Snow deposited on mountains can turn to ice that rolls down slopes, transforming the landscape in spectacular ways. Glaciers erode rock through multiple processes.
Glaciers transport rock debris of various sizes from large boulders to silt. This debris, known as glacial till, accumulates as moraines on their path through space.
Glacier erosion deposits vast amounts of sediment into rivers and oceans, altering biogeochemical balances globally and altering global sediment and biogeochemical equilibriums. Understanding glacial erosion allows us to predict its effects as climates shift.
Glacial erosion occurs when rock fragments embedded in the ice rub against rock surfaces, creating what’s known as abrasion. Over time, this creates intriguing marks known as striations; think of using sandpaper on wood; the harder you press down on it the faster it wears away.
Plumping and thrusting are other processes that contribute to glacier erosion rates, including breaking apart rock on its bed as it advances, leading to moraines, drumlins, kames and moulins being formed by its movements. A mountain range’s geology influences which glacial erosion processes are used, as well as which types of landforms they produce and the type of erosion caused.
Abrasion occurs when glacial ice rubs against bedrock, wearing away its surface layers. Depending on its hardness, concentration and velocity, erosion can range from light scuffing to intense rubbing that creates long scars in the bedrock known as striations scars.
Abrasion also erodes hills and mountains. Drumlins (hillocks or knobs of bedrock), often found in glacial landscapes, feature long streamlined drumlins topped by down-ice ends with steep side walls that feature steep sidewalls extending out towards them; drumlins may have either rounded, striated surfaces or blocky shapes depending on their environment.
Glacial erosion can also give rise to geomorphic structures like cirques and troughs, two geomorphic features common throughout mountain terrain. Cirques are bowl-shaped amphitheater-like depressions in mountains formed when glaciers erode into previously created valleys such as those formed by rivers; troughs form when glaciers flow into old river valleys with interlocking spurs into truncated spurs from glacier action; freeze-thaw weathering adds frictional resistance between rock and ice that increases frictional resistance against their formation resulting in their creation; freeze-thaw weathering adds further frictional resistance between rock and ice to help shape geomorphic features like these two formations.
Glaciers encountering bedrock must erode it before continuing down slope, with rainwater often percolating through the ice to accelerate this process and accelerate erosion rates. Snowfall further adds to erosion by encouraging glacier flow and shuffling sediment underneath it.
Freeze-thaw weathering is the main form of rock deterioration that takes place in regions with continental glaciation (such as in the Himalaya-Karakoram-Tibet region). Water infiltrates cracks in rocks, infiltrated through them by freezing/thawing cycles, eventually breaking them apart completely over time.
Abrasion in cold-based glaciers is less prevalent, since their ice must move at their base in order to scour over bedrock and create gouges or striations in bedrock. However, it can still happen if an iceberg moves slowly enough that it snags onto a rock or sediment and scrapes against it like sandpaper, leaving gouges or striations as its path goes over it.
Glacial erosion forms the land beneath glaciers and can produce features such as faceted clasts (rocks smoothed off by being rubbed against other rocks), grooves and striations in rock, glacial pavements and rock flour, U-shaped valleys, horns, cols, cirques and aretes.
These processes erode rocks much deeper than rivers do, creating distinctive landforms in areas with continental glaciation compared to those with alpine glaciation.
On a grander scale, glacial erosion creates cirques, troughs and rock basins known as fjords; on a more local scale it forms drumlins, eskers and kames. Furthermore, various stratified deposits commonly known as “varves” form in glacial regions; each layer of deposition represents one year. They provide us with insight into the history of glacial erosion.