Glacier

A glacier is a large body of natural ice that moves slowly on its own. It forms where lots of snow builds up faster than it melts away, often taking many years. Glaciers move and change shape because of gravity, making interesting patterns like cracks called crevasses.

Most of Earth’s glacial ice is located in huge ice sheets near the polar regions, but glaciers also exist in mountain ranges on every continent except Australia. They cover about 10% of Earth's land and store a lot of the world's fresh water. Glaciers also play a role in climate change and sea level.

Glacial ice often looks blue. This happens because the large amount of water makes it appear blue, and the pressure pushes out air bubbles, making the ice denser. The blue color helps us spot glaciers from far away.

Etymology and terminology

The word glacier comes from French. It began from a Latin word meaning "ice." When we talk about things related to glaciers, we call them glacial. The study of glaciers is called glaciology, and glaciers are part of something called the cryosphere, which includes all the ice on Earth.

Types

Glaciers can be grouped by their size, shape, and how they behave. Alpine glaciers form on mountain slopes. When a glacier fills a valley, it is called a valley glacier. A large mass of ice on top of a mountain, mountain range, or volcano is called an ice cap or ice field. Ice caps are smaller than ice sheets, which are huge glaciers that cover large areas like Antarctica and Greenland.

Glaciers also have different temperature states. A temperate glacier is at the melting point all year. A polar glacier stays below freezing all the time. A subpolar glacier has both temperate and polar ice.

Formation

Glaciers form where lots of snow and ice build up more than they melt away. They usually start in a special curved shape in the mountains called a cirque. The snow piles up, gets pressed down, and turns into something called névé, then firn, and finally glacial ice. When there is enough ice, it starts to move down the slope because of gravity.

Color

Glacial ice looks blue because it absorbs some red light. This is due to how water molecules vibrate. Liquid water can also appear blue for the same reason. While some think the blue color comes from tiny air bubbles in the ice, this is not true.

Structure

A glacier starts at a place called its glacier head and ends at its glacier foot, snout, or terminus.

Glaciers have different parts based on how much snow they gain and lose. The part where more snow is lost than gained is called the ablation zone. The upper part where more snow is gained than lost is called the accumulation zone. The line between these two zones is where the amount of new snow equals the amount of ice lost. The accumulation zone usually covers most of the glacier’s surface.

After a glacier melts, it often leaves behind a bowl-shaped hole. These holes can be as big as the Great Lakes or smaller mountain holes called cirques.

The accumulation zone can also be divided into parts based on whether the snow melts:

1. The dry snow zone where no melting happens, even in summer.
2. The percolation zone where some surface melting occurs, and meltwater moves into the snow.
3. The superimposed ice zone near the line where meltwater refreezes.
4. The wet snow zone where all the snow since last summer has reached 0 °C.

Since around 1850, after the Little Ice Age ended, glaciers around the Earth have retreated substantially. Glaciers are made of glacial ice, a special kind of rock mostly made of the mineral Ice I_h, with tiny bits of trapped air, dirt, and other materials frozen inside. In geology, glacial ice is a type of metamorphic rock that is very close to its melting point. It forms mainly from snow that falls, but also changes a little through melting and refreezing.

Motion

Glaciers move downhill because of gravity and changes in the ice. Ice is made of layers that can bend and flow like a soft solid when squeezed. Ice must be at least 30 meters thick to start moving, and it flows faster when it's thicker.

The slowest parts of a glacier are near the bottom and sides, where friction slows it down. The middle and top move faster. Thicker ice can flow faster, changing the shape of the land.

Glaciers can crack and break. The top part of a glacier can break into deep cracks called crevasses when moving over uneven ground. Below this, the ice is softer and doesn’t crack as easily.

Crevasses form in different ways, such as on steep slopes or where the glacier widens. These cracks make traveling on glaciers dangerous.

How a glacier moves depends mostly on where it touches the ground. The ground’s temperature, roughness, and softness decide if the glacier slides or moves through the soil. Water under the glacier can change its speed, depending on the slope, thickness, and amount of meltwater.

Some glaciers move very fast for short times, called surges. This can happen when meltwater builds up or the glacier gets too heavy. These fast movements can cause big earthquakes.

Ogives are patterns of dark and light bands on glacier surfaces. They show how the glacier moves each year, with each pair of bands matching one year’s movement.

Geography

Further informationon this topic: List of glaciers

Glaciers can be found on every continent except Australia and South Africa, though some faraway islands have glaciers. Large glaciers cover places like Antarctica, Argentina, Chile, Canada, Pakistan, Alaska, Greenland, and Iceland. Smaller glaciers are common in mountain ranges such as the Andes, the Himalayas, the Rocky Mountains, the Caucasus, Scandinavian Mountains, and the Alps. The Snezhnika glacier in Pirin Mountain, Bulgaria is the southernmost glacier in Europe. Although mainland Australia has no glaciers today, a small glacier existed on Mount Kosciuszko during colder times long ago. In New Guinea, tiny glaciers are located on Puncak Jaya, but they are shrinking quickly. Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya, and in the Rwenzori Mountains. Islands with glaciers include Iceland, islands near Norway such as Svalbard and Jan Mayen, New Zealand, and several subantarctic islands like Marion, Heard, Grande Terre (Kerguelen), and Bouvet.

Glaciers need lots of snow that stays all year to form. Where the land slopes, how much snow falls, and the wind all affect where glaciers can grow. Glaciers exist in many places around the world, except between 20° and 27° north and south of the equator, where it is usually too warm and dry for snow to build up. Even in very cold places, glaciers may not form if there is not enough snow. For example, some parts of the Arctic and Antarctica are so dry that glaciers cannot form, even though it is very cold there. Similarly, some very high and cold mountains in Bolivia, Chile, and Argentina do not have glaciers because they are in very dry areas with little snow.

Glacial geology

Glaciers change the shape of the land in two main ways: they pick up rocks and grind them down.

When glaciers move, they can pick up pieces of rock from the ground below. This happens when water freezes under the glacier and pushes rocks into the ice. These rocks then travel with the glacier.

The rocks carried by glaciers grind against the land, making it smooth. This process creates fine powder from the rocks, called rock flour. It also leaves deep scratches on the ground and can shape mountains and valleys over time.

Glaciers leave behind many different kinds of rocks and dirt when they move. Some of these form long ridges, while others create hills or valleys. These changes can stay long after the glacier is gone.

Retreat of glaciers due to climate change

South Cascade Glacier in Washington has been photographed from 1928 to 2003, showing how it has been shrinking quickly.

Human actions have increased gases in the air that trap heat. This has led to global warming, which is why glaciers are changing. As glaciers melt, they expose darker land and water, which absorbs more heat and causes even more melting. This process has been happening every year since 1988. Melting glaciers also add water to the oceans, causing sea levels to rise. This can harm coastal areas, islands, and ecosystems.

In 2023, Switzerland lost 4% of its glacier volume. The United Nations declared 2025 the International Year of Glaciers' Preservation to highlight the importance of protecting glaciers.

Glaciers as climate monitors

See also: Glacier Loss Day

Glaciers are very old and can help us understand Earth's climate history. Scientists study pieces of glacier ice, which contain tiny bubbles of air from different times. By analyzing these bubbles, they can learn about past changes in the atmosphere and temperatures, showing that carbon dioxide levels and global temperatures have been connected for at least a million years.

Isostatic rebound

Big masses like ice sheets and glaciers can push down the Earth's crust into the soft mantle below. This pushing down is usually about a third of the ice's thickness. When the ice melts, the mantle slowly moves back to its old position, lifting the crust up again. This slow lifting is called post-glacial rebound. It is happening now in places like Scandinavia and the Great Lakes region of North America.

A similar process on a smaller scale makes features called dilation-faulting. This occurs when rocks that were squeezed by ice go back to their old shape faster than they should. This can create cracks, like what might happen if the rock were hit by a large hammer. We can see dilation faulting in parts of Iceland and Cumbria that were recently freed from glaciers.

On other planets

The polar ice caps of Mars show signs that glaciers once existed there. The south polar cap looks like glaciers on Earth. Scientists think there were more glaciers on Mars in the past, especially between 35° and 65° north or south. Because Mars has very little air, these glaciers lose ice by turning straight into vapor instead of melting. Many Martian glaciers are covered with rocks that protect the ice underneath.

In 2015, the New Horizons spacecraft flew by Pluto and found a large area named Sputnik Planitia covered in nitrogen ice. Glaciers were also seen near the edges of Sputnik Planitia.