Cirque Glaciers Explained
Stand at the head of almost any alpine valley and look upward: behind you, above the last moraines, you will find one or more bowl-shaped hollows carved into the mountain face, often still holding snow or ice, with a rounded backwall and steep headwall rising above a flat or gently sloping floor. These features are cirques — also called corries in Scotland, cwms in Wales, and karer in German — and they are one of the most characteristic landforms produced by glaciation. Where ice still occupies them, the glaciers that created them are cirque glaciers: small, compact bodies of ice nestled in armchair-shaped basins that they have carved through thousands of years of erosion, freezing, and thaw.
Cirque glaciers are among the most widespread glacier types on Earth. They exist on every continent except Australia, on mountains from the tropics to the poles, and at elevations ranging from near sea level in high-latitude regions to above 5,000 metres in tropical mountain ranges. Their small size makes them sensitive indicators of climate — they respond to temperature and precipitation changes more rapidly than large valley glaciers — and their distribution across a mountain range can be used to reconstruct past snowline positions and climate conditions.
How Cirques Form
The formation of a cirque begins with a mountain hollow that preferentially accumulates snow. Topographic features that shelter snow from wind — concavities on north-facing or east-facing slopes in the northern hemisphere, where solar radiation is reduced — provide the initial setting. As snow accumulates year after year and compresses into ice, the nascent glacier begins to erode its bed through two main processes: abrasion, where rock fragments embedded in the basal ice scrape and polish the bedrock like sandpaper, and plucking (or quarrying), where the glacier freezes to the bedrock and pulls blocks away as it moves.
The headwall of a cirque is steepened by a process called bergschrund erosion. The bergschrund is the crevasse that opens between the glacier and the headwall in summer, when the glacier pulls away from the rock face. Meltwater enters this crevasse, percolates into cracks in the rock, and refreezes. The expansion of freezing water is sufficient to dislodge rock fragments, which fall onto the glacier surface and are incorporated into the ice, contributing to further abrasion at the bed. Over thousands of years, this combination of plucking at the headwall and abrasion at the floor produces the characteristic bowl shape.
The floor of a mature cirque is typically flattened by a process in which the glacier erodes more effectively at its centre than at its margins. The result is a rock basin — often occupied by a small lake called a tarn after the glacier retreats — with a raised lip of bedrock or moraine at the downslope end where erosion was less intense.
Glaciological Behaviour
Cirque glaciers are relatively simple glaciological systems compared to large valley glaciers. They lack the long transport pathways of outlet glaciers or valley glaciers; ice formed in the accumulation zone does not travel far before reaching the ablation zone at the downslope edge. Flow velocities are slow — typically metres per year rather than the tens or hundreds of metres per year achievable by larger glaciers with steeper gradients.
Despite their small size, cirque glaciers respond measurably to climate forcing. Because their area is small, a given temperature increase has a proportionally larger impact on their mass balance than on a large glacier. Many cirque glaciers that persisted through the twentieth century are now at risk of disappearing entirely within decades. In ranges such as the European Alps, the Rocky Mountains, the Pyrenees, and the East African highlands, the number of active cirque glaciers has declined sharply since the mid-twentieth century. In the Pyrenees, for example, the small glaciers of the massif — including the Aneto and Monte Perdido glaciers, which are themselves large by Pyrenean standards — have retreated to a fraction of their former extent.
Reading Relict Cirques
The majority of the world's cirques are no longer occupied by ice. Relict cirques — empty rock basins above treeline — record the positions of glaciers that existed during cooler periods, particularly the Last Glacial Maximum (approximately 20,000 years ago) and the Little Ice Age (roughly 1300 to 1850 CE). By mapping the distribution and elevation of relict cirques across a mountain range, palaeoclimatologists can reconstruct the equilibrium line altitude (ELA) of past glaciers — the boundary between accumulation and ablation — which is a proxy for past temperature and precipitation.
In tectonically stable mountain ranges, the distribution of cirques at similar elevations across a range reflects the snowline that prevailed during the most recent major glaciation. In ranges where tectonic uplift is ongoing, the interpretation is more complex, but the basic principle still applies: cirque floors cluster at the former ELA, and the elevation difference between fossil ELA and modern ELA quantifies the magnitude of climate change.
Iconic Cirques and Their Glaciers
Some of the world's most famous mountain landscapes are defined by their cirques. The Cirque de Gavarnie in the French Pyrenees, a UNESCO World Heritage Site, is a natural amphitheatre of three concentric headwalls over a kilometre high, still partially glaciated and home to one of Europe's highest waterfalls. The Cwm on Everest — the Western Cwm, as named by Mallory — is a vast cirque basin at the head of the Khumbu Glacier, bounded by the south faces of Everest, Lhotse, and Nuptse. The Cirque of the Towers in Wyoming's Wind River Range is a celebrated rock climbing destination occupying a classic U-shaped cirque above Lonesome Lake.
The glacier map illustrates the global distribution of glaciated and recently glaciated terrain, including the mountain ranges where cirque glaciers are most concentrated and best preserved. Exploring these locations on the map provides a useful geographic framework for understanding how the small glaciers of the world's mountain ranges fit into the larger pattern of global ice coverage.
What Their Loss Means
The disappearance of cirque glaciers has consequences beyond the aesthetic. Many cirque glaciers contribute meltwater to small streams that feed larger river systems; in dry years, this water is disproportionately important for downstream ecosystems. Some cirque glacier basins provide habitat for specialist cold-water invertebrate communities. The tarns left behind when cirque glaciers retreat are ecologically distinct environments, cold and oligotrophic, that support their own biological communities. The transition from glaciated cirque to ice-free basin is a measurable ecological change as much as a physical one.