Glacial erosion creates an intriguing variety of landforms. From rock drumlins with their distinct facets and striations patterns to vast mountain ranges with spectacular mountain peaks and U-shaped valleys.
Glacial erosion can take place at different rates. While no single term accurately describes it, several processes produce characteristic landforms and landscapes we see today.
Plucking
Glaciers shape landscapes by eroding and depositing rock and sediment. Additionally, glaciers break rocks apart called plucking before transporting them across their path – this creates characteristic landforms such as scalloped bedrock surfaces or glacial pavements and even produce features like facets, striations or grooves in rocks.
As glaciers move over bedrock, they drudge up bits of rock, sediment, and debris and grind it against its surface, known as abrasion – leaving behind scratches (striations marks) on its surface.
Recent work suggests that subglacial streams may erode fast enough to mitigate some of the erosion produced by glacier migration; this counters prior findings which concluded that sliding was dominantly responsible for glacial erosion.
Abrasion
Glacial erosion takes place through both processes of erosion, but one in particular stands out: abrasion. Glaciers travel over rock surfaces, wearing away their surface layers. This process leaves behind visible evidence on rocks as striations marks or scratches near where they have passed by, leaving behind evidence where glaciers have gone by.
Glacier abrasion is responsible for creating cirques, U-shaped valleys and rock basins; as well as long mountains, pointed mountain peaks called horns, deep fjords with side walls, and deep fjords with side channels.
Glacier abrasion is determined by multiple factors, with sliding velocity and basal friction playing an essential part. Quarrying and plucking also play an integral role.
Freeze-thaw weathering
Gletschers melt and refreeze to pull rocks away, known as plucking, from rocks it encounters, creating mechanical weathering processes such as plucking. However, this form of mechanical weathering only typically occurs where glacier ice can get close enough to ground surface to cause the process.
Ice scraping against rocks exposes rocks and minerals in its composition that act like sandpaper, wearing away at its surface and leaving glacial striations scratches behind. Such tools are held within the base of ice sheet as “tools.”
Freeze-thaw weathering creates crevices and cracks which provide niches and habitats for organisms, especially small invertebrates that find shelter and sustenance within these crevices. They also allow water to penetrate further into the rock, dissolving salts and chemicals more effectively as it penetrates deeper. Minerals released by rocks through soil become food sources for plants and sustain ecosystems. Freeze-thaw weathering produces landscape features like scree slopes and blockfields as it does so.
Excavation
Glacier melt water’s mechanical and chemical action can erode bedrock along its margin. The effectiveness of this erosion depends on both susceptibility to erosion (Reference Cook and Swift2012) as well as discharge regime factors, especially velocity and level of turbulent flow.
Quarrying, plucking and abrasion are key processes in creating various glacial landforms such as fjords, glaciated mountain valleys, and cirques whose bedrock has been worn away over long distances by glacier movement. These processes produce sediments of various grain sizes but most commonly coarse cobbles from abrasion and silt from quarrying.
Fluvial incision and subglacial avalanching are other mechanisms of erosion. Avalanching, or avalanche formation, occurs when preexisting cracks in rock are widen by stress from ice pressure, leading to rock fragments being swept downhill into basins along the edge of glacier or crevasses beneath.