Glaciers move across mountain terrain and scrape rocks of various shapes and sizes against it as they go, leaving behind distinctive landforms such as faceted clasts, grooves, striations lines, and rock flour deposits.
What term best characterizes glacial erosion? Your answer likely depends on whether the glacier is flowing or not.
Abrasion
Ice movement causes rocks beneath it to be scraped abrasively by its weight, sending debris tumbling onto the bedrock below and creating landforms such as ribbon lakes and U-shaped valleys as it does so. This action, known as quarrying, erodes away rock surfaces to form ribbon lakes and U-shaped valleys.
Glacial polish refers to rocks with smoothed off faces due to dragging across other rocks, while those with unevenly polished faces from sliding are known as faceted clasts.
Rate of Abrasion depends on concentration, hardness, velocity and mass of basal debris in an ice stream, along with its lithology. A high concentration of clasts in an ice stream increases friction against bedrock surfaces to slow basal sliding1. Unfortunately, direct measurements of glacier bed processes remain rare3,4. As such, most understandings of subglacial erosion come from theoretical models or observations of deglaciated areas4.
Plucking
Many glacial erosion processes, including quarrying (also referred to as plucking) and abrasion, are made possible by fluctuations in subglacial water pressure. Such fluctuations create shear stresses on bed surfaces which enable smaller particles to be ploughed through and into the glacier bed surface more quickly.
Bedrock erosion resistance depends heavily upon its lithology; rocks with numerous joints or fissures tend to experience faster rates of plucking than more compact, solid stones.
Mechanical weathering loosens rock from valley walls, which then falls to a glacier as glacial till. This unsorted mixture of boulders and silt may contain rocks of differing origin from that found nearby and are known as erratics; they then land onto it at valley edges where they create moraines forming lateral moraines indicating there is an apparent relationship between shear stress and glacier erosion rate; however this does not imply that basal sliding is determined solely by erosion rate.
Freeze-thaw weathering
Freeze-thaw weathering, a mechanical form of weathering, involves repeated freezing and thawing cycles of water that crack rocks. This typically takes place in arctic and alpine climates; once broken apart into small pieces, these fragments combine with plants, animal remains and bacteria to create soil.
Quarrying is more effective in glacial erosion than abrasion in producing sediment with cobble-sized particles, while abrasion generally produces silt-sized material.
Though much work has gone into improving our understanding of subglacial erosion, many unanswered questions still exist. For example, we don’t fully comprehend how moulins work to entrain substrate and time-variation in its extent of subglacial till. Furthermore, it remains unknown how quickly streams erode bedrock which significantly influences quarrying and abrasion rates.
Landforms
As glaciers flow, mechanical weathering loosens rock from valley walls that is carried onto it by mechanical weathering processes. Once carried along by the glacier, this debris eventually melts, depositing long unsorted piles of rock debris called moraines made up of different sizes of rocks and even different types than bedrock; larger boulders dropped by glaciers are known as glacial erratics.
Glaciers produce unique glacial landforms from their constant erosion of rock debris, from microscopic scratches (known as striae) and gouges up to centimeters deep and tens of meters long to smooth surfaces with a “glacial polish”. Cirques, troughs, rock drumlins as well as giant mountainous rocks known as “roches moutonnees” and whalebacks are just some examples of what can result from glacier action.
Glacial plucking, quarrying and freeze-thaw weathering all play their parts in creating alpine landscapes’ distinctive U-shaped valleys. This process happens where glaciers have scraped away valley floors over thousands of years to form their distinctive U-shaped cross sections – often filled with ribbon lakes.