Glaciers erode mountains to form fascinating landforms like lakes, peaks and distinctive aretes. Erosion results from various processes including plucking, abrasion and freeze-thaw weathering.
But why is glacial erosion difficult to observe in action? Let’s take a closer look at its processes, starting with abrasion.
Plucking
As glaciers move over bedrock, they slowly break off pieces of rock that they then carry away, known as plucking. Although not visible directly, plucking has an enormous impact on landscape formation; creating glacial landforms such as fjords.
Example of a typical stoss-and-lee glacier In a typical stoss-and-lee glacier, local pressure is higher on one side than on the other, creating a crack or joint across the surface of ice where it contacts bedrock. Freezing and thawing causes this crack to widen and loosen as its freeze/thaw cycle continues, leading to gradual removal of large pieces of bedrock known as joint blocks that accumulate into moraines that mark periods of glacial advance/retreat cycles.
Large rock fragments entrained in these deposits are further altered by glacial erosion processes such as polishing and grooving, giving rise to an array of glacial landforms and depositional glacial landforms.
Abrasion
Abrasion occurs when rocks of different sizes within a glacier come in contact with one another and scrape against each other, scraping away at the rock beneath and gradually eroding it away from within the glacier itself, thus leading to its creation of features such as corries, aretes, pyramidal peaks and other long features.
Conditions necessary for glacial abrasion to take place are extremely precise: only when there is warm base melting and basal sliding can it happen; furthermore, an ideal thermal regime with water below requires melting to occur.
Once this occurs, glacial abrasion creates long scratches called glacial striations on underlying rocks – visible signs that a glacier once covered an area. These scratch marks can help identify an area as having been covered by one.
An ice abrasion produces fine-grained material known as “rock flour,” which accumulates along glacial streams and gives them their characteristic dilute milky-white hue. Furthermore, rock flour forms part of the primary materials in creating stoss-and-lee topography as well as rock drumlins.
Freeze-thaw weathering
Freeze-thaw weathering is one of the primary erosion processes that shape periglacial landscapes. It involves repeatedly freezing and thawing water trapped within rocks or soil cracks or pores. Once frozen, expanding liquid can exert pressure that tears apart rock surfaces causing further erosion of this form.
By combining plucking and abrasion, glacial landforms such as U-shaped valleys, aretes, cirques, and moraines can be formed. Furthermore, such erosion contributes to ecosystem development by freeing minerals stored within rocks for uptake by plants.
Freeze-thaw weathering produces scree slopes and blockfields, which consist of piles of rock debris covering upland areas, usually found near glacial valley heads and formed through glacial erosion. This process also leads to patterns such as circles or stone stripes on the ground surface as well as slow downward movements of soil-water-rich material called solifluction lobes that create slow downslope movements of saturated soil and rock debris.
Sheeting
Glaciers serve as powerful natural erosive agents, creating spectacular landforms as they flow downhill. Moving slowly, glaciers collect rocks and sediment that they deposit elsewhere while simultaneously eroding rock surfaces they pass over, leaving behind features like polished bedrock surfaces, striations lines, grooves and gouges that mark glacial erosion as distinct processes.
Grinding action of the ice can produce similar effects by wearing away particles piecemeal – this process, known as abrasion, causes glacial polish, rock flour, and long, shallow grooves on bedrock surfaces that become known as glacial striations grooves.
Sheeting may not be the main contributor to erosion, but it plays an essential role in some instances. For instance, when glaciers pass through valleys carved by rivers they pucker up both the sides and bottom until converting them into U-shaped valleys; at times causing significant changes such as creating bowl-shaped depressions known as cirques or creating high altitude lakes known as tarns on their way through.