What is a glacier?

Glaciers form where snow accumulates faster than it melts, year after year, until the buried layers are compressed into dense glacial ice. Under the enormous weight, the ice behaves like a very slow-moving river, creeping downhill or spreading outward at rates from centimetres to several metres per day. As they move, glaciers carve out U-shaped valleys, gouge deep basins that become lakes, and grind rock into fine sediment. Glaciers store roughly 69 percent of the world freshwater and cover about 10 percent of Earth land area, mostly in Antarctica and Greenland. They are sensitive indicators of climate: most glaciers worldwide are retreating as global temperatures rise, contributing to sea-level rise and threatening water supplies for communities that depend on glacial meltwater.