How Do Glaciers Cause Erosion?
QUICK ANSWER
Glaciers cause erosion mainly through two mechanisms. Plucking happens when meltwater freezes onto bedrock and the moving glacier pulls rocks away. Abrasion happens when rocks embedded in the glacier's underside scrape against bedrock as the ice moves, like sandpaper on wood. Together these create distinctive U-shaped valleys, fjords, and polished bedrock surfaces.
Glacial erosion has shaped some of the world's most dramatic landscapes, from the U-shaped valleys of Yosemite to the deep fjords of Norway. Glaciers are extraordinarily powerful erosion agents, capable of carving solid bedrock through patient grinding over thousands of years. Understanding how glaciers cause erosion explains many distinctive landforms left behind by past ice ages, plus active landscape change in current glaciated regions.
What is glacial plucking?
Plucking is one of two main glacial erosion mechanisms. As a glacier flows over fractured bedrock, meltwater at the base of the ice seeps into rock cracks. When this water refreezes, it bonds firmly to the surrounding rock. As the glacier continues moving, it carries the frozen-in rock fragments along, effectively plucking pieces of bedrock out and incorporating them into the ice. Plucking removes large angular rock fragments from the bedrock surface, often producing steep cliffs on the down-slope side of obstacles. The plucked material then becomes available for additional erosion through abrasion.
What is glacial abrasion?
Abrasion is the second main glacial erosion mechanism. Rocks plucked from upstream become embedded in the ice at the glacier's base. As the glacier flows over bedrock, these embedded rocks scrape and grind against the surface below, much like sandpaper grinding wood. The constant grinding wears down the bedrock, creating polished surfaces and parallel grooves called striations that all point in the direction of flow. Abrasion produces fine rock flour that gets carried away by meltwater, eventually depositing in streams and lakes downstream. Striations on bedrock provide clear evidence of past glaciation.
What landforms do glaciers create?
Glacial erosion creates distinctive landforms not produced by any other process. U-shaped valleys form when glaciers carve through pre-existing river valleys, widening them and steepening their sides. Cirques are bowl-shaped basins at the heads of glaciers where snow first accumulates. Arêtes are sharp ridges between adjacent cirques. Horns are sharp peaks created when cirques erode mountains from multiple sides (the Matterhorn is the classic example). Fjords form when glaciers carve coastal valleys below sea level, which fill with ocean water when ice retreats. Other features include hanging valleys, glacial lakes, and roches moutonnées.
How fast does glacial erosion happen?
Glacial erosion rates vary widely depending on glacier characteristics. Fast-flowing temperate glaciers with abundant meltwater erode the fastest, sometimes removing several feet of bedrock per century. Cold-based polar glaciers (frozen to bedrock) erode much more slowly. The average rate for active glaciers is roughly 1 millimeter per year of bedrock erosion. While slow by human standards, this is much faster than typical rates of river erosion or weathering. Over thousands of years, glacial erosion has carved entire mountain ranges, with effects still visible thousands of years after the glaciers retreated.
Glaciers cause erosion mainly through plucking (lifting rocks from bedrock via meltwater that freezes and bonds rock into the ice) and abrasion (embedded rocks grinding against bedrock as the ice flows). These mechanisms create distinctive landforms including U-shaped valleys, cirques, arêtes, horns, and fjords. Glacial erosion rates average about 1 millimeter per year but can carve entire mountain ranges over thousands of years.
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