As glaciers make their way down a mountain, they erode away at the rocks beneath them, producing numerous landforms in their wake.
Glacial erosion occurs through two main methods, plucking and abrasion, as well as freeze-thaw weathering, creating landforms such as aretes and cirques through these physical processes.
Abrasion
Glacial erosion of rocks is often described as “abrasive”, with rock fragments (clasts) being carried along in a current and dragged across other clasts in a flow, creating features like faceted clasts, striations grooves and glacial pavements. Furthermore, glacial erosion contributes to rock flour’s creation – fine-grained sediment formed through grinding of clasts into dust particles.
The rate of abrasion depends on various factors, including bedrock lithology and contact area with the ice sheet. Abrasion rates tend to be highest where moulin-fed streams have direct access to bedrock without supercooling restraints – potentially higher even than under cold glaciers! Erosion by these streams is further accelerated by fluctuations in water pressure at the ice-water-rock interface due to surface melt and drainage (Hooke 1991; Gilbert 1906b).
Plucking
Glaciers erode the land under them in two ways. Their immense weight causes rocks and stones to be broken up into fine particles through “abrasion”, while whole rocks can also be picked up and carried downstream as “plucking”. Both forms of erosion can be particularly destructive for earth’s surface, leading to glacial moraine features like cirques (bowl-shaped valleys), moraine lakes, rock flour (talc) deposits and glacial polish (smoothened surfaces with smoothed and striated surfaces).
As is widely acknowledged, plucking is much more significant in glacial erosion than abrasion due to slow debris entrainment into basal ice and glacial erosion. Plucking can be found particularly prominently on slopes characterized by sheeting joints – something seen frequently on glaciated landscapes like stoss-and-lee topography with hills being extensively plucked off by glacier ice.
Freeze-thaw weathering
Water’s ability to freeze and thaw can be an extremely powerful weathering force. Rocks with porous and permeable surfaces allow water to seep into cracks within them, then expands 9% when frozen to produce significant pressure against rocks that eventually break them apart; this process is known as freeze-thaw weathering or frost wedging and it plays an integral part in glacial erosion of crystalline rocks.
Erosion rates depend on many variables, including ice thickness and availability of moisture as well as mechanical properties of rock being weathered by weathering processes. Erosion rates range across orders of magnitude; they cannot be explained solely through either quarrying or abrasion alone.
Landforms
Glaciers shape the landscape they leave behind through erosional processes like plucking, abrasion and freeze-thaw weathering, creating landforms such as aretes, horns, truncated spurs ribbon lakes and roche moutonnee. Ice can also alter V-shaped valleys into U-shaped ones while creating striations rock flour glacial pavements eskers and drumlins.
Glaciated basins often show higher sediment yield rates than typical fluvial basins due to subglacial erosion which can quickly create large amounts of erosional debris in an instant.
Glacial erosion rates depend on three key variables: temperature, location and formation. Glaciers usually form in cold climates at high elevations on mountain ranges where glacial runoff has an adverse impact. Therefore, erosion occurs more quickly on mountain tops than lower altitudes, yet still has sufficient time to form landforms such as till plains (wide areas covered with glacial drift), kame terraces or glacial drumlins.