The nature and factors influencing glacial erosion

Glacial erosion involves the breakdown and removal of rock material through the actions of ice, embedded rock debris, and meltwater. This process shapes landscapes dramatically, as glaciers exert immense pressure due to their weight, giving them greater erosive power than rivers. Warm-based glaciers, which have liquid water at their base allowing faster movement, typically erode more effectively than cold-based glaciers, which are frozen to the bedrock and move more slowly.

Factors that increase the rate and amount of erosion

• Thickness of the glacier - A thicker glacier applies more pressure on the bedrock, leading to greater erosion.
• Composition of debris - Debris made of resistant rock types withstands wear and contributes more to scraping actions.
• Quantity of debris - More debris embedded in the ice increases the abrasive effect, similar to coarser sandpaper.
• Resistance of bedrock - Bedrock composed of less resistant rock erodes more readily under glacial action.

These factors combine to determine how powerfully a glacier can wear down the underlying and surrounding rock surfaces.

Entrainment of debris in glaciers

Entrainment refers to the process where rock debris is picked up and incorporated into a glacier, allowing it to act as a tool for erosion. This debris travels with the glacier and scrapes against valley walls and floors, enhancing abrasive effects.

Types of entrainment

• Supraglacial entrainment - Rock debris is carried on the surface of the glacier, often falling from surrounding slopes.
• Subglacial entrainment - Rock debris is held at the base of the glacier, where it erodes under the ice's weight and pressure; some may loosen and be carried away by meltwater.

The finest debris, known as rock flour (particles less than 0.1 mm in diameter), polishes and smooths the bedrock.

Key processes of glacial erosion

Glaciers erode landscapes through several mechanical and hydraulic processes, often working in combination. These processes depend on the glacier's movement, the presence of debris, and interactions with the bedrock.

Plucking

Plucking, also known as quarrying, is a process where glaciers remove chunks of rock from the bedrock.

How plucking works:

1. Bedrock develops joints or cracks due to fracture (breaking under stress) and traction (dragging force of the ice).
2. Ice in contact with these surfaces thaws slightly and refreezes around protruding rocks.
3. As the glacier advances, it pulls or plucks these rocks away, resulting in a highly jagged rock face.

This process is particularly effective on jointed bedrock and contributes to steepening valley walls.

Abrasion

• Abrasion happens when debris embedded in the glacier scrapes against the bedrock, wearing it down over time.

• Debris acts like sandpaper, grinding the valley walls and floor as the glacier moves.

• This scraping leaves striations (scratches or grooves) on the rock surface, which indicate the direction of glacial movement.

Crushing

• Crushing occurs under the immense weight of the glacier, which exerts pressure on the bedrock.

• The pressure scatters and chips the rock into large fragments.

• Variations in glacial weight cause dilation, where existing fractures deepen or expand.

Basal melting

• Basal melting involves erosion by meltwater at the glacier's base, functioning similarly to river erosion but under high pressure.

• Meltwater causes abrasion through debris-laden flow, hydraulic action (forceful impact of water squeezed through bedrock), attrition (rocks within the water eroding each other), and corrosion (dissolution of soluble rock).

• High-pressure meltwater carves channels and hollows beneath the glacier.

This process is common in warm-based glaciers where basal water is present.

Subaerial processes contributing to erosion in glaciated areas

In addition to direct glacial actions, subaerial processes (those occurring in the open air) aid erosion in glaciated regions by weakening rock faces exposed above the ice.

Freeze-thaw weathering

• Freeze-thaw weathering, also called frost shattering, breaks down rock through repeated freezing and thawing.

• Water enters cracks in the rock and freezes, expanding and widening the cracks.

• Over time, this causes rock fragments, known as scree, to separate from the bedrock.

This process is prevalent on exposed slopes and contributes to debris supply for glaciers.

Mass movement

Mass movement involves the downhill shift of rock material under gravity, eroding slopes in glacial areas. Examples include landslides, rock falls, and avalanches, which dislodge and transport rock fragments.

Landforms formed by valley glaciers

Valley glaciers, confined to mountain valleys, create distinctive erosional landforms as they grow and move downslope. These typically form on the shadier, colder side of a mountain where snow accumulation exceeds ablation (melting and evaporation).

Corries

Corries, also known as cwms or cirques, are bowl-shaped depressions formed by glacial erosion.

How corries form:

1. Snow accumulates in a pre-existing hollow on a slope and compacts into ice.
2. Basal slip (sliding at the base), abrasion, and plucking deepen the hollow into an armchair-shaped basin; abrasion intensity depends on the amount of eroded rock material.
3. When thick enough, the ice flows over the lip and downhill as a glacier.
4. Freeze-thaw weathering and plucking steepen the back wall.
5. After melting, a corrie lake or tarn often remains, bounded by a rock lip where the ice was thinner and erosion less intense.

Arêtes

Arêtes are narrow, steep-sided ridges formed between parallel valleys.

How arêtes form:

1. Two glaciers in adjacent valleys erode the valley sides.
2. This sharpens the intervening ridge through abrasion and plucking.

Pyramidal peaks

Pyramidal peaks are pointed mountain summits with at least three steep sides.

How pyramidal peaks form:

1. They form where three or more corries erode back-to-back.
2. Their steep back walls converge to create the peak.

Glacial troughs

Glacial troughs, or U-shaped valleys, are steep-sided valleys with flat bottoms.

How glacial troughs form:

1. A glacier erodes an existing V-shaped river valley, widening and deepening it.
2. The inflexible ice moves straight through interlocking spurs (ridges projecting into the valley), truncating them via plucking and abrasion to form truncated spurs.
3. After melting, a ribbon lake may form in areas of softer rock eroded more deeply than surrounding harder rock.

Hanging valleys

Hanging valleys are side valleys left at a higher elevation than the main valley floor.

How hanging valleys form:

• Tributary glaciers, being smaller, erode their valleys less deeply than the main glacier.
• After melting, waterfalls often cascade from these elevated valleys into the deeper glacial trough.

Landforms formed by ice sheets

Ice sheets, vast expanses of ice not confined to valleys, erode on a broader scale, creating landscapes known as knock and lochan terrains. Knocks are hills of harder rock that resist erosion, while lochans are small lakes in eroded patches of weaker rock.

Roches moutonnées

• Roches moutonnées are masses of resistant rock on the valley floor, shaped by glacial passage.

• The upglacier side (stoss) is smoothed by abrasion as ice flows over it.

• The downglacier side (lee) is steep and rough due to plucking.

Crag and tail

• Crag and tail formations consist of a resistant rock outcrop with a sloping extension.

• The upglacier crag (often a volcanic plug) resists abrasion, becoming steep and slowing the ice.

• This protects the softer rock on the downglacier side from erosion, leaving a long, sloping tail.