Ice is an effective agent of erosion, creating stunning landforms such as ribbon lakes, moutonnee rocks and pyramidal peaks.
Glacial action also leaves scratches called glacial striations on rocks, while it creates cirques, troughs and rock basins – sometimes even creating the spikey arete when two corries come together!
Freeze-thaw weathering
As with almost everything in nature, rocks can wear away over time to become sediment. This process is known as erosion. While water is the most commonly known eroding agent, other materials such as freeze-thaw weathering and abrasion may also erode rocks in similar fashion.
Freeze-thaw weathering occurs when water seeps into cracks in rock during the day, then freezes at night. Over many cycles, this ice slowly wears away at the rock – particularly effective if its surface has an open porous structure.
Glacial abrasion is one of the more visible forms of glacial erosion, producing distinct striations marks on bedrock surfaces. Unfortunately, measuring rates of glacial abrasion is challenging as it requires monitoring long-term observations closely spaced over an extended period. As this task can be complicated and requires extensive effort on behalf of researchers to account for effects like fluvial erosion – making gaining a comprehensive knowledge about glacial erosion difficult at best.
Abrasion
Ice moving along its surface scrapes the rocks, carrying away bits of sediment with it, to form landforms; some are obvious such as cirque basins while others more subtle are formed through erosion and abrasion processes, creating what are known as aretes.
Abrasion tends to work better when rocks are harder than the ice itself. Therefore, glaciers tend to erode softer rocks more rapidly than harder ones.
On a regional scale, erosion rates are determined by both geology and climate. Abrasion is limited in areas of continental glaciation by lack of rock strength variations and recent tectonic activity; alpine glaciation typically produces lower erosion rates due to more uniform rock strength distribution and features like cracks or ridges in bedrock that prevent erosion patterns from being sustained over time. Abrasion plucking may also produce distinctive landforms like striations or Roches moutonnees.
Plucking
One of the primary processes of glacial erosion is plucking (also referred to as quarrying). When moving over rock surfaces, glacier ice freezes onto any loose sections before pulling them away with it – leading to corries, aretes, pyramidal peaks and U-shaped valleys being formed as a result.
Ice has the capability of scratching rock surfaces, leaving long, parallel lines known as striations on them that can give geologists an idea of where and what was happening with the ice movement. These markings give geologists clues as to where ice was moving within a given rock body.
Glaciers produce large quantities of sediment that is carried downstream and dumped somewhere. This may help fill in cirque lakes or build mountains such as Matterhorn and Mont Blanc.
Glacier police
Glaciers tend to be large and deep because it takes so much snowfall to build them up than can melt away; over time, however, their surface snow compacts into ice (imagine trying to roll a ball of snow uphill!). This process can cause erosion to the rock layers beneath a glacier.
Erosion plays an integral part in creating many features found throughout northwest Montana and elsewhere, such as aretes: sharp ridges that form sides of U-shaped glacial valleys; cols: low points on aretes that serve as passes between adjacent valleys; horns: steep peaks eroded through both freeze-thaw weathering and glacier erosion; and cirques: bowl-shaped basins which form at the head of glacial valleys.
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