How Is A Corrie Formed

How is a Corrie Formed? A Comprehensive Guide to Glacial Landforms

Corries, also known as cirques or cwms, are breathtaking amphitheatre-shaped hollows carved into mountainsides by glaciers. These impressive landforms are a testament to the immense power of ice and represent a crucial element in understanding glacial geomorphology. This comprehensive guide will delve into the fascinating process of corrie formation, explaining the contributing factors, the stages involved, and the resulting features that make corries so unique. Understanding corrie formation offers a window into the dynamic interplay between ice, rock, and time, ultimately shaping the landscapes we see today.

Introduction: The Genesis of a Corrie

Corries are iconic features of glaciated landscapes, often found high in mountainous regions. Their characteristic bowl shape, with steep back walls and a lip at the lower end, is a direct result of glacial erosion. But the story of corrie formation is not simply a matter of ice carving away at rock. It's a complex process involving several contributing factors and stages of glacial activity. This article will explore these elements in detail, providing a clear and comprehensive understanding of how these impressive landforms are created. Understanding corrie formation requires us to consider the interplay of pre-existing weaknesses in the bedrock, freeze-thaw weathering, and the erosive power of glacial ice.

The Role of Pre-existing Weakness: Setting the Stage

The formation of a corrie doesn't begin with a blank slate. Pre-existing weaknesses in the bedrock, such as joints, faults, and bedding planes, play a crucial role in determining the location and shape of the future corrie. These weaknesses provide lines of least resistance for erosion. Glaciers don't randomly carve out hollows; they exploit existing vulnerabilities in the landscape. A slight depression or a pre-existing hollow, even a small one, can act as a nivation hollow—a small, shallow depression— which provides a focal point for snow accumulation. This initial depression is critical because it initiates the process that will eventually lead to the formation of a much larger corrie.

Freeze-Thaw Weathering: The Power of Repeated Freezing and Thawing

Even before significant glacial activity begins, freeze-thaw weathering significantly contributes to the initial stages of corrie formation. This process, also known as frost shattering, involves the repeated freezing and thawing of water within cracks and joints in the bedrock. Water expands by approximately 9% when it freezes, exerting immense pressure on the surrounding rock. This repeated expansion and contraction progressively widens the cracks, eventually leading to the breakdown of the rock into smaller fragments. These fragments are then removed by other processes like meltwater, contributing to the deepening and widening of the initial depression. The effectiveness of freeze-thaw weathering is dependent on factors such as the frequency of freeze-thaw cycles, the presence of readily available water, and the type of rock. Rocks with a high porosity are particularly susceptible to this type of weathering.

The Glacial Process: Erosion and Excavation

Once a significant amount of snow accumulates in the pre-existing hollow, the formation of glacial ice begins. This snow transforms into firn (compacted granular snow) and eventually into glacial ice. The weight of the accumulating ice exerts immense pressure, causing the glacier to flow outwards and downwards. The movement of the glacier, combined with the abrasive action of the ice and incorporated rock debris, is the primary mechanism responsible for the erosion and excavation that shapes the corrie. This process involves several key mechanisms:

• Abrasion: Rock fragments embedded within the basal layer of the glacier act like sandpaper, scraping and grinding against the bedrock. This abrasive action polishes and smoothens the rock surfaces, contributing to the overall deepening and widening of the corrie. The intensity of abrasion depends on factors like the amount and size of the rock fragments, the velocity of the glacier, and the hardness of the bedrock.

• Plucking: As the glacier moves, it can pull or pluck out pieces of rock from the bedrock. This occurs when meltwater penetrates cracks and joints in the rock, refreezes, and expands, forcing pieces of rock to detach from the bedrock and become incorporated into the glacier. This process is particularly effective in areas with pre-existing weaknesses in the rock. The size of the plucked fragments can vary considerably, ranging from small pebbles to large boulders.

• Meltwater Erosion: Meltwater flowing beneath and within the glacier also plays a role in corrie formation. This meltwater can erode the bedrock through both hydraulic action (the sheer force of the water) and solution (dissolving soluble minerals). The meltwater often follows existing cracks and joints, further enhancing the erosion process.

Stages of Corrie Development: A Step-by-Step Process

The formation of a corrie is a long and gradual process, spanning millennia. While the precise timeline varies depending on several factors, we can broadly outline the key stages:

1. Nivation Hollow Formation: The process begins with the formation of a small, shallow depression, often in a sheltered location on a mountainside. This depression provides a suitable location for snow accumulation.

2. Snow Accumulation and Ice Formation: Snow accumulates within the nivation hollow, eventually transforming into firn and then glacial ice. The weight of the accumulating ice causes the glacier to flow.

3. Erosion and Excavation: The glacier, through abrasion, plucking, and meltwater erosion, progressively deepens and widens the initial depression. The steep back wall of the corrie is formed through the erosional process.

4. Lip Formation: As the glacier erodes the bedrock, the lower end of the corrie develops a lip or threshold. This lip is formed because the erosional processes are less effective at the lower end of the glacier, where the ice is thinner and moves more slowly.

5. Corrie Maturation: Over time, the corrie continues to evolve, reaching its mature form, characterized by its amphitheatre shape, steep back wall, and a lip at its lower end.

Beyond the Corrie: Associated Landforms

The formation of a corrie often results in the creation of other associated glacial landforms, further enhancing the dramatic landscape. These include:

• Aretes: Sharp, jagged ridges formed between two adjacent corries.

• Pyramidal Peaks (Horns): Sharp, pointed peaks formed when three or more corries erode back into a single mountain summit. The Matterhorn is a classic example of a pyramidal peak.

• Hanging Valleys: Smaller valleys that join a larger valley at a significantly higher elevation, often forming stunning waterfalls. These are formed when smaller glaciers occupying tributary valleys merge with a larger main glacier. The main glacier erodes more deeply, leaving the tributary valleys hanging high above the main valley floor.

Frequently Asked Questions (FAQs)

Q: How long does it take to form a corrie?

A: The formation of a corrie is a process that occurs over thousands of years, even tens of thousands of years, depending on several factors like the rate of glacial erosion, the type of rock, and the climate.

Q: Are corries only found in high mountains?

A: While corries are most commonly associated with high-altitude mountainous regions, they can also form at lower elevations under suitable climatic conditions.

Q: What are some examples of famous corries?

A: Many mountainous regions around the world feature spectacular corries. Examples include the numerous corries in the Scottish Highlands, the Lake District in England, and the Alps.

Q: What happens to a corrie after the glacier disappears?

A: After the glacier melts, the corrie often becomes occupied by a lake, known as a corrie lake or tarn. The lake forms because the lip at the lower end of the corrie acts as a natural dam, trapping meltwater. The floor of the corrie often displays evidence of glacial erosion in the form of striations and polished surfaces.

Conclusion: A Legacy of Ice

Corries are remarkable landforms, showcasing the immense power and artistry of glacial processes. Their formation is a complex interplay of pre-existing geological weaknesses, freeze-thaw weathering, and the relentless erosional power of glacial ice. Understanding the mechanisms behind corrie formation not only enhances our appreciation of these stunning features but also provides valuable insights into the history of past glaciations and the dynamic processes that shape our planet's landscapes. The next time you encounter a photograph or experience the majesty of a corrie in person, remember the immense geological forces and the vast timescale involved in creating this breathtaking testament to the power of nature. The study of corries remains a vital area of research in geomorphology, continuing to contribute to our understanding of glacial processes and their impact on the Earth's surface.