Ice is a formidable force capable of carving out vast tracts of land into specific landforms such as corries, aretes, pyramidal peaks, and U-shaped valleys. But which term best defines glacial erosion’s rate of action?
Answering that question depends on which processes or mechanisms are at work. This article will explore three of the most frequently seen glacial erosion processes: abrasion, quarrying and plucking.
Freeze-thaw weathering
Freeze-thaw weathering, or frost wedging, occurs when water seeps into a crack in rock and freezes overnight, expanding by approximately 9% as it expands outward. This forces additional stress onto the rock surface that may eventually fracture it causing repeated cycles to weaken it further and create scree slopes, talus piles or boulder fields over time.
Rock crevices and fractures created by freeze-thaw weathering provide niches for plants and animals that have adapted to these environments. Over time, weathering causes rock to break down further, carrying sediment across landscapes and creating valleys, gorges and other landforms in its wake.
Breakdown of rocks releases minerals and nutrients into the environment through nutrient cycling, helping maintain ecosystems while contributing to soil fertility. This process supports ecosystems while contributing to soil fertility.
Abrasion
Glacial erosion forms landscapes by dislodging and transporting rocks and sediment, leaving landforms such as U-shaped valleys, horns, moraine hills and more in its wake.
Glaciers’ bases usually contain bits of rock, sediment and debris which abrasion the rock surface and cause scores or striations; this process is called abrading.
Glacial retreat can exacerbate bedrock abrasion by drawing in basal sediment from melting or fractured bedrock surfaces; however, these processes do not significantly increase rates of bedrock abrasion beyond what would occur under natural conditions.
Abrasion rates depend on two primary factors: speed of glacial movement over bedrock and debris concentration in basal ice. Therefore, they tend to fluctuate significantly during glacial retreat periods, often exceeding what would be observed during steady moving times.
Plucking
Glaciers work to erode bedrock through two methods of erosion: abrasion and plucking. Abrasion involves debris in the glacier scraping against rocks below, leaving behind facets, grooves, and striations on their surfaces; in plucking, cracks expand due to pressure from expanding ice packs, fragments break off of them then are carried off by glacial waters as part of their movement.
Glacial erosion is responsible for many features that characterize mountain landscapes. A glacier can move rocks from river valley sides and bottoms, widening and deepening them as it travels, to form U-shaped valleys; also creating end moraines along its route before retreating; when several glaciers converge at once from different parts of a mountain they can form cirques with steep-sided bowl-shaped basins like Tuolumne Meadows in Yosemite National Park which are popular destinations among rock climbers.
Quarrying
Glacial erosion is one of the key processes responsible for shaping glacial landscapes, yet measuring or reconstructing erosion rates related to glaciation can be challenging. Results from various methods indicate that glaciers and ice sheets erode at rates ranging from 0.001 mm per year in cold-based ice up to 100 mm year 1 for temperate ice sheets.
Erosion rates depend on several factors, including bedrock susceptibility and velocity of ice flow. Furthermore, erosion rates may also be limited by subglacial water-pressure fluctuations and backpressure from terminal lakes.
Large-scale glacial erosion creates some of the most iconic geomorphic landforms, including cirques, troughs, rock basins and fjords. Smaller forms include eskers (winding ridges of sand and gravel that point in the direction of movement of the ice) and drumlin fields (asymmetrical hills made up of rock). Glacial erosion also transforms sediments into rocks or materials which are transported by ice and eventually dumped elsewhere.