Glaciers moving down mountain sides erode rock through physical processes such as abrasion, plucking and basal slip, creating distinct features like striations lines and chatter marks, as well as creating features like cirques, hanging valleys, horns and moraine.
Why is glacial erosion so difficult to observe in action?
Freeze-thaw weathering
Frozen-thaw weathering, which also plays a key role in erosion, also sculpts landscapes by mechanically breaking rocks apart and producing mineral-rich soils. As part of its role as an erosional process, freeze-thaw weathering helps form valleys, canyons, gorges, and other distinct landforms.
Cirque environments experience glacier frontal retreat, creating steeper surface slopes in the ablation zone (Reference Cuffey and Paterson2010). But rapid erosion is limited by factors that restrict subglacial erosion: adverse bed slopes, concentration of water required for driving erosion and backpressure from terminal water bodies.
Frost wedging (freeze-thaw weathering), commonly referred to as frost wedging, is an integral component of creating talus slopes and boulder fields. Its repeated cycles of expansion and contraction exert pressure on cracks, fissures, fractures and fissures which widen them while simultaneously weakening rock strength; making talus slopes so slippery and difficult to climb upon. Furthermore, this process contributes to ecosystem evolution by encouraging pioneer plants to colonize broken-down rock fragments allowing more complex environments to form.
Plucking
As glaciers move across the land, they gather up and transport large chunks of rock, known as plucking, which helps shape the landscape by creating unique features like U-shaped valleys and mountain ridges.
An essential factor in the effectiveness of plucking is the composition or lithology of bedrock. Blocks with numerous joints and cracks tend to respond better than ones that feature fewer or wider-spaced fractures for this technique.
Plucking is also affected by cavities or crevices in the bedrock, where meltwater seeps in through cracks in ice sheets and then freezes, expanding pressure against rock beneath. When this process repeats itself frequently enough, fluctuations in water pressure within these cavities alters how a glacier affects bedrock, leading to plucking. This phenomenon is enhanced when its movement speed allows frequent fluctuations in pressure which influence plucking. For this reason, plucking tends to occur more in regions with relatively low altitude and seasonal melting patterns.
Abrasion
Traditional understandings of glacier erosion involve quarrying and abrasion as the means by which landforms such as U-shaped valleys, truncated spurs, and ribbon lakes are formed.
These landforms are created through quarrying blocks (plucking) and abrasion of smaller fragments, which produces finer-grained sediments with silt-sized particles as a result of this abrasion process. Abrasion may also create scoring of bedrock surfaces called striations.
Modern glaciers exhibit signs of abrasion as well, but its causes remain elusive. Ice may move obliquely over rock surfaces or till may be removed beneath them (Cuffey and Alley 1996 for further study on this). Till not only suppresses abrasion but it can also reduce stress concentrations on surface slopes as well as dampen water pressure fluctuations (recall sawdust and swarf model). Friction between surface and bed might even play a part in driving abrasion.
Basal slip
Glaciers move through basal slip, in which a thin layer of meltwater reduces friction between their bed and themselves by means of basal slip. Pressure melting at bedrock obstacles causes pressure melting; as it moves downglacier from these obstacles it forms zones of low pressure where ice can easily slide over them, producing what is known as regelation.
As basal sliding is controlled by various factors dependent on its overall glaciological context, making accurate predictions regarding erosion rate is challenging (Hooke 1991). Recent analysis indicates that basal sliding rate depends heavily on surface slope as well as focused loading around cavities in the bed (Reference Herman Koppes Hallet).
Since till, or swarf, is deposited beneath many glaciers, it also means abrasion can take place. Abrasion rates tend to be very slow but could be enhanced in certain instances by rock fragments carried along by glaciers (Cuffey and Alley 2002). Furthermore, the tectonic context can influence net erosion rates by controlling availability of sediment transport pathways (Alley et al 2019) – particularly high rates in desert environments like Himalayan regions where erosion rates can reach as much as 100%!