Glaciers move across land surfaces, scraping away bare soil while dislodging rocks and sediment before transporting it elsewhere for deposit.
Glacial erosion is an intricate and dynamic process, capable of shaping distinctive landscapes with incredible variation and dramatic change.
Freezing and thawing water play a critical role in glacial erosion. This happens through frost wedging in rock cracks of various sizes through which freezing takes place, leading to frost heaves in rock surfaces that affect glacial movement.
As water expands 9% during freezing, it has the power to push outward against cracks with incredible force, widening and eventually breaking apart rocks progressively. This acts as a powerful weathering agent and one of the primary causes for fractured rocks.
An additional cause of erosion are roots from plants and animals, which burrow deep into rocky soil to alter its structure as they expand. Furthermore, they put pressure on rock joints which weaken them further causing them to eventually break off completely.
Weathering like this is typical on mountain slopes where cold air and ice accumulates. One telltale sign of this process are rounded rocks at the edges of ridges or at the foothills of peaks – an indication of its power.
Weathering of rocks can also be caused by other external forces, including glacier movement. When glaciers push down on rocks and push away soil particles from under their weight, cracking away sections. Plant roots also contribute to weathering processes by breaking them apart with their growth; and altering their shape over time.
Alterations to temperature and other environmental changes can also influence weathering on rocks, altering their mineral makeup or molecular structures to cause corrosion or even cause them to crack open and fracture.
Surface spalling refers to weathering which occurs on a rock’s surface and poses serious threats, including life and property loss.
However, when combined with freeze-thaw weathering effects, surface spalling and freeze-thaw can produce even more severe forms of weathering that can erode rock to such an extent that its original purpose no longer applies.
Glacial erosion has many names; most commonly it’s believed to be controlled by sliding and basal shear forces.
Sliding can be a quick and effective method for erosion due to its production of low shear stresses at glacier bases (Cuffey and Alley 1996). Basal shear stresses, on the other hand, are slower processes but nonetheless produce significant erosion.
Erosion plays an essential role in shaping glaciers and other landforms. A bowl-shaped cirque may form when a glacier moves downward from a mountain peak and erodes rocks on both sides of a valley wall, leaving behind an impression.
Tarns are another form of erosion caused by meltwater seeping into cracks in bedrock and freezing around it, eventually breaking away sections of rock from its foundation and creating a lake-like area.
Glaciers can also erode rocks through abrasion, scraping grooves into them below, creating features known as striations which make an amazing landscape feature.
Glacier erosion rates vary greatly depending on its water supply and whether or not it has been moved downhill. Rapid erosion may occur if there is not enough liquid available, while more abundant sources may have slower results.
Glaciers contain many different kinds of rocks, enabling them to erode them all and form fascinating landforms such as hanging valleys when two U-shaped valleys meet.
Glaciers can erode landforms through various mechanisms. Examples include creating rocky spurs at the ends of aretes where their ice has worn away rock into steep triangle-shaped cliffs; or several glaciers flowing in different directions on one mountain can combine their forces and produce multiple cirques to form sharp ridges known as aretes that in turn erode into sharp triangle-shaped cliffs.
Glaciers can be powerful tools for shaping the landscape, yet it is vital that we understand their various forms of erosion so as to preserve the environment and prevent damages to land.
Abrading is often used to refer to glacial erosion. This process may occur through various means including rock fragments embedded within it, or by moving glaciers over rocks below.
Glacial erosion occurs principally through abrasion of rocks beneath glaciers, but not entirely on its own. Abrasion takes place through multiple processes that are assisted by volumetric expansion of frozen water (Matsuoka and Murton 2008).
Glaciers are powerful agents of erosion. By moving over bedrock and other sediment layers, glaciers can erode away at them resulting in landforms such as roche moutonnees, crag and tails being formed below them.
Frost-cracking, another abrasion process on glaciers, occurs by means of cracks in ice that allows temperature fluctuations beneficial for frost-cracking and freeze-thaw weathering (Cuffey & Alley 1996). As this fractured block travels from its source through subglacial streams it can be exposed by flowing meltwater, or be removed through headwall avalanches sweeping into its crack and denuding it further.
Abrading is an integral component of glacial landform formation and erosion processes in valley systems; for instance, U-shaped valleys like those found at Yosemite National Park in California can be directly attributable to this process.
Cirque headwall retreat, when glaciers retreat into higher topography in the mountains, typically occurs horizontally compared to downward erosion beneath non-cirque glaciers. This process offers some striking geological features associated with glaciers.
Stream erosion caused by non-cirque glaciers is typically weak; however, under mountain range cirques where avalanches and rockfalls erode headwalls and surrounding strata is likely to be much stronger, potentially spreading across basins by means of weathering and hillslope transport processes.
Glacial erosion is an intricate process influenced by various physical forces and geochemical reactions such as dissolving and regualating meltwater. While there may be several physical forces at work during glacial erosion, as well as geochemical processes which affect it, such as weathering, fractures and high water pressure; erosion also plays an important part.
Transport plays an essential role in glacial erosion rates. This includes current and longshore drift transport of sediment from outside areas into former glaciated regions.
Fine-grained particles are generally carried along rivers until they reach lakes or the sea where calm conditions allow them to settle and deposit themselves near an ice margin.
Deglaciation processes can transport large volumes of material away from one glacier at once, including rock fragments that cannot easily be broken down through simple methods like abrasion and plucking.
There are also various other factors that play into glacial erosion’s rate, such as changes in climate, soil moisture and groundwater availability. For instance, in the Arctic region snowfall accumulation rate has an impactful influence on glacier erosion rates.
Other factors influencing glacial erosion include bedrock coverage by sediments. When large volumes of debris created from glacial erosion is left deposited on the surface, it may create an impediment to further erosion.
Calculating how much bedrock has been covered by sediments can be challenging due to their self-generated nature, but you can get an approximate idea by measuring how thick minerogenic Quaternary deposits are in an area which was once glaciated.
Calculating the average thickness of these deposits requires using a mathematical model which takes into account erosion from pre-glacial regolith and bedrock erosion by glaciers; typically this rock used by glaciers as deposit material within their former glaciated areas is also the same rock eroded away by them.
Estimating thick deposits of minerogenic Quaternary sediments to cover about 10% of bedrock surface area would equate to an erosion rate between 0.2-4 meters per glacial cycle.