How does a cirque form in a glaciated landscape?

A cirque forms in a glaciated landscape through the erosional activity of a glacier over a long period of time.

A cirque, also known as a corrie or cwm, is a bowl-shaped, amphitheatre-like depression in the landscape, often found on the side of a mountain. It is one of the primary landforms created by the erosional activity of glaciers. The formation of a cirque begins with a small area of a mountainside where snow accumulates more than it melts, forming a niche glacier. Over time, the weight of the accumulated snow compresses into ice, which begins to move downhill due to gravity.

The glacier erodes the landscape through two main processes: plucking and abrasion. Plucking occurs when the glacier freezes onto the bedrock, and as it moves, it pulls away large chunks of rock. Abrasion happens when the rocks and debris frozen into the base of the glacier scrape against the bedrock, like sandpaper, smoothing and polishing the surface. The rotational movement of the glacier, also known as ice flow, further helps in deepening and widening the hollow, giving it a characteristic armchair shape with a steep back wall.

The back wall of the cirque is often subject to freeze-thaw weathering, where water seeps into the cracks in the rock, freezes, expands, and eventually breaks the rock apart. This process, known as frost shattering, contributes to the steepness of the back wall. The debris from the back wall accumulates at the base of the cirque, forming a rock lip, which is often the highest point of the cirque floor.

Over thousands of years, the glacier retreats, leaving behind a semi-circular, steep-sided hollow – the cirque. If the cirque is deep enough and conditions are right, it may hold a small, often circular lake, known as a tarn.

In summary, the formation of a cirque is a slow and complex process involving glacial erosion, ice movement, and weathering processes. It is a testament to the powerful and transformative effects of glaciers on the landscape.