Glacial Erosion Processes
Glacial erosion reshapes bedrock and sediment through a combination of mechanical and chemical processes. Understanding how glaciers erode is foundational to reading glaciated landscapes, since the landforms left behind are direct records of the processes that created them.
Mechanical and Chemical Processes
How effectively a glacier erodes depends on several interacting factors:
- Ice velocity and ice thickness control how much force the glacier exerts on its bed
- Ice temperature determines whether the base is frozen to bedrock (cold-based) or sliding over it on a film of meltwater (warm-based)
- Bedrock properties like rock type, jointing, and fracture density affect how easily material is removed
These factors produce enormous variation in erosion rates. Cold-based glaciers, frozen to their beds with minimal sliding, may erode less than 0.1 mm/year. Warm-based, fast-flowing glaciers can exceed 10 mm/year, removing material orders of magnitude faster.
Abrasion, Plucking, and Quarrying
Three main mechanical processes do the work:
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Abrasion occurs when rock fragments embedded in the base of the ice are dragged across the bed, scouring the underlying surface. This produces striations (parallel scratches aligned with ice flow direction) and glacial polish (smooth, reflective rock surfaces created by fine-grained abrasion).
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Plucking (also called glacial quarrying at smaller scales) happens when meltwater refreezes around rock fragments on the glacier bed, bonding them to the ice. As the glacier moves, it pulls those fragments free. This tends to create rough, jagged surfaces on the downstream side of bedrock obstacles, which is why a roche moutonnée has a smooth, abraded upstream face and a rough, plucked downstream face.
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Quarrying at larger scales removes entire blocks of bedrock, especially where pre-existing joints and fractures allow the ice to exploit weaknesses. This process forms steep cliff faces like cirque headwalls.
Erosional Landforms
Cirques and Arêtes
A cirque is a bowl-shaped depression carved into a mountainside. Cirque formation begins with nivation, where snow accumulates in a pre-existing hollow. As the snow compacts into ice and a small glacier forms, plucking steepens the back wall while abrasion deepens the floor. Freeze-thaw weathering above the glacier (especially around the bergschrund, the crevasse between the glacier and the headwall) helps loosen rock that the glacier then removes.
When two cirques on opposite sides of a ridge erode headward toward each other, the ridge between them is narrowed into an arête, a sharp, knife-edge ridge. Crib Goch in Snowdonia, Wales, is a classic example.
Glacial Valleys and Associated Features
U-shaped valleys (glacial troughs) form when a glacier occupies a pre-existing river valley and transforms it. Unlike rivers, which primarily erode downward, glaciers erode both the valley floor and walls, widening and deepening the valley into a broad, flat-bottomed trough with steep sides. Yosemite Valley, California, is one of the most recognizable examples.
Hanging valleys form where a smaller tributary glacier joins a larger main glacier. The main glacier erodes its valley much deeper than the tributary can, so after the ice retreats, the tributary valley is left suspended high above the main valley floor. Streams flowing out of hanging valleys often form waterfalls, like Bridalveil Fall in Yosemite.
Glacial horns are sharp, pyramidal peaks that form when three or more cirques erode headward into the same mountain from different sides. The Matterhorn in the Swiss Alps is the textbook example.
Subglacial Meltwater's Role
Meltwater beneath a glacier is not just a byproduct of melting. It actively enhances erosion by lubricating the bed (increasing sliding and abrasion), transporting sediment, and carving its own distinctive features under high pressure.
Erosional Processes and Features
Pressurized subglacial meltwater creates several distinctive landforms:
- Potholes: cylindrical holes drilled into bedrock by sediment-laden water swirling in a vortex
- P-forms: smooth, undulating rock surfaces sculpted by pressurized water flow, often with scalloped or sinuous shapes
- Flutes: elongated ridges on the bed, aligned parallel to ice flow
Tunnel valleys are long, overdeepened troughs carved beneath ice sheets by pressurized subglacial meltwater. They often follow pre-existing bedrock weaknesses and can be tens of kilometers long. Well-studied examples occur across northern Europe and North America.
Depositional Features and Hydrological Impacts
Subglacial meltwater also builds landforms by depositing sediment:
- Eskers are sinuous ridges of sand and gravel deposited by meltwater streams flowing in tunnels within or beneath the glacier. After the ice melts, these ridges remain as preserved casts of the former drainage channels. Finland and Canada have extensive esker networks.
- Subglacial lakes form where meltwater accumulates beneath ice sheets, insulated by the overlying ice. These lakes influence ice dynamics by reducing friction at the bed. Lake Vostok in Antarctica, the largest known subglacial lake, lies beneath roughly 4 km of ice.
The interaction between meltwater and sediment also produces streamlined subglacial bedforms:
- Drumlins: elongated, teardrop-shaped hills aligned parallel to ice flow, with a steeper upstream end
- Mega-scale glacial lineations: extremely long (sometimes tens of kilometers), low-relief parallel ridges that record fast ice flow, often found in areas of former ice streams
Glacial Erosion's Impact on Landscapes
Landscape Modification and Denudation
Glacial erosion is the dominant landscape-shaping force in alpine and high-latitude regions, producing landforms with no equivalent in non-glaciated areas.
Glaciers also reorganize drainage. By carving new valleys and blocking old ones, they alter pre-existing fluvial drainage patterns and create new watershed boundaries. The Great Lakes basin in North America, for instance, was largely excavated and shaped by repeated glaciations.
Glaciers are remarkably efficient at removing rock. Studies suggest glaciers can erode mountains up to 10 times faster than rivers and other surface processes, though this varies with latitude, elevation, and climate. The effect is most pronounced in high mountain ranges like the Himalayas and in polar regions like Greenland.
Long-term Landscape Evolution
After glaciers retreat, the landscapes they leave behind continue to evolve:
- Overdeepened basins fill with water to become lakes, fundamentally altering local hydrology and ecology. The lakes of the European Alps and Patagonia formed this way.
- Glacially steepened slopes are prone to mass wasting (landslides, rockfalls), and rivers begin reworking glacial sediment through fluvial erosion and transport.
- Sediment supply to rivers and coasts is affected for thousands of years after deglaciation, influencing channel form, coastal morphology, slope stability, and even ecosystem development and biodiversity patterns.
The legacy of past glacial erosion is written across much of the mid- and high-latitude world, shaping landscapes that billions of people live in today.