Earth’s glaciers are incredibly varied in their size and shape, ranging from small ice masses that cling precariously to steep mountain sides, to vast ice sheets that submerge entire continents below kilometres thick ice1,2.
The form, shape and structure – known as the morphology – of these two extreme examples, as well as all glacier types in between, is a function of two key variables: climate and topography.
Climate
Climate controls the annual temperature cycle of a region as well as the amount of precipitation that falls as snow. Because of this, climate governs the annual mass balance of glaciers and hence their size (a key part of glacier morphology).
Where climatic conditions lead to mass inputs (e.g. snowfall) that are larger than mass outputs (e.g. melting) a glacier will grow. Conversely, where mass outputs exceed mass inputs a glacier will shrink.
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All other factors being equal, therefore, it follows that the coldest places on Earth’s surface, the polar regions, will contain the largest and most extensive glaciers. However, climate is only one part of the story.
Topography
Topography is also a major control on glacier morphology. Topography not only provides the land surface (e.g. high altitude mountains) on which glacial ice can develop, but it also controls the physical dimensions of glaciers and how they flow.
Consider this example. A deep valley that is several kilometres in length will contain a thicker and longer ice mass than a small mountain cirque. Because of its greater thickness, this hypothetical valley glacier will, in turn, flow more rapidly because thicker ice increases driving stresses at the glacier bed and raises basal temperatures3, which increase the rate of ice deformation and basal sliding.
Types of glacier
Bearing in mind the combined influences of climate and topography in shaping glacier morphology, the broad range of glacier types at Earth’s surface fit into two main groups, known as unconstrained glaciers and constrained glaciers1,2, which are defined as follows:
- Unconstrained glaciers have a morphology and flow pattern that is in the most part independent of underlying topography, whereas;
- Constrained glaciers have a morphology and flow pattern that is strongly dependent on underlying topography.
Unconstrained glaciers
Ice sheets and ice caps
Ice sheets and ice caps take the same basic form, having a broad, upstanding, and slowly moving ice dome at their centre, with channels of faster moving ice that transfer mass to their margins.
However, they differ in terms of scale2. Ice sheets are larger, being more than 50,000 km2 in size, with ice domes that can be more than 3000 m thick. In contrast, ice caps only reach thicknesses of several hundred metres.
There are two more key features of ice sheet and ice cap morphology. Firstly, they almost completely submerge the landscape, with only the tips of mountain peaks (known as nunataks) piercing the ice surface.
Secondly, their flow patterns are (at least in the most part) unaffected by underlying topography. The exception to this general rule are the fast-flowing ice streams and outlet glaciers that often reside within glacial troughs closer to the periphery of ice sheets and ice caps4,5.
Ice streams
Ice streams are corridors of rapidly moving ice in an ice sheet4. A feature unique to ice streams is that they are bordered on either side not by bedrock, but by slowly moving ice.
Ice streams are extremely large (>20 km wide and >150 km long) and when viewed from space, we observe that they are fed by numerous tributaries that are connected to a central ice dome6,7. Ice streams are critically important to the overall dynamics and mass balance of ice sheets as they control the vast majority (~90% in Antarctica) of ice and sediment discharge to the oceans4,6,7.
Constrained glaciers
Ice fields
Unlike ice caps, ice fields do not have a simple dome-like structure. Instead, their morphology and flow are controlled by topography. Ice fields (such as the Patagonian ice fields) develop in mountainous terrain where the land surface reaches an altitude that enables snow and ice to accumulate. They are drained by large valley glaciers.
Valley glaciers
Valley glaciers (as their name suggests) exist within bedrock valleys and are overlooked by ice-free slopes. They are found in many alpine and high mountain environments, including the European Alps, Southern Alps of New Zealand, the Andes, and the Himalayas (to name just a few).
Valley glaciers are fed in their upper parts by ice and snow discharged from surrounding ice fields or cirques (see the Aletsch Glacier above) in addition to snow and ice avalanches from overlooking slopes. In terms of morphology, valley glaciers can be single features or made up of a branching network of tributaries (see image below), and range in length from several kilometres to over 100 kilometres.
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tributary glaciers feeding the main trunk of Columbia Glacier (centre).
Transection glaciers
Transection glaciers are, in essence, a system of interconnected valley glaciers that flow in several different directions, often in a radiating (or web-like) pattern. Transection glacier networks develop where bedrock valleys are deeply dissected, allowing ice to overflow the cols between adjacent valleys.
Examples of active transection glaciers can be found in Greenland, Svalbard (see above), and Alaska. Such systems also developed during the last glacial period in the European Alps8, and parts of the Loch Lomond Stadial ice cap in Scotland are also thought to have formed transection glacier networks9.
Piedmont glaciers
Piedmont glaciers have a distinctive form characterised by large terminal ice lobes that splay outwards onto lowland terrain after exiting a confining bedrock valley. Topography, therefore, exerts varying degrees of control on piedmont glacier morphology and flow at different points along the glacier length.
Another common feature is that large areas of a piedmont glacier are situated below the equilibrium line altitude in the ablation zone. The Malaspina Glacier in Alaska (see image above) is the most famous example of a piedmont glacier. This glacier, which drains the Mt. St. Elias ice field, has a terminal lobe that is around 40 km long and almost 65 km across at its widest.
Cirque glaciers
Cirque glaciers are among the most common types of glacier on Earth, being found in nearly all alpine landscapes that support ice accumulation. Cirque glaciers are either localised to armchair-shaped bedrock hollows on a mountain side (see image below), or to the uppermost parts of a glacial trough, where they flow into larger valley glaciers.
The morphology of a cirque glacier largely depends on the topography in which it sits. The cirque basin itself dictates the size and shape of the cirque glacier and directs its flow, while the terrain surrounding a cirque basin is an important source of wind-blown snow and therefore glacier mass balance10.
Niche glaciers
Smaller in size than cirque glaciers, niche glaciers form where ice accumulates in a mountain side recess (or niche), such as a rock bench, couloir, or depression. Niche glaciers represent the early stages of glacier development and are commonly found in climatically favourable settings, such as in shaded north-facing slopes of mountains in the Northern Hemisphere11. Similar to niche glaciers, but adhering to steep mountain sides, are ice aprons.
