A glacier is a large body of ice that moves slowly under its own weight. It may look frozen in place, but over time ice flows like an extremely thick liquid. Over thousands of years, that slow motion carves some of the most dramatic landscapes on the planet.
Key takeaways
- Glaciers form where more snow falls than melts, year after year.
- Snow compacts into dense ice that begins to flow under gravity.
- Glaciers move by internal deformation and by sliding over their beds.
- They erode rock, carrying debris that carves valleys and leaves distinctive landforms.
- Many glaciers are retreating as the climate warms, which is closely monitored.
From snow to ice
In cold places, fresh snow that survives the summer is buried by the next year's snowfall. The weight of the layers above squeezes out air and gradually turns the snow into granular firn and then into solid glacier ice. This process can take years to centuries depending on temperature and snowfall. Once a mass of ice becomes thick enough, typically tens of metres, it starts to deform and move.
How ice moves
Glaciers move in two main ways. In internal deformation, the crystals within the ice slowly shift and slide past each other under pressure. In basal sliding, meltwater at the bottom acts as a lubricant, letting the whole glacier slip over the rock beneath it. Speeds vary widely, from a few centimetres to many metres per day for fast outlet glaciers. The top of a glacier can crack into crevasses where the ice is stretched, which is one reason travelling on glaciers is hazardous.
Accumulation and ablation
A glacier is usually described as having two zones. The accumulation zone at higher elevation gains snow, while the ablation zone lower down loses ice through melting and other processes. If accumulation matches loss over a long period, the glacier stays about the same size. If loss exceeds gain, it retreats, and if gain exceeds loss, it advances.
How glaciers shape landscapes
Moving ice acts like a conveyor belt and sandpaper. It plucks rocks from the bed and scrapes the surface with embedded debris. Mountain glaciers carve bowl-shaped hollows called cirques and wide, U-shaped valleys that look very different from the V-shaped valleys cut by rivers. Where valleys meet the sea, they can become fjords. When glaciers melt, they leave behind piles of rock and sediment called moraines, long ridges known as eskers, and lakes filling hollows.
Glaciers and sea level
Glaciers on land, along with ice sheets in Greenland and Antarctica, hold large amounts of fresh water. When land ice melts and flows into the ocean, it adds to sea level. Floating ice shelves and sea ice do not raise sea level when they melt in the same way, because they are already displacing water.
Why they are monitored
Scientists track glaciers with satellites, aircraft and field measurements because changes reveal information about climate over time. Many mountain glaciers are shrinking, which affects local water supplies that depend on seasonal meltwater.
Types of glaciers
Glaciers come in several forms. Valley glaciers flow down mountain valleys, like many in the Alps, the Himalaya and the Andes. Cirque glaciers sit in bowl-shaped hollows high in the mountains. Piedmont glaciers spread out where a valley glacier reaches flat ground. Ice caps are domes of ice that cover highland areas, and ice sheets are the vast continental-scale masses of Greenland and Antarctica. Tidewater glaciers flow into the sea, where chunks break off in a process called calving to form icebergs.
Famous glaciers and ice sheets
Some glaciers are well known. Perito Moreno in Argentina is a spectacular glacier that ends in a lake, and it has been unusually stable compared with many others. Jakobshavn Glacier in Greenland is one of the fastest-flowing and has been studied extensively as a source of icebergs. Thwaites Glacier in West Antarctica is nicknamed the doomsday glacier because its collapse could raise sea levels significantly, so it is a focus of research. The Lambert-Fisher glacier system in Antarctica is among the largest in the world.
Ice ages and glacial history
Glaciers have advanced and retreated many times. During the Pleistocene epoch, which began about 2.6 million years ago, cycles of cold glacial periods and warmer interglacials occurred, driven largely by slow changes in Earth's orbit. At the last glacial maximum, about twenty thousand years ago, ice sheets covered much of Canada, northern Europe and parts of Asia, and sea level was well over a hundred metres lower than today. Many landscapes, such as the Great Lakes and Scandinavian fjords, were shaped by this ice.
Retreat and water supplies
Many mountain glaciers are shrinking, which affects people who rely on their meltwater. In the Himalaya, the Andes and the Alps, glaciers store water in winter and release it in the warm season, helping rivers flow in dry periods. As glaciers shrink, meltwater at first increases, a stage sometimes called peak water, and then declines. Communities downstream may face reduced supplies, and the lakes that form at glacier edges can burst and cause flooding. Planning for these changes is a growing challenge.
How scientists monitor glaciers
Researchers measure glacier mass balance, the difference between accumulation and loss, by drilling stakes into the ice and tracking their height over the year. Satellites and aircraft measure changes in area, thickness and speed, and radar can map the ice below the surface. Ice cores, drilled from deep within glaciers and ice sheets, contain air bubbles and dust that record past climate. Together these methods give a detailed picture of how ice is changing.
Frequently asked questions
Is glacier ice different from freezer ice?
Glacier ice is formed from compressed snow and often contains tiny trapped air bubbles, which can make it look blue because dense ice absorbs red light.
Do glaciers move backwards?
Ice always flows downhill, but the front edge can retreat when melting at the end is faster than the ice supply from above.
How old is glacier ice?
It varies. Ice at the bottom of the thickest ice sheets can be hundreds of thousands of years old.
How fast do glaciers move?
Speeds vary widely. Many valley glaciers move tens of metres a year, while fast-flowing outlet glaciers in Greenland can move many metres a day. Speed depends on slope, ice thickness, temperature and the amount of meltwater beneath.
What is calving?
Calving is when large chunks of ice break off from the front of a glacier or ice shelf that ends in water, forming icebergs. It is a natural process, but it can speed up when glaciers thin and lose support.
Why do glaciers look blue?
Dense glacier ice absorbs red light more than blue light, so the light that is scattered back to our eyes is blue. The effect is strongest in compact ice with few air bubbles.
Sources and further reading
Written by
FactNest Editorial Team
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