Erosion rates of warm-based glaciers depend on basal sliding and produce photogenic rock features like faceted clasts, striations lines and grooves in bedrock and glacial pavements.
Erosion is responsible for producing many glacial landforms such as ribbon lakes, cirques and pyramidal peaks – so what term best describes the rate at which glacial erosion takes place?
Abrasion
Abrasion occurs when rocks being pulled along by glaciers rub against other rocks, creating unique glacial features like facets, striae and glacial polish. Abrasion also results in U-shaped valleys, horns and moraines being produced from this process.
Glacial abrasion depends on a number of variables, including its geometry and material properties (including debris accumulation and surface smoothness) as well as sliding velocity (Hallet, Hallet1979).
Assumptions about erosion often make assumptions that quarrying accounts for most erosion; however, in deglaciated bedrock and sediment environments where subglacial fluvial erosion produces features like Nye channels incised into bedrock as well as silt. Yet available data suggest that most erosion in these environments comes from quarrying of cobble-sized mode rather than from abrasion1. Furthermore, glacial abrasion appears less efficient when its basal debris load increases friction, slowing its sliding.
Quarrying
Glaciers use their rubbing action to transport rocks and sediments over bedrock, leaving striations marks that indicate where glacial erosion occurred. Debris created from glacier erosion is then transported along its course by glaciers to form distinct landforms and landscapes such as U-shaped valleys, horns, moraine or bowl-shaped depressions known as cirques.
Subglacial erosion depends on the availability of till that can lubricate ice-bed contact, while an adequate supply of concentrated water restricts erosion rates without outburst floods.
Although significant progress has been made in our understanding of quarrying, much remains to be done. Quarrying is tightly linked with abrasion and may be accelerated by basal friction from abrading clasts; thus it likely follows a velocity-linked law of erosion (Cohen and others, Reference Cohen2005). Fluctuations in water pressure play an essential role in driving quarrying as they also affect basal flow laws and basal earthquake generation – progress in understanding quarrying is therefore key for understanding glacial erosion as a whole.
Plucking
As glaciers advance, cooling ice scoured over proglacial sediments remobilizes them rapidly (as fast as 3 meters per year under Taku Glacier; Nolan, Motyka, Echelmeyer and Trabant1995); this process is in balance with the advancing ice rather than driving bedrock erosion. Erosion by melting ice provides sediment flux to downstream regions as well as shaping glacial valley geometry but it should not be considered the driving force in shaping overall landscape features.
Subglacial erosion rates depend heavily on water availability for transporting sediment, with effective mechanisms including entrainment during regelation across or into the bed, frost heave, meltwater flow from bergschrunds or filling of basal crevasses being among them. Furthermore, cataclysmic subglacial outburst flooding (such as 1996’s Jokulhlaup triggered by Grimsvotn’s eruption) can dramatically increase sediment transport rates causing tunnel-channel erosion over a wider area than ever imagined before.
Freeze-thaw
As the ice grinds over surface rocks, it forms frozen masses around any loose or weak sections (known as plucking), pulling these away along with it as it advances. This process is faster than abrasion and produces finer-grained sediments.
Erosion within a glacier typically takes place both supraglacially, where erosion takes place on its sides or within its main body, and subglacially at its bed. Supraglacial erosion typically produces coarser-grained sediment than abrasion, often yielding cobble-sized pieces; subglacial erosion typically generates finer-grained forms (with silt particles being the norm).
At a continental ice sheet scale, erosion rates are determined by both environmental and internal influences such as lithology, topographic relief and dynamically shifting thermomechanical ice-sheet configuration. Reducionist analyses of glacial erosion patterns often ascribe them to latitudinal or precipitation-related proxies; however, comparative analyses between glacier types reveal no statistically robust link between glacial erosion rates and either climate metric; however at least some patterns do seem related to timing of deglaciation events.