Glacial erosion is an impressive natural process. It erodes rock, forms landforms, and leaves characteristic scratch marks called glacial striations on bedrock surfaces.
Geomorphological processes form large geomorphic features like cirques, troughs and rock basins as well as mountain peaks visible to the naked eye but hard to recognize.
Abrasion
Glaciers operate like giant magnets, scraping rock and sediment from surfaces with their feet to loosen particles from their surroundings, which then travel with the glacier and deposit themselves at new locations, shaping new landforms in its wake.
These processes erode the bedrock, producing the distinctive scratches (known as striations marks) so characteristic of glacially-carved rock formations like roche moutonnee. Furthermore, these scratches serve to indicate where glaciers once passed by.
Many theories of glacial erosion rely heavily on ice sliding velocity as a primary influencing variable, as faster moving ice drags more erosional particles over the bedrock at once.
However, erosion rates are more complex than simply determined by sliding velocity alone. Other factors influencing erosion include subglacial till (lubricating’sawdust’), which acts as a source of sand and gravel for quarrying, as well as fluvial incision which may directly erode bedrock. Furthermore, climate impacts also play a part in this process by altering glacier size, retreat direction and meltwater supply.
Plucking
Glacier ice can be an efficient erosion machinery despite being quite soft (Mohs hardness 1.5 at 0degC). Like a belt sander, glacier ice slowly grinds away rocks and sediment it rests upon.
Glaciers also “pluck” pieces of rock from the surface and carry them away, creating long, parallel grooves known as striations in bedrock.
Glaciers erode bedrock by quarrying and plucking, as well as through abrasion. Their impact may widen existing cracks in rocks as the ice moves over them, or gouge basins into the bedrock creating lakes. Furthermore, glaciers transport rocks of all sizes – even large boulders! – which they deposit on moraines which sometimes have different origins or rock types than that which surrounds it – also known as “erratics.”
Freeze-thaw action
Freeze-thaw action is a major cause of erosion in environments with temperatures regularly falling below freezing, occurring when groundwater, rainwater or melting snow seep into cracks in rocks and freeze. When this ice expands by 9 percent it applies pressure that gradually erodes away at them over time – often leading to mass wasting of hillsides as well as scree slopes.
This process is driven by both segregation of ice, as well as its unique property of expanding when frozen, as it melts during the day and fills back up when freezing at night. When this occurs in cracks or crevices, its opening expands wider; later that night when more ice arrives to fill those cracks further. Over time this can even break apart larger rocks – a process known as frost shattering; an essential aspect of glacial environments as it contributes to erosion of relict glacial landforms.
Basal slip
Glacier basal surfaces often slip and slide across their beds, often on a thin film of melt water lubricated by pressure melting at its base on rough beds and by refreezing processes known as regelation (refreezing). Basal slides are particularly common beneath continental glaciers which cover poorly compacted sediments.
Glacial erosion models that incorporate an intuitive relationship between erosion rate and basal surface velocity produce overdeepenings that appear realistic, as demonstrated in Figure 11.5.
But it goes deeper than that; the rate at which a glacier erodes is also dependent upon how much debris has become embedded within its layers, with higher amounts slowing its rate due to increased friction between ice and its bed, thus increasing frictional resistance against erosion and making roches moutonnees, whalebacks and rock drumlins more likely in cold climates than warm ones.