Ice is an effective tool that can erode landscapes, creating amazing landforms such as ribbon lakes, U-shaped valleys, aretes and pyramidal peaks.
But which term best characterizes glacial erosion? Unfortunately, it’s difficult to give an answer since there are multiple variables at play here.
Plucking
Glacier erosion produces many beautiful rock features, from faceted clasts (small bits of eroded rock scraped across bedrock by glaciers), to glacial pavements and rock flour (formed when rocks are abraded by glacial flow), not to mention U-shaped valleys, horns and moraine formed as landforms from glacier action.
Glaciers tend to erode mountain landscapes at a faster rate than rivers flowing through similar terrain nonglaciated regions due to a combination of abrasion and plucking processes.
Glaciers tend to hit rocks first on one side (the stoss side), leaving behind smooth surfaces shaped by abrasion while rougher lee sides are worn away by plucking erosion. When this process continues, rougher lee sides become pushed forward under pressure of an advancing glacier until eventually being ploughed under. This process creates distinctive rock surfaces known as glacial polish. A similar phenomenon is often observed on rock debris (known as talus) deposited by glaciers.
Abrasion
Glaciers shape landscapes through two fundamental processes: plucking and abrasion. This erosion produces U-shaped valleys, U-horns, rock drumlins and glaciated cirques – usually decametric or hectometric in size but some of the world’s most famous glacial landscapes may even reach kilometric proportions.
Glaciers scrape over bedrock like sandpaper, producing scratches (striations marks) on its surface while leaving behind powdery particles known as rock flour.
Rock-flour sediment is carried down glaciers through a process known as “rock grinding,” an essential step for effective glacial erosion, however certain conditions must be fulfilled for its success. Abrasion works best under temperate glaciers because their release of melt water helps lubricate basal sliding; on cold glaciers where erosion rates may be lower.
Basal Slip
Glacial erosion is often enhanced by basal sliding, wherein ice beneath the surface of a glacier moves over its bed using meltwater as a lubricant to move. This method is prevalent among warmer glaciers while cold glaciers move primarily via internal deformation.
This process, known as quarrying, involves the removal of blocks of rock from bedrock by means of sliding ice (Fig. 6.1). As velocity of sliding ice increases, so will its rate of quarrying; further acceleration may come from bedrock steps or undulations which increase fracture stress on bedrock surfaces.
Glacial erosion is highly variable. Climate factors impacting bedrock temperature and meltwater volume may alter basal slip rates and have an effect on basal slip rates; as such, glacial erosion tends to occur near its equilibrium line altitude (ELA). Numerical landscape evolution models that take these effects into account reveal that glacial erosion occurs primarily near this altitude line.
Basal Crust
Glaciated basins often experience vigorous subglacial fluvial erosion by subglacial fluvial processes (e.g., subglacial Nye channels formed by retreat of continental glaciers; see Hallet1979 and 1980 for references), which can erode both rock and bedrock directly.
Glaciers typically erode bedrock via abrasion and quarrying, with fluvial erosion producing finely grained material with mode sizes exceeding those created by quarrying and abrasion; consequently, in basins dominated by rapidly eroding glaciers, fluvial action can help delay bedrock erosion by increasing its supply of finer-grained sediments. This may help slow bedrock abrasion.
Till must be removed efficiently from beneath an ice sheet for rapid bedrock erosion to occur, usually by means of meltwater flows remobilizing proglacial sediments and migration of moulins up-glacier to reach steep downsloping beds (e.g. Alley and others 1997, Cuffey 2000). Such streams tend to erode at rates well beyond proglacial erosion rates until adverse bed slopes arise from overdeepening and thus limit transport capacity and tie glacial geomorphology with slower proglacial evolution.