Landforms formed from glacial erosion are some of the most visible features on any landscape, including lakes, cirques, troughs, rock basins and fjords.
Glaciers produce distinct landforms like mountain ridges, hanging valleys and striations. Glaciers transport an assortment of rocks – some large, others very small – which differ significantly from those found nearby bedrock.
Freeze-thaw weathering is a physical form of erosion caused by water seeping into cracks, crevices, or porous material in rocks. Once inside these crevices, the frozen water freezes and expands by approximately 10% when frozen; creating pressure against surrounding rocks which then fracture into smaller fragments over time.
Weathering of rock depends upon its exposure and proximity to moisture sources. For instance, weathering rates on limestone bedrock tend to be one order of magnitude lower than sites featuring crystalline bedrock.
Rain and snowfall can enhance glacial erosion rates by lubricating the interface between the ice and rock, enabling it to slide faster downhill, which leads to an increase in erosion yield; however, rates can differ depending on glacier size, climate conditions (elevation sums, precipitation sums, aspect, slope) as well as individual glacier size and aspect/slope conditions.
Mechanical weathering (abrasion) wears away sharp rocks by rubbing against other rocks or sediments, gradually making jagged edges smooth and rounder over time. Abrasion also wears down and smoothes rock fragments carried along river currents; its friction eventually disintegrating rock into smaller pieces to form what is known as “grus”.
Mechanical weathering agents include moving water, wind, and glaciers. Earthworms and other burrowing animals such as earthworms may break apart existing rocks to expose fresh surfaces for weathering; more surface area exposed means faster wear-and-tear wear-off according to “surface area over volume”, as larger rocks have less exposed area than smaller ones.
Glacial erosion by abrasion results from basal ice clasts rubbing against bedrock, creating a surface marked by scores or grooves similar to how sandpaper scratches wood surfaces.
Erosion rates are determined by both basal sliding velocity and bedrock lithology, with variations across orders of magnitude often being observed despite being controlled for by either basal sliding velocity or bedrock lithology alone.
Till (swarf, sawdust and fault gouge) can provide an effective buffer between glacier movement and quarrying and erosion of bedrock beneath glaciers, but thin enough till can allow direct bedrock contact; rapid erosion often results when two valleys or glacial cirques have been separated by glacial movement forming an arete or roche moutonnee in between them.
Wind can also play an essential role in erosion. Its winds carried dust over vast distances, impacting millions of tons of soil away and taking livelihood away from farmers who relied on agriculture as their livelihood source. During the Dust Bowl era, this phenomenon contributed to millions of tons being washed away and devastated agriculture communities throughout the Midwest and Southwest United States.
Theories and direct observations indicate that glacier’s rate of till removal depends on several factors. Entrainment occurs rapidly near cavities at glacier bed where water pressure compensates for any overburden stress, providing faster till removal rates.
Physical erosion also creates unique landscapes. For instance, when glacial movements erode two valley headwalls to form a ridge in between them and subsequently create an arete– a pyramidal peak–it results in nunatak formation. A similar scenario happens when two glacial cirques erode adjacently and produce an arete or narrow ridge formation.
Moving over rock
Glaciers have long been responsible for altering Earth’s surface through erosion. Yet their power remains mysterious due to being hidden under layers of ice that cover them from view. Understanding glacial erosion requires delving deeper than ever, even with modern science’s advancements.
Glacial erosion rates can best be assessed through direct observation, which typically requires drilling or tunnelling directly under an active glacier. Such observations typically include regular measurements of microtopography of rock surfaces beneath an glacier as well as fixed reference plates under its base.
Observations at the Vorab glacier forefield reveal an intriguing spatial pattern in terms of erosion rates (Figure 11). Erosion is at its lowest close to present-day glacier front while highest at center. This could possibly be related to rock structure.