Glaciers
Glaciers are large, long-lasting masses of ice that form from compacted snow and move under their own weight. They store freshwater, carve landscapes, feed rivers, record past climates, and contribute to sea-level rise when they lose more ice than they gain.
What glaciers are
A glacier is a large body of ice that persists for many years and moves under its own weight. It begins where snow survives the melt season, accumulates, compresses into firn, and eventually becomes glacial ice. Unlike seasonal snowfields, glaciers deform and flow, carrying ice, sediment, and sometimes meltwater through mountain valleys or across polar landscapes.
How glaciers form
Glaciers need more snow accumulation than melt over long periods. Fresh snow is gradually buried, squeezed, and recrystallized into firn and then dense ice. The balance between input and loss is called mass balance. A glacier with positive mass balance gains ice; one with negative mass balance loses ice and may retreat even though ice inside it still flows forward.
How they move
Glacier motion comes from gravity. Ice can deform internally like a very slow-moving solid, and some glaciers also slide at the base where pressure, meltwater, and bed conditions allow movement. The surface often cracks into crevasses because different parts of the glacier move at different speeds or over uneven ground.
Types and settings
Mountain glaciers occupy valleys, cirques, and high basins. Tidewater glaciers flow into the ocean and can lose ice by calving icebergs. Ice caps cover broad uplands, while the Greenland and Antarctic ice sheets are continental-scale ice masses. Each setting responds differently to temperature, snowfall, ocean contact, slope, and bedrock shape.
Landscapes shaped by ice
Glaciers erode, transport, and deposit rock. They can carve U-shaped valleys, polish bedrock, deepen basins, and leave moraines, outwash plains, drumlins, and erratic boulders. Many mountain lakes and fjords owe their shape to past glaciation. These landforms are evidence of where ice once existed, even after the glacier has vanished.
Water and hazards
Glacier meltwater can support rivers, agriculture, hydropower, and ecosystems, especially in dry seasons. But glaciers can also create hazards. Ice avalanches, crevasses, unstable moraines, and sudden outburst floods from glacier-dammed lakes can threaten communities downstream. Risk changes as glaciers thin, retreat, and expose new terrain.
Climate and sea level
Glaciers respond to climate because temperature and precipitation control how much ice they gain or lose. Worldwide glacier retreat is one of the clearest visible signs of a warming climate. Melting mountain glaciers and ice sheets add water to the ocean, contributing to sea-level rise, while loss of bright ice also changes how much sunlight landscapes reflect.
Why it matters
Glaciers connect local landscapes with global systems. They influence water supply, hazards, tourism, ecosystems, sea level, and climate records. Ice cores and glacier layers can preserve information about past atmosphere and snowfall, while modern glacier change helps scientists, planners, and communities understand how warming is reshaping frozen regions.