what is a glacier

A glacier is a large, persistent body of dense ice that is constantly moving under its own weight. It forms where the accumulation of snow exceeds its ablation over many years, often centuries. Glaciers are formed from compressed snow that moves under its own weight and moves very slowly. They are usually divided into two groups: alpine glaciers, which form on mountainsides and move downward through valleys, and continental ice sheets, which spread out and cover larger areas. Glaciers are important components of the global cryosphere and are the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the worlds freshwater. They are also important features in the hydrologic cycle and affect the volume, variability, and water quality of runoff. Glaciers are found in regions of high snowfall in winter and low temperatures in summer, such as Alaska, Patagonia, and the polar regions.