Glaciers, the giant rivers of ice, play a significant role in shaping the Earth's surface. These slow-moving masses of ice, formed from compacted layers of snow, have been a powerful force in sculpting the landscape over millions of years. This section delves into the various processes by which glaciers transform the land and identifies the distinct landscape features found in glaciated areas.
Introduction to Glaciers
Glaciers are dynamic systems that move and change in response to climate and gravity. They can be found in various sizes, from small valley glaciers to vast ice sheets. As they flow, they interact with the landscape, leading to profound geological changes.
Glacial Processes
Erosion
Glaciers are potent agents of erosion, reshaping the land as they move.
- Plucking or Quarrying: This process involves the glacier freezing onto the bedrock and then pulling chunks of rock away as it advances. The effectiveness of plucking depends on the freeze-thaw cycles and the nature of the bedrock.
- Abrasion: As glaciers move, they drag embedded rocks across the bedrock beneath them. This action grinds the surface below, smoothing it and carving grooves or striations that indicate the glacier's movement direction.
Transportation
Glaciers are natural conveyors, moving vast amounts of rock and sediment.
- Englacial Transport: Some debris is carried within the ice, either because it has fallen into crevasses or has been engulfed by the glacier.
- Supraglacial Transport: Debris that falls on the glacier's surface, such as rockfall from surrounding cliffs, is transported on the glacier's top layer.
Deposition
When glaciers lose mass faster than they gain it, they retreat, leaving behind the debris they carried.
- Till: This unsorted material is deposited directly by the melting ice and can range from fine silt to large boulders.
- Outwash: Meltwater from the glacier can carry and deposit finer materials away from the glacier's edge, forming stratified and sorted deposits.
Landscape Features in Glaciated Areas
Cirques/CWorries
Cirques, also known as corries or cwms, are bowl-like hollows found at the head of a glacier.
- Formation: They form through a combination of plucking and abrasion, where ice accumulates in a natural hollow and deepens it over time.
Glacial Lakes
- Origin: These lakes are often formed in depressions that were either eroded by the glacier or dammed by glacial deposits.
Pyramidal Peaks/Horns
- Formation: Occur where several cirques erode a mountain from different sides, leaving a sharp, pointed summit.
Arêtes
- Characteristics: These are sharp ridges that form between two adjacent glaciers or cirques. They are often jagged and steep.
Image courtesy of coolgeography.co.uk
Glacial Troughs
- Appearance: These are U-shaped valleys, in contrast to the V-shaped valleys formed by rivers. They are a classic sign of glacial activity.
Moraines
- Types:
- Lateral Moraines: Form along the sides of the glacier, marking the height of the ice.
- Medial Moraines: Appear as dark lines running down the centre of a glacier, formed when two glaciers with lateral moraines join.
- Terminal Moraines: Found at the furthest point of the glacier's advance, forming a ridge of debris.
Image courtesy of VectorMine
Erratics
- Description: These are large boulders that have been transported by a glacier, often found sitting in isolation and differing in composition from the bedrock upon which they rest.
Conclusion
Understanding glacial processes and the resultant landscapes is crucial for comprehending the historical and ongoing impact of glaciers on the Earth's surface. From the majestic U-shaped valleys to the towering pyramidal peaks, each feature tells a story of a landscape sculpted by the relentless force of moving ice. This knowledge not only enriches our appreciation of natural landscapes but also informs our understanding of past and future climatic conditions.
FAQ
Drumlins are streamlined, elongated hills composed of glacial till, with a steep stoss side (up-glacier side) and a gently sloping lee side (down-glacier side). They are significant in interpreting glacial environments as they indicate the direction of glacier movement. Drumlins form beneath a glacier as it reshapes and redeploys the subglacial till. Their aligned, teardrop shape points in the direction the glacier was moving. By studying drumlins, geographers can determine the flow patterns of past glaciers, providing insights into the dynamics of ice movement and the characteristics of the subglacial environment.
Fjords are long, narrow, deep inlets of the sea, flanked by steep cliffs or mountains, created by the deep glacial erosion of valley floors. Their formation begins with a glacier moving down a valley to the sea. As the glacier erodes the valley through plucking and abrasion, it carves a deep U-shaped valley. When the glacier retreats and melts, sea water floods the valley, forming a fjord. The depth and steep sides of fjords are evidence of the glacier's erosive strength. Studying fjords helps in understanding the extent and power of ice age glaciers and the subsequent rise in sea levels that flooded these valleys.
Eskers are long, winding ridges of sand and gravel that form within glacial landscapes. They are created by streams that flow underneath, within, or on top of glaciers. These streams carry sediment that is deposited along their course. When the glacier melts, the stream’s course becomes a ridge, marking where the stream once flowed. Eskers can be several kilometers long and vary in height and width. They are important in understanding the subglacial hydrology of glaciers, revealing the routes of meltwater channels and providing evidence about the nature of the glacier's retreat, such as whether it was steady or punctuated by pauses. Eskers also serve as indicators of the direction of glacial movement.
Hanging valleys are a striking feature in glaciated landscapes, often seen as elevated secondary valleys that join a primary, deeper glacial valley. They form when a smaller tributary glacier flows into a larger main glacier. The main glacier, being deeper and more powerful, erodes the valley floor more vigorously, creating a deeper U-shaped valley. Meanwhile, the tributary glacier, with less erosive power, creates a shallower valley. When the glaciers retreat, the tributary valley is left hanging high above the main valley floor, often leading to the creation of spectacular waterfalls. Hanging valleys are important indicators of the relative size and erosive power of past glaciers, highlighting the dynamics of glacial erosion.
The freeze-thaw cycle is a significant factor in glacial erosion, particularly in the process of plucking. It occurs when water seeps into cracks in the bedrock beneath or at the edges of a glacier. As temperatures drop, the water freezes and expands, exerting pressure on the surrounding rock. This expansion widens the cracks and loosens fragments of the rock. When temperatures rise, the ice melts, allowing the glacier to move. The glacier can then pluck these loosened rock pieces and carry them along. This process is particularly effective in areas with frequent temperature fluctuations around the freezing point, leading to repeated cycles of freezing and thawing, thereby intensifying the erosive power of the glacier.
Practice Questions
Glaciers play a pivotal role in shaping U-shaped valleys, a distinctive feature of glaciated landscapes. As a glacier moves through a pre-existing V-shaped river valley, its immense weight and movement exert significant erosive force on the valley floor and sides. The process of plucking, where the glacier pulls pieces of rock from the valley sides, combined with abrasion - the grinding down of rocks trapped in the glacier's base - effectively widens and deepens the valley. Over time, the glacier transforms the narrow, steep-sided V-shaped valley into a broader, U-shaped valley with a flat floor and steep sides. This transformation is a clear testament to the power of glacial erosion in reshaping landscapes.
Moraines are key indicators of past glacial activity, providing valuable insights into the movement and dynamics of glaciers. Composed of unsorted debris ranging from fine silt to large boulders, moraines are deposited by melting glaciers. Lateral moraines, formed along the glacier's sides, indicate the height of the ice; medial moraines, appearing as dark lines in the glacier's centre, signal the convergence of two glaciers; and terminal moraines, found at the glacier's furthest advance, mark its maximum extent. The composition, size, and distribution of materials in moraines can reveal the glacier's velocity, direction of movement, and changes in climate that influenced its advance and retreat. These features serve as crucial records for reconstructing glacial history and understanding climatic changes over time.
