Ice is an effective agent of erosion. It erodes different types of rocks differently, leaving distinct marks across the landscape known as glacial landforms.
Corries and aretes (spiky ridges), pyramidal peaks, ribbon lakes and truncated spurs are some examples.
But, what are these mysterious landforms like and what processes give them their characteristic features?
As glaciers flow, they scrape against rocks and erode sediment beneath them – this process is known as abrasion – leaving behind features such as faceted clasts, striations grooves, rock flour and glacial pavements – as well as U-shaped valleys, horns and moraine.
Mechanical weathering also erodes rock along the sides of glaciers, and these loosening bits fall away to become glacial erratics that are different in size, shape or origin from their bedrock surroundings. Melting glaciers collect all these erratics together into an accumulation known as glacial till.
Abrasion and plucking are the two key processes by which glaciers sculpt their land, but these erosional processes do not form all terrain under their influence; for instance, soft rock may be less likely to be eroded than hard and stiff ones – this difference creates what is known as the trim line.
Glacial erosion differs significantly from more familiar fluvial erosion. Glaciers erode rock more intensely due to being constantly moving and possessing sharper ice than rivers do.
This movement causes abrasion that wears down rocks and divides them into smaller pieces, as well as plucking rocks from bedrock for transport along its path. The result often creates series of ridges known as cirques containing small lakes called tarns at their bases and sometimes two adjacent cirques form an arete between them forming an arched bridge-like feature separating them – often producing beautiful views along its journey.
Recent years have witnessed increased research into glacial erosion rates. This includes studies of ice discharge rates and sedimentation rates at continental scale. The studies revealed a variety of complex factors affecting erosion. They also led to models incorporating climate/altitudinal thresholds in mountain glacier erosion capacity.
Glaciers transform the landscape around them through their movement, carving U-shaped valleys, ribbon lakes, cirques and aretes as well as depositing rock different from that found nearby in piles known as till.
Glaciers often erode bedrock by lifting chunks out, known as joint blocks. Glacial meltwater seeps into cracks in the rock surface and freezes, weakening its bond with other pieces and making it easier for glaciers to pick up loose rocks along their paths and carry them along behind.
Rock particles engulfed by this process tend to become polished, creating a glossy appearance on their surfaces. Long parallel lines found on these rocks’ surfaces are known as striations lines, and can provide geologists with vital clues as to where and when a glacier traveled before scooping out rocks from its path or landscape.
Ice can shape incredible landscapes, yet the effectiveness and mechanisms governing glacial erosion remain poorly understood and highly contentious over geological timescales1. 2. This is particularly evident on Polar continental shields with cold subglacial environments1.
Meltwater seeping through cracks in glacier’s ice allows rocks to break off or ‘plucked’ from their rockbed, creating glacial striations on bedrock surface and creating U-shaped valleys, horns and moraine formation.
However, meltwater’s erosional potential is limited by various factors. Meltwater cannot penetrate deeply into glacier ice to produce mechanical glacifluvial erosion – where water flows over an uneven surface and produces high impact forces – but also by low supplies of water and no surface tension between bedrock and ice; furthermore it is often polluted with sediments which prevent its proper erodibility.