Many of the same processes responsible for erosion in rivers and streams can also be observed in glacial erosion; however, some differences become more noticeable over time.
Gular erosion includes plucking, abrasion and freeze-thaw weathering processes that produce many distinctive landforms such as cirques (bowl-shaped valleys), aretes (sharp peaks), glacial striations (long parallel scratches carved into bedrock), glacial striations and hanging valleys.
Abrasion
Glaciers are extremely powerful agents of erosion. They can alter a landscape in numerous ways, including abrasion and plucking. Abrasion occurs when ice and rock fragments carried by glaciers scrape against bedrock – much like using sandpaper on wood; only thicker with much more embedded grit, creating long scratches called “striations” along its surface.
Glaciers require a constant source of abrasive material from basal ice and debris being transported downward, for this process to continue effectively. If these factors don’t match, erosion rates could decrease; additionally, stick-slip motion of the ice may impact erosion rates as well as sediment production rates; climate warming is expected to alter erosion rates further and have profound ramifications on ecosystems and global biogeochemical cycles.
Plucking
Glaciers are powerful tools, capable of scraping away rocks, sediment and rockslide deposits to form vast ice sheets which blanket entire landscapes; alternatively they may flow downhill through mountains along existing valleys to deposit sediment and create different landforms. Continental glaciers form these massive ice sheets while Alpine or valley glaciers may follow existing valleys downhill through mountains forming valley glaciers – these powerful forces also deposit sediment, creating different kinds of landforms along their paths.
Glaciers move over easily eroded rocks, wearing away at them with erosional erosion. When moving over hard to erode rocks they pluck off chunks called clasts which cause long scratches on their surface known as glacial striations.
Striations on rocks may indicate where a glacier once traversed; their direction indicates its movement. Sometimes these striations form other landforms such as roches moutonnees (rock and mountain beds) or crag and tails. When several glaciers erode mountaintops this way, creating pointed mountain peaks known as “horns”.
Freeze-thaw weathering
Freeze-thaw weathering is a physical process in which rock breaks apart through repeated freezing and thawing cycles. Liquid seeps into cracks, fissures and pores in rocks during the daytime before freezing overnight, eventually expanding by approximately 9% overnight to exert pressure against it and eventually disintegrating it altogether.
This process leaves behind surface markings on bedrock called striations lines, abrasion scars or polished surfaces; in addition to creating classic glaciated landscape features such as eskers, moraines and drumlins.
As a result of freeze-thaw weathering erosion, small invertebrates and plants are able to take advantage of microhabitats created by freeze-thaw weathering erosion to invade crevices and other sheltered environments. As pioneer species establish themselves in these new soil conditions, mountain ecosystems thrive over time. However, climate change could alter this dynamic as freeze-thaw weathering changes the frequency and intensity of freeze-thaw weathering weathering events, potentially altering glacial erosion rates significantly.
Movement
As glaciers move across a rock surface, they scrape, scratch, and cut at it, leaving characteristic landforms such as U-shaped valleys, horns, and moraines behind. Erosion occurs most rapidly where there has been no tectonic activity for millions of years before erosion occurred.
Below the brittle zone, where the ice is weakest, its movement tends to grind and scour rock surfaces below it. Although not particularly efficient at erosion itself, rock fragments clinging tightly to its surface are very abrasive – this process is known as quarrying where large pieces of bedrock are removed in one go.
Glacial erosion produces unique landforms, including glacial polish on granite surfaces and striations on granitic rocks. Its rate can be further hastened by subglacial melt water escaping at high pressure from within the ice sheet itself.