Processes of Glacial Erosion

How do glaciers erode the landscape?

Glaciers are powerful agents of erosion. As a glacier moves downhill under the influence of gravity, it erodes the rock beneath and around it.

The two main processes of glacial erosion are:

• plucking
• abrasion

These processes remove rock and help create distinctive glacial landforms.

What is plucking?

Plucking is the process by which a glacier freezes onto rock and pulls pieces of it away as the ice moves.

Rock beneath and alongside a glacier often contains joints, which are cracks in the rock. Water can enter these cracks and freeze. This helps loosen blocks of rock.

Meltwater beneath the glacier can also freeze onto the rock. As the glacier moves, the frozen rock becomes attached to the ice and is plucked away from the bedrock.

The glacier then transports the pieces of rock downhill.

Water enters cracks → water freezes → rock becomes attached to the ice → glacier moves → rock is plucked away

Plucking is particularly effective where the bedrock is well jointed, as blocks of rock can be removed more easily.

What is abrasion?

Abrasion occurs when pieces of rock and sediment carried at the base of a glacier scrape and grind against the bedrock as the glacier moves.

The material carried by the glacier acts like sandpaper, wearing away the rock beneath it.

Rock debris becomes embedded in the ice → glacier moves → debris scrapes over the bedrock → bedrock is worn away

Larger fragments can gouge grooves and scratches into the bedrock. These scratches are known as striations.

Striations can provide evidence of the direction in which a glacier once moved.

How are plucking and abrasion linked?

Plucking and abrasion often work together.

Plucking removes rock → rock becomes incorporated into the glacier → the rock scrapes against the bedrock → abrasion increases

This means that material removed by plucking can become a tool that causes further erosion through abrasion.

What affects the rate of glacial erosion?

The amount of erosion caused by a glacier varies. Erosion is generally greater when:

• The glacier moves faster – debris at its base is dragged across the bedrock more rapidly.
• The glacier contains more rock debris – more material is available to scrape and grind against the bedrock.
• The ice is thicker – greater pressure at the base can increase contact between the glacier, its debris and the bedrock.
• The bedrock contains many joints – blocks of rock can be removed more easily by plucking.
• Over thousands of years, these processes can significantly change the shape of the landscape.

Plucking or abrasion?

A useful way to remember the difference is:

Plucking = pulling rock away

Abrasion = scraping rock away

Both processes occur as glaciers move and are responsible for creating many of the erosional landforms found in glaciated upland areas of the UK.