Glacial erosion works primarily through two powerful processes: plucking and abrasion. These mechanisms, driven by the immense weight and movement of glaciers, carve out landscapes over time.
Plucking
Plucking, also known as quarrying, is the process where a glacier freezes onto bedrock and then rips away pieces as it moves. Here's a breakdown:
- Water Seepage: Meltwater from the glacier seeps into cracks and fractures in the bedrock beneath and around the glacier.
- Freezing: When the water freezes, it expands, exerting pressure on the surrounding rock. This process, known as frost wedging, weakens the rock.
- Adhesion: The ice then freezes to the weakened rock.
- Glacial Movement: As the glacier advances, it pulls the ice and the attached rock fragments along, effectively "plucking" them from the bedrock.
- Transport: These plucked rock fragments are then carried within the glacier, often contributing to further erosion.
Abrasion
Abrasion is the grinding and polishing action of a glacier as it scrapes along the underlying bedrock. This occurs because glaciers carry rock debris, including the plucked fragments mentioned above.
- Glacial Sediment: The ice contains a mixture of rock fragments ranging in size from fine silt to massive boulders. This material acts like sandpaper.
- Scouring: As the glacier moves, the embedded rock fragments scrape against the bedrock, grinding it down.
- Polishing: Finer sediments within the ice polish the bedrock, creating smooth surfaces.
- Striations and Grooves: Larger, harder rocks can carve striations (scratches) and grooves into the bedrock, indicating the direction of glacial movement. These markings provide valuable evidence of past glacial activity.
Summary
In summary, glacial erosion is a two-pronged process involving:
- Plucking: Freezing onto and ripping away bedrock fragments.
- Abrasion: Grinding and polishing the bedrock with embedded rock debris.
These processes, combined with the sheer mass of the ice, result in the significant shaping of landscapes over geological timescales, creating features like U-shaped valleys, cirques, and fjords.
