Glaciers erode rock by plowing and abrading. If a glacier passes over an unstable surface such as rocks, for instance, it can create long grooves called striations in the bedrock that run parallel with its path – this is called glacial erosion.
These features can sculpt and shape the land. For instance, they can convert V-shaped stream valleys to U-shaped ones by widening their bottom and steepening the sides of their valley.
Why is it hard to see glacial erosion in action?
Glaciers move as they collect rocks from beneath the surface, known as plucking, creating moraines of debris along their path. Glaciers also transport rocks from different origins which form glacial erratics as they pass.
Plucking and abrasion are the primary ways glaciers erode a landscape, creating landforms such as striations, glacier police ridges and horns in their wake. Striations refers to long, parallel scratches in rock surfaces while glacier police are narrow saw-toothed ridges with sharp teeth. Horns form when cirques have been eroded on three or more sides, becoming pointy peaks that create distinctive visual features in landscapes ravaged by glacier action.
River valleys typically take on a V-shape; however, glaciers reshape them by carefully chipping away at bedrock along their bases and lower sidewalls – creating U-shaped valleys with flat bottoms and steep sidewalls. Glaciers also erode cirques on mountain sides.
Plucking
Glaciers move over rocks and other surfaces, leaving behind visible marks known as striations marks.
Abrasion depends on the rate at which glaciers move across a surface, with slower-moving glaciers being less capable of eroding surface rock than faster moving ones. Lithology type also plays an important role; hard lithologies tend to be eroded more readily than soft ones and sharp debris fragments tend to penetrate bedrock more readily than their rounder counterparts.
Direct observation of abrasion is difficult due to tunneling under a glacier and inspecting its base, but some scientists have observed glaciers scraping bare bedrock surfaces; for instance, Boulton planted two slabs of marble and basalt bedrock at the base of an Icelandic glacier before retrieving them after 9.5m of tool-studded ice had passed over them.
Abrasion
Glaciers erode bedrock through a process called abrasion, by dragging rocks or pebbles along their surface. This produces various landforms including faceted clasts (rocks smoothed off from rubbing against other rocks), glacial striations grooves and pavements which smooth and polish rock surfaces, fine-grained products such as rock flour that gives glacial streams their characteristic dilute-milk white colouring.
Abrasion depends on many variables, including hardness and concentration of rocks and minerals held within the glacier base; pressure exerted by ice; speed at which glacier moves; surface texture of bedrock, with areas harder or softer than other parts protruding from it, eventually leading to formation of knob-and-tail landforms – tail protrusions facing towards glacier edges while knobs form on its lee side.
Freeze-thaw weathering
Freeze-thaw weathering, like many natural processes, gradually wears away rocks over time. Repeated cycles of freezing and thawing cause rock joints, fissures, and pores to expand and contract, leading to fragmentation of rocks into smaller fragments while weakening cohesion between mineral grains.
Mechanical weathering is most effective in regions with fluctuating temperatures around the freezing point, such as cold climates or high altitude areas. It works particularly effectively in rocks with well-joined cracks and fissures where water penetrates during warmer daytime hours before expanding as it solidifies into ice during cooler evening hours, exerting pressure that enlarges cracks to cause them to widen further, leading to the separation of rock pieces and eventual breakdown.
Mechanical erosion processes like abrasion and plucking work together to shape the landscape, leaving behind distinctive glacial landforms such as U-shaped valleys and cirques. Eroded rock fragments and sediments transported by glaciers become loess deposits elsewhere on land surfaces, eventually becoming loess deposits.