Glacial erosion refers to the process by which glaciers erode rock, including quarrying blocks and abrasion, subglacial erosion from melt water and subglacial erosion by melting glaciers.
This chapter explores these processes that result in glacial landforms such as striations lines, moraines and geomorphic features.
Glaciers cause erosion through two main methods. Plucking involves rocks caught up in the glacier acting like large scouring pads to scrape away at bedrock beneath it, leaving behind striations marks known as tribologies on it – these scrapes eventually becoming part of its flow, being carried off as debris by the glacier as it travels.
Or the glacier can erode bedrock through its own abrasion. This form of erosion is the primary mode, with evidence often seen in the form of striations marks or finely ground material found deposited as glacial streams – commonly referred to as rock flour but rarely understood fully.
Glacial erosion is responsible for many of the iconic mountain landscape features such as cirques, troughs, rock basins and fjords, while it also creates medium-sized landforms such as whalebacks, mountain ridges and rock drumlins.
Glacial plucking or quarrying, also referred to as glacial quarrying, involves dislodging larger fragments and boulders from glaciers by dislodging larger rocks or boulders from them. It is particularly prevalent on lee sides of mountain ranges (qv), downstream side of glacial stairs and inner and deeper parts of bergschrund crevasses in cirque glaciers. Like abrasion, plucking leaves long scratches known as glacial striations marks or gouge marks on bedrock which remain.
Freeze-thaw weathering occurs by way of a process known as freeze-thaw weathering. This occurs when water in cracks of rocks freezes and expands, opening up their passageway. This may cause rocks to break off of them or dislodge large blocks known as joint blocks that have become connected over time. Plucking can lead to the creation of moraines, roche moutonnees, glacial erratics and drumlin fields – as well as near glacier margins where plucking may also occur. It does so due to an inconsistent relationship between ice velocity, which provides tractive force, and effective pressure of ice on bed (ice overburden pressure less water pressure), leading to preexisting or new microcracks opening up that allow large blocks of rock to be lifted off their beds and lifted away by forces generated by overburden pressure less water pressure.
A quarry is an outdoor location where stone is extracted from the earth for use as building material, aggregate or other purposes. Excavation usually uses mechanical means like draglines, shovels and bulldozers; although sometimes caves or mines also serve as quarries.
Glacial erosion occurs as a result of glacier movement, leaving behind various marks on bedrock. Striations – long lines which appear scratched-through the rock surface – is one such mark, giving geologists insight into both speed and movement of glaciers.
Plucking is another form of glacial erosion. This happens when a valley glacier moves into a mountain and forms an amphitheater-like depression with it, gradually eroding away the sides and bottom. In doing so, large fragments or boulders of bedrock are carried off while changing the contours of the area by widening, widening or expanding joints or cracks in rock surfaces.
Glacial erosion produces some of the most complex and variable geomorphic structures on Earth. They include rock facets (striations grooves and glacial polish), glacial pavements, rock flour sediment, and other features formed from grinding action of rocks in glaciers – often appearing where once there was glaciation such as in cirques and trunk valleys in alpine mountains.
Glaciers utilize a process known as abrasion in which large volumes of broken rock and sediment rub against one another, producing U-shaped valleys, horns, aretes, moraine as a result. Abrasion also results in distinctive surface textures ranging from microscopic scratches to gouges several meters deep that date back centuries or millennia. Abrasion is closely tied to basal sliding which depends on temperature and mass balance regime; many assume glacial erosion scales with velocity but observations and modeling show otherwise1.