Ice ages are periods when glacial ice covers large areas of the Earth's surface that are not normally covered by ice. Ice ages are characterized by fluctuations of climatic conditions: a cycle of several glacial periods contains interglacial periods, perhaps of a few tens of thousands of years, when the climate may be as temperate as between ice ages. It is not known if the Earth is currently between ice ages or merely passing through an interglacial period.
There seem to have been several ice ages in the Precambrian, and certainly a major one immediately prior to the start of the Cambrian. There were a number in the Paleozoic, including a major ice age with a complicated cycle running through the Mississippian, Pennsylvanian, and early Permian. The ice age that we know most about, however, is that of the Quaternary, continuing through most of the Pleistocene and whose last glacial period ended about 10,000 years ago, denoting the start of the Holocene. At their greatest, the Pleistocene glaciers covered about a third of the Earth's surface, or some 45 million square kilometers, and may have been up to 3 kilometers thick in places. They covered most of Canada, northern Europe, northern Russia, northern parts of what is now the United States, and, in the Southern Hemisphere, Antarctica, parts of South America, and some other areas.
Theories about the cause of ice ages include that the Sun's energy output varies, that the Earth moves with respect to its axis, that continental drift may alter global climatic conditions, and that volcanic dust in the atmosphere could reduce the amount of solar heat received by the surface. One or more factors may be involved.