Glacial Erosion (Leaving Cert Geography): Revision Notes

Glacial Erosion

Glacial erosion occurs when glaciers move across the landscape, wearing away rock and other materials at their base and sides. This powerful process shapes some of the most dramatic mountain landscapes we see today, creating distinctive landforms that tell the story of past ice ages.

Main processes of glacial erosion

Glaciers erode the landscape through two primary mechanisms that work together to reshape valleys and mountainsides. These fundamental processes - plucking and abrasion - operate simultaneously to create the distinctive features we associate with glaciated landscapes.

Plucking

The plucking process works most effectively on rocks that already have well-developed joints and cracks, particularly sedimentary rocks. Here's how it works:

• Pressure from the glacier's weight causes ice at the base to melt slightly
• This meltwater flows into existing cracks and joints in the bedrock
• When temperatures drop, the water refreezes and expands
• This expansion weakens and loosens chunks of rock
• As the glacier continues moving downslope, it plucks these loosened rock pieces away
• The rock fragments become part of the glacier's load (the material it carries)

Abrasion

The abrasion process creates several distinctive features:

• Rock fragments embedded in the glacier smooth and polish the bedrock surface
• Larger rock pieces can carve deep scratches called striae or striations into the bedrock
• These scratches provide evidence of past glacier movement and direction
• The grinding action further aids the plucking process by creating new weaknesses in the rock

Factors affecting the rate of erosion

The speed and intensity of glacial erosion depends on three main factors that work together to determine how effectively a glacier can modify the landscape:

• Glacial thickness: Thicker glaciers create more pressure, leading to greater rates of erosion. The deeper the ice, the more powerful the erosive force
• Gradient: Glaciers moving down steeper slopes flow faster and have greater erosive power, similar to how fast-flowing rivers erode more than slow ones
• Rock resistance: Softer rocks erode much more quickly than harder, more resistant rock types

Landforms of glacial erosion

Glacial erosion creates two main types of landforms: cirques and glaciated valleys, each with their own distinctive characteristics and associated features. These landforms represent different stages and scales of glacial activity.

Cirques

Cirque formation involves several stages that occur over thousands of years:

• Most cirques in Ireland formed on north or north-east facing slopes where they received the least sunshine, allowing snow to accumulate
• Snow gathers in pre-existing hollows on mountainsides
• The expansion of ice through freeze-thaw action gradually enlarges these hollows
• As snow compresses into glacial ice, the growing glacier begins to move downslope
• The weight of the ice causes pressure melting at the base
• This meltwater seeps into bedrock cracks and refreezes, causing plucking of the back wall
• This plucking creates a deep crevasse called a bergschrund in the glacier
• Freeze-thaw action causes scree to fall into the bergschrund, which gets compressed into the ice
• As the glacier moves in a rotational sliding motion, it deepens the cirque through continued plucking and abrasion

When cirques fill with water after glaciers melt, they form small round lakes called tarns or corrie lakes.

Related cirque features

• Arêtes: When two cirques form on either side of the same mountain, they create a narrow, steep-sided ridge of rock between them
• Pyramidal peaks: When three or more cirques form around the same mountain, they create a steep, pointed peak. Carrauntouhil, Ireland's highest mountain, exemplifies this formation

Glaciated valleys

Most glaciated valleys began as V-shaped river valleys that provided the easiest route downslope for glaciers. The glacier occupation dramatically transformed these valleys through a systematic process:

• Large glaciers widened and deepened the original river valleys through vertical and lateral erosion
• The glacier's immense weight caused pressure melting at its base
• This meltwater seeped into bedrock cracks before refreezing and expanding
• The glacier plucked loosened rock from the valley floor, deepening it further
• Rock fragments stuck in the glacier caused abrasion, creating a smooth valley floor
• Unlike rivers that flow around obstacles, glaciers eroded straight through interlocking spurs, leaving triangular-shaped cliffs called truncated spurs

Additional valley features

The complex processes of glacial erosion create numerous subsidiary features within glaciated valleys:

Rock basins and ribbon lakes: In areas of softer rock or deep soil, abrasion creates deep hollows along the valley floor. When glaciers melt, these basins fill with water to form ribbon lakes. When connected by streams, they become paternoster lakes.

Hanging valleys: Smaller tributary valleys along the main valley contained smaller glaciers with less erosive power. This means they didn't erode as deeply as the main valley. When glaciers melted, these tributary valleys were left "hanging" at a higher level than the main valley floor, often marked by waterfalls where they meet.

Rock steps: Due to varying rock resistance and intense vertical erosion, glaciated valley floors often descend in a series of steps. These are frequently found below sea level due to the glacier's powerful erosive action.

Fiords: When glaciers melted and sea water flooded the glaciated valleys, these drowned glacial valleys became fiords. Killary Harbour in County Mayo provides an Irish example.