Glaciers take over V-shaped stream valleys and transform them by preferentially eroding the base and lower sidewalls, giving rise to their distinctive parabolic shapes.
Glaciers also erode bedrock through two processes – plucking and abrasion – leaving behind landforms such as glacial striations lines or rock flour as evidence of their action.
Plucking
Glacial erosion is driven by two primary processes: plucking and abrasion. Abrasion uses scrapers or brushes to scrape away at rocks like sandpaper, leaving behind what are known as glacial striations lines.
Plucking occurs when the force of a moving glacier causes rock fragments to break off and be drawn into its body by gravity, similar to how snowflakes do. This method tends to work better when rock is soft or weaker than its surroundings.
Rock fragments that fall onto glaciers are then either directly deposited by them (till), or after some further modification by meltwater streams which carry and deposit sediment along their bed, creating what’s known as glacial drift deposits. These glacial drift deposits play an essential role for human civilization by providing fresh water; often also containing valuable minerals; shaping landscape features like deep U-shaped valleys sharp peaks ridges as well as mounds of granitic rock called moraines; providing freshwater access; shaping landforms in many different ways!
Abrasion
Abrasion occurs when glacier ice encounters bedrock and grinds against it, creating various landforms including stoss-and-lee topography, rock basins, glacial pavements, striations grooves and fine material called rock flour that accumulates on glacial streams.
Glacier ice is instrumental to the creation of some of the most distinctive mountain landscapes, such as those with sharp, jagged peaks known as horns and the aretes that divide them, as well as providing erosional mechanisms which lead to knob-and-tail formations on rocks.
Conditions may limit glacier’s ability to abrade rock. Most significantly, for this effect to take place effectively it must move at its base, most likely happening with warm-based glaciers exhibiting basal slippage.
Freeze-thaw weathering
At its heart, glacial erosion involves loosening and transport of rock and sediment by glaciers to form classic landforms such as U-shaped valleys, cirques, aretes, moutonees roches moutonees roches moutonees hanging valleys rock drumlins.
Freeze-thaw weathering occurs when exposed rocks are porous or permeable and contain multiple cracks, where water seeps into these openings during warmer days and freezes at night, expanding and forcing itself into surrounding rocks, eventually forcing it apart.
This form of erosion works best in climates where temperatures regularly dip below freezing, such as Antarctica. Although glacier erosion doesn’t work quite so effectively in other locations such as continental-style climates or extremely warm places like Antarctica, it remains the predominant form of glacier erosion – responsible for creating many erosional landforms around the world and contributing to creating steep mountain landscapes carved by glaciers today. Furthermore, millimeter scale to kilometers-scale erosion events have all been reported over time.
Deposition
Glaciers can be formidable beasts, and as they erode the landscape they scrape against, specific landforms known as glacial landforms may appear. These forms result from processes such as plucking, abrasion, freeze-thaw weathering and plucking/abrasion/freeze-thaw weathering that occur as glaciers grind across their pathway.
Landforms that form are usually quite distinct and leave behind distinctive ‘tool-marks’ ranging from microscopic scratches to centimetre-deep gouges, including shapes such as striations lines, gullies and ridgelines.
As glaciers move across the landscape they can also pick up rock materials and sediments ranging from minute clay particles to large boulders – known as glacial till. Once picked up by the glacier it is carried along until reaching its destination where it gets dumped out as erosion by water (this process creates river valleys or kettle lakes). Once there it remains in one form or another until being recycled back into new landforms like river valleys and kettle lakes.