Glacial erosion is an immensely powerful force that dramatically changes the landscape, yet remains one of the more complex geological processes to fully comprehend.
We know this by observing glacial striations patterns on bedrock – long parallel grooves known for causing erosion by both abrasion and plucking – which reveal their presence.
But what causes it? A team of researchers may have discovered an answer.
Abrasion
Ice is an extremely powerful material, and when scraped across the ground it can create distinctive landforms. Abrasion is the basic form of glacial erosion in which ice rubs against rocks to wear away rock surfaces (think of it like using sandpaper on wood surfaces). Abrasion leads to some classic glacial landforms: stoss-and-lee topography, ribbon lakes, U-shaped valleys and roches moutonnee.
But for abrasion to work effectively, glacial ice must be moving downhill at an adequate rate – something cold-based glaciers (which tend to remain attached to bedrock) cannot do. For this abrasion process to take effect, bits of rock, sediment, and debris must be transported through its basal ice onto bedrock beneath it, grinding away at it while producing scratches called striations marks in its path – thus explaining why rates of abrasion vary so widely depending on various factors.
Plucking
Glacial plucking or quarrying refers to the process by which large chunks of bedrock are eroded away by glacial movement and transported downriver by valley glaciers.
As ice flows over rocks, its effects vary depending on whether they’re soft or hard. When flowing over soft rocks (called lee sides), more easily eroded parts such as lee sides are abraded by abrasion; on harder, rougher parts (known as stossss), plucking is used.
At its heart, this process accounts for why rocks across landscapes vary so much: their interaction between abrasion and plucking. It’s an underlying mechanism affecting all glaciers from small cirque glaciers to continental ice sheets; furthermore, this phenomenon interacts with freeze-thaw weathering processes (which we will cover next) producing lakes and peaks that make for interesting landforms.
Freeze-thaw weathering
Glaciers transform landscapes by eroding and transporting rocks and sediment, through various erosion methods such as abrasion, plucking or freeze-thaw weathering. Freeze-thaw weathering occurs when water seeps into cracks in rocks during the day before freezing over night; this causes widening cracks that put pressure on them as water enters through these crevasses, eventually breaking apart their structure altogether.
Glacial erosion occurs from both abrasion and plucking. Abrasion happens when the rough surface of a glacier scrapes against bedrock, creating grooves or striations in its surface. Plucking occurs when rocks held at the base of a glacier move forward, pulling away bits of rock from underlying bedrock that cause additional erosion.
Cracked rock surfaces exposed to repeated cycles of freezing and thawing will experience significant weathering due to repeated cycles of water freezing and thawing that only works where night temperatures remain cold in wintertime.
Landforms
Glaciers move over rocks at great speed, and in doing so erode it through a combination of abrasion, plucking, smoothing and scouring processes that create characteristic landforms such as the stoss-and-lee topography; one such landform features gentle upstream sides with steep downstream sides shaped in such a way as to resemble a trough shaped like an inverted U with evidence such as rock flour and glacial striations scars on its surfaces indicating their passage over time.
Glaciers can shape mountains with glacial forces to form features such as cirques, ribbon lakes, talus slopes, truncated spurs, drumlins, eskers and mounds of debris. Glaciers may even form horns and aretes on mountain tops. When two opposite-side cirques erode an arete between them to converge on one mountainside to form U-shaped valleys which then can erode away at their walls to become hanging valleys containing rocks, debris and even meltwater lakes!