Studies of glacier erosion rates using different measurement techniques have revealed that they vary depending on climate conditions.
Glacial quarrying (also referred to as plucking) is the process by which large, fracture-bounded blocks are consumed by moving ice. It is one of the fastest rates of bedrock erosion seen in geomorphic environments.
Freeze-thaw weathering
Physical weathering occurs when rocks are repeatedly exposed to changing temperatures, with water seeping into cracks in rock over time and eventually breaking it apart – an essential process in creating soil formation, shaping landscapes and ecosystem dynamics.
Freeze-thaw weathering is most effective in environments that experience dramatic temperature swings, such as mountainous or polar regions. When water freezes it expands 9%, creating high pressures against surrounding rocks that exert enough force to fracture and fracture them into small fragments.
Moisture availability is central to this process of glacial erosion; when water can seep into cracks and freeze again, causing rocks to fragment, leading to soil formation. As such, glacial erosion rates differ with season. For example, erosion beneath Ovre Beiarbre tends to peak some years and decline during others.
Plucking
Glaciers act like giant slow bulldozers; over centuries they obliterate everything they come across, including vegetation, topsoil, rocks and gravel. Although some sediment is buried below the ice sheet itself, most is left behind as layers of silt and cobble-sized debris that remain scattered along their paths.
Abrasion and plucking transform landscapes by pulling bits of rock up from the ground, smoothing surfaces, and leaving behind distinctive patterns of striae (scraping grooves). Abrasion also produces unique landforms like horn-shaped mountain peaks, U-shaped valleys, hanging valleys and fjords that stand out against other environments.
At a smaller scale, glacial erosion also produces narrow canyons incised into bedrock (‘inner gorges’) in certain deglaciated areas (e.g. Durst-Stucki and others; Reference Durst-Stucki, Schlunegger, Christener & Otto2012). However, this process primarily relies on quarrying of blocks for quarrying purposes as well as subglacial fluvial action which may limit rate-limiting in rugged mountains.
Abrasion
Rapid erosion of abrasive ice and sediment is one of the most easily identifiable glacial geomorphic features. Abrasion plays an integral part in creating photogenic, striated surfaces often seen across glacial landscapes.
The rate at which glaciers erode depends on a combination of factors. While velocity-linked laws usually hold true, abrasion also plays an integral role. At Engabreen in Norway for instance, frictional forces between debris-bearing ice and subglacial bedrock were observed that suggested an inverse relationship between velocity and erosion rate; faster sliding speeds leading to less erosion.
However, this does not hold true for all glaciers; large-scale erosion in cirques, rock basins and fjords often occurs much faster than anticipated, due to factors such as entrainment creating a lubricating till layer and backpressure from terminal water bodies. As a result, quarrying becomes an important glacial erosion process here.
Basal slip
Under glaciers and ice sheets are numerous sediment transport processes at work. Basal sliding is common in glaciated environments; combined with quarrying and abrasion processes it can produce high rates of bedrock erosion (Referring back to Koppes and Montgomery 2009).
Basal slip depends on numerous factors, including the thickness of deforming till, which acts as a lubricating sawdust or fault gouge (Alley et al. 2019). Furthermore, glacier velocity changes depending upon valley gradient and steering effects from subglacial topography – these variables influence basal slip significantly.
Basal slip is generally faster than abrasion and quarrying, yet slower than subglacial fluvial action. Its rate of occurrence depends on factors that trigger it; such as steepening of lower ablation zones or migration of moulins up-glacier that allow them to access previously inaccessible regions of bed previously limited by supercooling threshold. We measured basal-sliding motion by monitoring displacement of metal stakes inserted in boreholes on Ice Stream B in West Antarctica using our “tethered stake” apparatus (Fig 1).