce some astonishing landforms, including corries, aretes, U-shaped valleys, hanging valleys, truncated spurs, ribbon lakes and even mountainous regions like roche moutonnee. A key form of erosion involves ice scraping along the surface and carrying bits of rock away with it; this practice is known as quarrying.
Quarrying can produce characteristic glacial features, including faceted clasts, striae, grooves and glacial pavements (see image below). Furthermore, it produces a range of grain sizes with silt being the dominant constituent (Gurnell and Clark 19887).
At the same time, quarrying can also serve as a rate-limiting mechanism. When ice encounters an extremely rough or damaged surface, it freezes around loose sections before pulling them loose from their beds (see animation below). Therefore, quarrying remains important in most situations but unlikely to dominate subglacial erosion rates (e.g. Zoet and others’ Reference of Zoet Alley Anandakrishnan and Christianson2013a).
3. Freeze-thaw
As glaciers move downhill, they erode rocks using two primary processes: abrasion and plucking. Abrasion involves scraping bedrock with ice to produce distinctive marks called striations marks; plucking involves temporarily melting around larger rocks before pulling them out as the glacier advances, producing sediment with cobble-sized particles as it goes.
Freeze-thaw weathering also contributes to erosion by weakening rocks. This process occurs when water seeps into cracks and crevices of rocks, freezes, expands, causing the rock to break apart; further acceleration may come from plants or animals burrowing under it and breaking it apart, thus furthering this form of weathering.
Glaciers also erode by transporting and depositing sediments, an activity known as sediment transport or deposition. Sediments may be transported downstream via glacial meltwater in streams known as “outwash plains”, or deposited as ridges of sand and gravel in eskers or drumlin fields. The rate at which glacier erosion processes occur depends on numerous factors, including climate conditions and amount of debris embedded into basal ice; for instance, lower erosion rates are expected for glaciers with greater debris embedded into basal ice as this increases friction between them both and bed which reduces basal sliding rates1.
4. Basal slip
Glaciers move slowly across bedrock, slowly sliding their ice across it to form Nye channels incised into rock and sediment (e.g. Gilbert1906b). While streams also erode landforms, their basin-averaged erosion rates tend to be much less rapid than glaciers.
Glaciers that reach pressure melting point across their beds experience rapid erosion due to basal slip. On the other hand, valley glaciers often exhibit variable rates of erosion over time.
This can be explained by differences in water flow through the glacier. Seasonal surface melt increases subglacial drainage system water pumps, increasing ice flow downslope1. This results in greater erosion along the top center, where there is less frictional resistance to flow compared to bottom center where flow speeds are fastest, leading to crevasses being formed here.