A glacier is a body of ice naturally formed on land. To qualify as glacier a feature needs to persist over many years and be built of glacial ice – that is ice, which forms in processes of snow accumulation and transformation.
Glaciers take different shapes and sizes. The smallest glaciers may be less than 1 square kilometer in size, while the largest, called ice sheets, can cover entire continents. Glaciers are most often formed in mountains, places, where precipitation is high and temperatures low enough to allow snow to survive summer. As snow accumulates over the years, lowermost layers transform to firn and then to glacial ice, under the pressure of overlying snow. This process is called snow metamorphism. Despite their static appearance, glaciers are in constant motion. Ice masses this big behave differently, than a small ice cube – they are elastic to a degree, which allows the glacier to flow under its own weight – albeit very slowly. The flow causes stresses in the ice body, which may cause cracking and appearance of crevasses.
Glaciers and ice sheets are the largest global reservoir of fresh water. In many areas of the world, especally South-East Asia and South America, human populations depend on water flowing from melting glaciers for water supply, irrigation and even hydropower. The global climate changes impact glaciers significantly, threatening these communities.