Glaciers are slow-moving rivers of ice found on every continent and in about 47 countries. They can be hazardous when they surge, causing rock and snow avalanches. Glaciers have caused sea levels to lower by 100m and have even covered the planet in ice during the Cryogenian period.
A glacier is a large, slow-moving river of ice made up of many layers of packed snow. The speed of movement varies greatly depending on ambient temperature, ice depth, underlying slope, and other factors. The movement varies from several meters per hour to several meters per century. Sometimes, when conditions are right, glaciers will experience a surge, accelerating their movement speed by up to 100 times. When glaciers surge, they can be a hazard to humans, causing rock and snow avalanches.
Glaciers are found on every continent and in about 47 countries. Most mountains higher than 4,500 meters (14,800 feet) have them, because the temperature tends to drop rapidly with altitude. There are two main categories of glaciers: alpine glaciers, in the mountains, and continental glaciers, in the plains 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 the planet’s fresh water is found in the Antarctic ice sheet alone.
Glaciers are present year-round, but vary in their rate of melt. For an ice pack to qualify as a glacier, it exists continuously rather than just seasonally. For reasons that aren’t entirely clear, the planet has experienced several major ice ages in its history, when glaciers reached as far away as New York, US, and Paris, France. So many glaciers have accumulated that sea levels have been lowered by 100 m (328 ft), opening up vast areas of land such as the North Sea, the Bering Strait and connecting New Guinea to the Southeast Asian mainland.
At some point in the distant past about 700 million years ago, during the Cryogenian period, some scientists believe that glaciation may have been so severe that the entire planet was covered in a layer of ice. This has been called the Snowball Earth hypothesis and is controversial, especially among scientists who doubt the geophysical viability of a completely frozen ocean. What is known is that the glaciers at that time were extremely large in extent, reaching the Equator at least in some areas. Interestingly, the first complex multicellular organisms, the Edicaran biota, appear in the fossil record almost immediately after the Cryogenian glaciations.
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