Glacier Formation, Movement, and Mass Balance
A glacier is a large, long-lasting mass of ice that forms from compacted snow on land and flows slowly downhill under its own weight; its mass balance is the difference between accumulation (snow gained) and ablation (ice lost by melting and other processes) over a year. In brief: snow turns into firn and then dense glacial ice as it is buried and compressed. Glaciers move by internal deformation and by sliding on a base lubricated with meltwater. If accumulation exceeds ablation, the glacier gains mass and advances; if ablation exceeds accumulation, it loses mass and retreats.
Glaciers and ice sheets store about 69% of the world’s fresh water and respond sensitively to climate. This guide explains how glacial ice forms, the main types of glaciers, how they move, how mass balance is measured, compares glacier types in a table, works through calculations, outlines the history of glaciology and corrects common mistakes.
How do glaciers form?
A glacier forms where more snow falls in winter than melts in summer, year after year. Fresh snow is light and fluffy, with a density of roughly 50 to 100 kg/m3. As more snow buries it, the snowflakes are compressed, partly melt and refreeze, and become rounded grains called firn (névé), with a density of about 400 to 830 kg/m3. With more burial, air is squeezed out and the grains fuse into glacial ice, with a density of about 830 to 917 kg/m3. The process takes years in a warm, wet climate and decades to centuries in cold, dry places such as Antarctica. Once the ice is thick enough, usually about 30 m or more, it begins to flow.
What types of glaciers are there?
- Valley (alpine) glaciers: rivers of ice in mountain valleys, such as in the Alps, Himalaya and Alaska.
- Cirque and hanging glaciers: small glaciers in hollows or on steep slopes.
- Piedmont glaciers: where valley glaciers spread out on a plain at the foot of mountains.
- Ice caps: domes of ice covering highlands, such as in Iceland (Vatnajökull).
- Ice sheets: continental masses of ice larger than 50,000 km2. Only two exist today: Antarctica, which holds about 90% of the world’s ice, and Greenland. If Greenland melted completely, sea level would rise by about 7 m, and for Antarctica, about 58 m.
Glaciers can also be classified by temperature. Warm-based (temperate) glaciers are at the melting point at the base, so they have liquid water and slide, and move relatively fast. Cold-based (polar) glaciers are frozen to their bed, move mainly by internal deformation and erode little. About 10% of the land surface is covered by glacial ice today, compared with about 30% at the peak of the last ice age.
How do glaciers move?
Gravity causes ice to flow from high to low ground. There are two main mechanisms. Internal deformation (creep) occurs as ice crystals deform and slide past each other, and layers move at different speeds, so the ice at the surface moves faster than at the bed. Basal sliding happens when the glacier slides over its bed on a thin film of meltwater, which is made by pressure and by heat from friction and from the ground. Ice may also move over soft sediment that deforms underneath. Rotational flow occurs in corries, where ice moves in a curved path.
Most glaciers move only a few meters to a few hundred meters per year, but some, such as the outlet glaciers of Greenland, flow at several kilometers per year. A surging glacier advances rapidly for a short period after a long quiet phase. The speed is highest at the center of the surface, away from the friction at the valley walls, and in the middle section near the equilibrium line. Tensional stress opens crevasses in the brittle upper ice where the glacier flows over steps or bends.
What is glacier mass balance?
A glacier has an accumulation zone in its upper part, where snow gain exceeds loss, and an ablation zone lower down, where melting, evaporation (sublimation) and calving of icebergs exceed gains. The boundary between them is the equilibrium line (the equilibrium line altitude, ELA). Net mass balance = accumulation − ablation, usually given in meters of water equivalent per year. A positive balance leads to growth and advance, and a negative balance leads to thinning and retreat. A glacier is in equilibrium if the net balance is about zero over several years.
Because the ice takes time to flow, the terminus (snout) responds with a delay to a change in mass balance. Since the 1980s, most glaciers around the world have lost mass, which contributes to sea-level rise and to changes in river flow and water supply in regions that depend on meltwater. Glaciologists measure mass balance with stakes in the ice, snow pits, satellite altimetry and gravity measurements.
Glacier types compared
| Type | Setting | Typical size | Example |
|---|---|---|---|
| Valley glacier | Mountain valley | Kilometers to tens of km long | Aletsch Glacier, Switzerland |
| Ice cap | Highland dome | Under 50,000 km2 | Vatnajökull, Iceland |
| Ice sheet | Continental | Over 50,000 km2 | Greenland, Antarctica |
| Piedmont glacier | Mountain foot | Broad lobes | Malaspina, Alaska |
Worked example: density, speed and mass balance
- Density to water equivalent: 1 m of fresh snow with a density of 100 kg/m3 contains 100 kg of water per m2, equal to 0.1 m (10 cm) of water, because water has a density of 1,000 kg/m3.
- Firn to ice: a column of 10 m of snow at 100 kg/m3 turns into ice at 900 kg/m3 with a thickness of 10 × 100 ÷ 900 = 1.1 m, a ninth of the original thickness.
- Mass balance: a glacier gains 1.8 m of water equivalent in the accumulation zone and loses 2.3 m through ablation in the same year (averaged over the glacier). The net balance is 1.8 − 2.3 = −0.5 m of water equivalent, so the glacier is losing mass.
- Volume change: if the glacier has an area of 20 km2 (20,000,000 m2), the loss is 0.5 × 20,000,000 = 10,000,000 m3 of water, which is about 10 million tonnes.
- Travel time: ice moves at an average of 50 m per year. Ice that falls as snow at the top of a 10 km (10,000 m) glacier takes 10,000 ÷ 50 = 200 years to reach the snout.
- Retreat rate: if the snout moves back 25 m per year while the ice flows forward at 50 m per year, the ice is melting at the front at a rate of 50 + 25 = 75 m per year, because the net retreat is the difference between melting back and forward flow.
How did we come to understand glaciers?
In the early 19th century, naturalists in the Alps, including Ignace Venetz and Jean de Charpentier, proposed that glaciers had once been much larger, and Louis Agassiz developed the idea of an Ice Age in the 1830s and 1840s. James Forbes and John Tyndall studied glacier flow in the 1840s and 1850s. In the 20th century, ice cores from Greenland and Antarctica gave records of past climate for hundreds of thousands of years, and satellites began to monitor ice sheets. Modern measurements show that the ice sheets of Greenland and Antarctica are losing mass, and mountain glaciers are retreating in most regions.
Which mistakes are common?
Mistake 1: Thinking that glaciers are static
Glaciers flow continuously, and a glacier that looks still is moving, though slowly.
Mistake 2: Saying that a retreating glacier is flowing backward
Ice always flows downhill. Retreat means that the front melts back faster than the ice flows forward.
Mistake 3: Mixing up glaciers and sea ice
Glaciers form on land from snow. Sea ice forms from frozen seawater. Only melting land ice raises sea level substantially.
Mistake 4: Assuming that a glacier responds quickly to every change
Because ice flow is slow, a glacier may keep retreating for decades after the climate has stabilized.