Glacier

A glacier is a large, long-lasting mass of ice that forms from compacted snow and flows slowly downhill under its own weight. Glaciers and ice sheets together cover about a tenth of Earth's land and store roughly two thirds of its fresh water.

Snow that does not melt in summer builds up in the accumulation zone, where it compacts into dense ice. The ice deforms and slides on its bed, carrying it down to the ablation zone, where melting and calving exceed snowfall. The line between the two zones is the equilibrium line, and a glacier advances or retreats according to the balance of the two. Types include valley glaciers, cirque glaciers and tidewater glaciers that end in the sea.

Moving ice carves U-shaped valleys, fjords and bowl-shaped cirques, and it leaves deposits of rock called moraines. Glacial meltwater feeds rivers and groundwater, supplying drinking water and irrigation to hundreds of millions of people in places such as the Andes and South Asia.

Glaciers outside the two big ice sheets have been losing roughly 270 billion tonnes of ice a year in recent decades, contributing to sea level rise. Dark, bare ice reflects less sunlight than snow, which accelerates melting, as described under ice-albedo feedback. The study of ice is called glaciology, and glaciers are monitored as part of the cryosphere. The largest bodies are the Greenland and Antarctic ice sheets.