Glacier

A glacier is a large, long-lasting body of natural ice. Glaciers are a form of rock that is always moving under its own weight. A glacier forms where snow builds up faster than it melts away (ablation), often over centuries. Glaciers slowly flow and change shape because of gravity. They can also develop features such as crevasses and seracs.

As a glacier moves, it wears down the rock and debris beneath it. This creates glacial landforms such as cirques, moraines, and fjords. A glacier may flow into water and can include ice shelves and ice tongues. Still, it starts on land. This makes it different from the much thinner sea ice and lake ice that form on freezing water.

Most of the ice on Earth, about 99%, is held in huge ice sheets (also called "continental glaciers") in the polar regions. Other glaciers are found in mountain ranges on every continent except the Australian mainland. They can also be found on oceanic island countries with high latitudes, such as New Zealand. Between 35°N and 35°S, glaciers appear only in the Himalayas, the Andes, and a few high mountains in East Africa, Mexico, New Guinea, and Zard-Kuh in Iran. Pakistan has more glacial ice than any other country outside the polar regions. Glaciers cover about 10% of Earth's land. Nearly all of Antarctica is covered by continental glaciers. Patagonia and Greenland also have large areas of them.

Glacial ice is the largest store of fresh water on Earth. Together with the ice sheets, it holds about 69% of the world's freshwater. Many glaciers in temperate, alpine, and seasonal polar climates store water as ice in cold seasons. They release it later as meltwater when summer warmth melts them. This creates a water source that is very important for plants, animals, and people when other sources are hard to find. Glaciers are sensitive to long-term changes in precipitation, mean temperature, and cloud cover. Because of this, glacial mass changes are among the clearest signs of climate change. They are also a major cause of changes in sea level.

Glaciers appear blue because, like large amounts of water appear blue, water molecules absorb other colors more strongly than blue. The lack of air bubbles also adds to the blue color. Air bubbles make ice look white, but pressure squeezes them out as the ice becomes denser.

Etymology and terminology

The word glacier is a loanword from French. It came through Franco-Provençal from the Vulgar Latin word glaciārium. That word came from the Late Latin glacia, and in the end from Latin glaciēs, which means "ice".

Things caused by or related to glaciers are called glacial. The process of a glacier forming, growing, and flowing is called glaciation. The study of glaciers is called glaciology. Glaciers are an important part of the cryosphere, which is the part of Earth covered in ice and snow.

Types

Glaciers are grouped by their shape, temperature, and behavior. Alpine glaciers form on mountain tops and slopes. A glacier that fills a valley is called a valley glacier, an alpine glacier, or a mountain glacier. A large mass of glacier ice on a mountain, mountain range, or volcano is called an ice cap or ice field. Ice caps cover less than 50,000 km² (19,000 sq mi). Bigger masses are called ice sheets or continental glaciers. They are several kilometers deep and cover the land beneath them. Only nunataks stick out above the surface. Only two ice sheets exist today, covering most of Antarctica and Greenland. They hold so much water that if both melted, sea levels would rise by over 70 m (230 ft). Parts that reach into water are called ice shelves, and fast-moving sections are called ice streams. Many ice streams in Antarctica flow into large ice shelves. Others flow directly into the sea, like Mertz Glacier, often with an ice tongue. Glaciers that end in the sea are called tidewater glaciers. These include most glaciers flowing from Greenland and Antarctica, and glaciers in Baffin, Devon, and Ellesmere Islands in Canada, Southeast Alaska, and the Northern and Southern Patagonian Ice Fields. Pieces of ice break off as icebergs. They often fall from above the sea and hit the water hard.

Glaciers can also be grouped by temperature. A temperate glacier is at its melting point all year, from top to bottom. A polar glacier stays below freezing from top to bottom, though its snowpack may melt in summer. A subpolar glacier has both temperate and polar ice, depending on depth and position. Glaciers can also be described by their base temperature. A cold-based glacier is frozen to the ground beneath it. A warm-based glacier can slide along the ground. Sliding ice helps a glacier erode its bed, because it helps plucking rock loose from the surface below. A glacier that is partly cold-based and partly warm-based is called polythermal.

Formation

Glaciers form where more accumulation of snow and ice happens than ablation. A glacier usually starts in a cirque, which is also called a corrie or a cwm. This is a bowl-shaped dip, like a hollow between mountains, with edges called arêtes. The cirque collects snow that falls into it. Gravity presses the snow down. The snow freezes and packs together into névé, which is grainy snow. As the snowflakes are pressed further and air is pushed out, the snow becomes firn and then "glacial ice." The ice fills the cirque until it spills over a weak spot in the edge, called the "lip" or threshold. When the mass of snow and ice is thick enough, it starts to move because of the slope, gravity, and pressure. On steep slopes, this can happen with as little as 15 m (49 ft) of snow and ice.

In temperate glaciers, snow repeatedly freezes and thaws and turns into granular ice or névé. The weight of the layers above presses this grainy snow into denser firn. Over many years, the firn is packed even more tightly and becomes glacial ice. Glacier ice is slightly denser than ice made from frozen water because it has fewer trapped air bubbles.

Color

Glacier ice has a blue tint because it absorbs some red light. This happens due to an overtone of the infrared OH stretching mode of the water molecule. Liquid water looks blue for the same reason.

Some people think the blue color of glacier ice comes from Rayleigh scattering of bubbles in the ice, but this is a common misunderstanding.

Structure

A glacier starts at its glacier head and ends at its glacier foot, snout, or terminus. Glaciers are divided into zones based on snow and melting. The upper part is the accumulation zone, where more snow is gained than lost. The lower ablation zone is where the glacier loses more ice than it gains. The equilibrium line separates the two zones. The accumulation zone usually covers 60–70% of a glacier's surface. After a glacier melts, it often leaves a bowl-shaped dip. These dips can be large basins, like the Great Lakes, or small mountain hollows called cirques. In parts of the accumulation zone, meltwater refreezes into ice lenses and layers.

Scientists check the health of a glacier by looking at its glacier mass balance or by watching how its terminus moves. Healthy glaciers have large accumulation zones and a terminus that flows strongly. After the end of the Little Ice Age around 1850, glaciers around the Earth have retreated substantially. Glaciers are made of glacial ice, a type of rock made mostly of the mineral Ice I_h, with small amounts of trapped gases and debris. Glacial ice forms mainly from snowfall, a sedimentary process, with smaller igneous processes such as melting and refreezing. Scientists who study structural geology use its methods to describe and understand glaciers.

Motion

Glaciers move downhill because of gravity. The ice also slowly bends and changes shape inside. Ice needs to be at least 30 m (98 ft) thick to start flowing. When it is thicker than about 50 m (160 ft), it can move in a plastic flow, which lets the glacier spread across the land. The ice moves slowest near the bottom and sides, where friction slows it down. It moves fastest at the surface. Thicker ice moves faster, so glaciers wear away the land most where the ice is deepest. This helps explain why fjords can be so deep, while nunataks hardly wear away at all. Scientists first understood this movement in the 1840s, thanks to James Forbes.

The top 50 m (160 ft) of a glacier is stiff and mostly moves as one piece. When different parts move at different speeds, crevasses (deep cracks) can open up. Some crevasses form a bergschrund at the edge of the glacier. Hidden crevasses covered by snow bridges can be dangerous to cross. Meltwater flows through streams and moulins inside the glacier. Glaciers can also slide over the ground on a thin layer of water. This is called basal sliding. Friction and geothermal heat can melt the ice at the bottom, and this meltwater helps the glacier slide faster. Some glaciers suddenly speed up in surges. Very large movements can cause glacial earthquakes. A large part of a glacier can also break away and slide down the slope in a glacier collapse.

Further informationon this topic: List of glaciers

Glaciers are found on every continent and in about fifty countries. Australia and South Africa have glaciers only on far-off subantarctic islands. Large glaciers are found in Antarctica, Argentina, Chile, Canada, Pakistan, Alaska, Greenland and Iceland. Mountain glaciers are common in the Andes, the Himalayas, the Rocky Mountains, the Caucasus, the Scandinavian Mountains and the Alps. The Snezhnika glacier on Pirin Mountain in Bulgaria is the southernmost glacier in Europe. Mainland Australia has no glaciers today. A small one on Mount Kosciuszko existed during the last glacial period. Small glaciers on Puncak Jaya in New Guinea are shrinking quickly. Africa has glaciers on Mount Kilimanjaro in Tanzania, Mount Kenya and in the Rwenzori Mountains. Island glaciers are found on Iceland, on islands off Norway including Svalbard and Jan Mayen, in New Zealand, and on the subantarctic islands of Marion, Heard, Grande Terre (Kerguelen) and Bouvet. During the glacial periods of the Quaternary, Taiwan, Hawaii on Mauna Kea and Tenerife also had large mountain glaciers. The Faroe and Crozet Islands were completely covered by ice.

Glaciers need snow that stays all year. This depends on the slope of the land, how much snow falls, and the winds. Glaciers can be found at almost all latitudes, except between 20° and 27° north and south of the equator. There, the Hadley circulation dries the air so much that, with strong insolation, snow lines rise above 6,500 m (21,330 ft). Between 19˚N and 19˚S, more rain and snow fall, and mountains above 5,000 m (16,400 ft) usually have permanent snow. Even at high latitudes, glaciers do not always form. Parts of the Arctic, such as Banks Island, and the McMurdo Dry Valleys in Antarctica are polar deserts. Very little snow falls there, even though it is bitterly cold, because cold air cannot hold much water vapor. Even during the Quaternary glacial periods, Manchuria, lowland Siberia, and central and northern Alaska were too dry for glaciers to form. Some high, cold mountains and volcanoes in Bolivia, Chile and Argentina also lack glaciers, because they get too little precipitation. They are near or inside the very dry Atacama Desert, which is also hyperarid.

Glacial geology

Glaciers wear away land in two main ways: plucking and abrasion. In plucking, water seeps into cracks in the rock, freezes, and expands, lifting blocks of rock into the ice. In abrasion, ice and rock pieces slide over the bedrock like sandpaper and smooth it. The fine powder this makes is called rock flour. Scratches called glacial striations and lines called chatter marks can show which way the glacier moved. Rocks carried far from where they started are known as "glacial erratics." Glaciers leave behind till and other material in piles called moraines, including ground moraines, also called glacial drift. The word moraine comes from peasants in the French Alps, who used it for ridges near the edges of glaciers. Drumlins are canoe-shaped hills, and many grow in groups called drumlin fields, such as one east of Rochester, New York. Glaciers also change mountain valleys. A "V" shape valley can become a U-shaped valley, with truncated spurs, paternoster lakes, and hanging valleys left behind. If the valley reaches the sea, it can become a fjord. Mountain tops can be shaped into arête ridges, mountain pass gaps, pyramidal peaks, and horns. Ice can also shape bare rock into a roche moutonnée, or "sheepback" rock.

As meltwater flows away from a glacier, it drops fine sediment, which can build an alluvial plain. In a valley, this is called a valley train. In an estuary, the sediment is called bay mud. Small lakes called kettles can form when buried ice blocks melt. When a glacier shrinks, its meltwater leaves stratified deposits such as terraces, which remain as "glacial deposits." Hill-shaped deposits are called kames, and long, winding ones are called eskers. Some kames are built by deltas of meltwater. Very fine sediment can be carried by wind over long distances, forming deep loess deposits in places such as China and the Midwestern United States. Katabatic winds can help this process. Water flowing from melting ice also carries sediment in fluvial ways, and wind-blown sediment is called eolian.

Retreat of glaciers due to climate change

Further information: Retreat of glaciers since 1850

South Cascade Glacier in Washington has photos from 1928 to 2003. They show how much the glacier has shrunk. Around the world, glaciers have been losing ice for many years, and each year they lose more. If countries keep their current promises, scientists expect that global warming will cause about half of Earth's glaciers to disappear by 2100. This could also raise sea level by about 115 mm (about 4.5 in.), not counting melting ice sheets. People have added carbon dioxide and other greenhouse gases to the air. These gases trap heat and cause global warming. Human activity is the main reason for changes to the cryosphere, which includes glaciers.

See also: Glacier Loss Day

Glaciers can make the planet warm even more. When temperatures rise, glaciers melt and reveal darker land and sea. These dark areas soak up more sunlight and get warmer. These positive feedback loops are part of ice–albedo feedback. Reference glaciers tracked by the World Glacier Monitoring Service have lost ice every year since 1988. Melted glacier water is adding to the rise in global sea level. The IPCC describes this as a "slow onset" event. In 2023, Switzerland lost 4% of its glacier volume, which was much faster than in earlier years. In 2025, the United Nations announced the International Year of Glaciers' Preservation to recognise that climate change is melting glaciers. Some glaciers are hundreds of thousands of years old, so scientists use them to learn about the climate over long periods. Researchers study ice cores, where deeper layers show earlier times. Gas bubbles trapped in these layers show how the air has changed. They also suggest that for at least the last million years, global temperatures have followed carbon dioxide levels.

Isostatic rebound

Large masses of ice, such as ice sheets or glaciers, can push the Earth's crust down into the mantle. The crust sinks by about a third of the ice's thickness. After the ice melts, the mantle slowly flows back to its original place, and the crust rises again. This post-glacial rebound is very slow, but it can be measured today in Scandinavia and the Great Lakes region of North America.

A similar process on a smaller scale creates a feature called dilation-faulting. It happens when squeezed rock springs back to its original shape faster than it can hold together without breaking. It is a bit like the rock being hit by a large hammer. Dilation faulting can be seen in parts of Iceland and Cumbria where glaciers have melted away recently.

On other planets

The polar ice caps of Mars have features that look like glaciers. The south polar cap is especially similar to glaciers on Earth. Landforms and computer models suggest that Mars had more glaciers in the past. The Martian air is very thin, so ice near the surface turns straight into gas through sublimation, instead of melting. Like many glaciers on Earth, many Martian glaciers are covered by a layer of rocks that protects the ice. A radar instrument on the Mars Reconnaissance Orbiter found ice under a thin layer of rocks in formations called lobate debris aprons.

In 2015, the spacecraft New Horizons flew past the Pluto–Charon system. It discovered a huge basin called Sputnik Planitia on Pluto. The basin is covered in a layer of nitrogen ice. Much of its surface is split into irregular shapes separated by narrow troughs. These are thought to be convection cells, which are powered by heat from inside Pluto. Glacier-like flows were also seen near the edges of the basin. They appear to move both into and out of the basin.