Glaciers move down mountain valleys, eroding rocks and earth as they go. This process of glacial erosion is known.
Glacial erosion creates distinct landscapes. These include cirques, rock basins and fjords – some are kilometric in scale whereas smaller landforms like moraines or rock drumlins exist as well.
Abrasion
One of the primary factors affecting glacial erosion is abrasion. This process occurs when two surfaces come into contact and rub together, eventually wearing away one surface in turn.
Glaciers constantly rub against the bedrock beneath them, which wears away and smoothens it over time, leading to its distinctive, polished surface in many glaciated landscapes.
However, for abrasion to occur successfully there must be certain conditions present. First and foremost is moving ice at the base of a glacier; therefore abrasion tends to occur more in warm-based glaciers with basal slip than cold-based glaciers.
Abrasion depends on the type of rock being rubbed against. Different rocks have differing hardnesses; therefore, harder rocks tend to abrade bedrock more effectively, creating fine-grained mineral fragments known as rock flour that are carried away by meltwater streams and give their characteristic dilute-milk whitish color; additionally it is responsible for long, straight lines etched by glaciers known as striations lines.
Quarrying
Many glaciers traverse bedrock ranging from hard crystalline rocks to soft sedimentary ones, and erosion occurs primarily via quarrying rather than abrasion (refer to Siman-Tov, Stock, Brodsky and White 2017 for more info). (Please see Figure below from recent study for evidence of this).
Subglacial till acts as a lubricating ‘fault gouge’ and fluctuating basal water pressure can intensify quarrying operations, with rates varying across orders of magnitude across bedrock steps and differential stress peaks at drowned step boundaries amplifying rock fracture rates five times greater than their respective ice overburden pressures. This results in faster quarrying rates due to the addition of subglacial till lubricating “fault gouge”.
Fluctuations in effective pressure and sliding velocity also enhance quarrying by decreasing frictional forces exerted by ice on bedrock, thus making quarrying more sensitive to changes in these parameters than abrasion alone. Furthermore, these factors help explain why glacial erosion varies significantly.
Melt Water
Glacial erosion is hastened by melt water generated from mechanical weathering of glacier walls and melting ice pressure, and collected as subglacial streams that scour across decayed ice surfaces, often at rates exceeding 3 meters per year (Reference Nolan).
Rock debris from quarrying can also be transported downriver by glaciers and carried away, creating eskers on valley sides and drumlins in their center, along with wide expanses of stratified drift that form broad areas to support future fjords or deep valleys extending out from their source.
Rapid erosion requires that sufficient till be cleared away. This is usually accomplished through subglacial streams fed by surface meltwater seeping down through moulins into the bed through moulins; however, their flow can sometimes be limited by adverse bed slope and backpressure from terminal water bodies (Reference Alley), restricting their path towards ablation zones that reach through crevasses into the bed surface.
Subglacial Erosion
The type of bed rock underneath determines the rate of erosion, with more solid beds supporting more intense erosion than soft beds. Preexisting weaknesses also impacting erosion potential; an example would be when glaciers move quickly enough through bedrock with numerous locations suitable for cavities to form; then this phenomenon leads to stoss-and-lee topography being formed in their beds as a result.
These factors influence both small cirque glaciers and large continental ice sheets; erosion is an integral component of glacial systems and interacts strongly with both mass balance regime and glacier thermal regime. Erosion plays an especially powerful role in shaping landscapes associated with glacial cirques, troughs, rock basins and fjords at kilometric scales; it also plays a key role in medium-sized landforms such as roches moutonnees and whalebacks – and subglacial erosion is responsible for creating tunnel valleys containing sediment (e.g. Nye channels).