A glacier is a large, slow-moving river of ice, formed by many layers of compacted snow. The rate of motion varies greatly, depending on the ambient temperature, depth of the ice, the underlying slope, and other factors. Movement ranges from several meters per hour to several meters per century. Sometimes, when conditions are just right, glaciers experience a surge, accelerating their rate of movement by as much as 100 times. When glaciers surge, they can be a danger to humans, triggering avalanches of rock and snow.
Glaciers can be found on every continent and in about 47 countries. Most mountains taller than 4,500 meters (14,800 ft) have them, because temperature tends to quickly drop with altitude. There are two main categories of glacier: alpine glaciers, on mountains, and continental glaciers, on flat land where it is very cold. Continental glaciers almost completely cover Greenland, parts of Iceland, northern Siberia and Canada, and most of Antarctica. About 70% of fresh water on the planet can be found in the Antarctic ice sheet alone.
Glaciers are present year-round, but vary in their rate of melting. For a pack of ice to qualify as a glacier, it exist continually rather than just seasonally. For reasons that are not completely clear, the planet has experienced several major Ice Ages in its history, when glaciers extended as far south as New York, USA and Paris, France. So many glaciers piled up that the sea level was lowered by 100 m (328 ft), opening up large areas of land such as the North Sea, the Bering strait, and connecting New Guinea to the Southeast Asian mainland.
At one point in the distant past, about 700 million years ago, during the Cryogenian Period, some scientists believe glaciation may have been so severe than the entire planet was covered in an ice sheet. This has been called the Snowball Earth Hypothesis, and it is controversial, especially among scientists who doubt the geophysical feasibility of a completely frozen ocean. What is known is that the glaciers at this time were extremely large in extent, reaching the Equator in at least some areas. Interesting to note is that the first complex multicellular organisms, the Edicaran biota, appear in the fossil record almost immediately after the glaciations of the Cryogenian.