Any geologic period during which thick ice sheets cover vast areas of land. Such periods of large-scale glaciation may last several million years and drastically reshape surface features of entire continents. Larger ice sheets reflect more of the sun’s energy into the atmosphere, cooling the planet to an even greater degree.
There have been at least five documented major ice ages during the 4.6 billion years since the Earth was formed — and most likely many more before humans came on the scene about 2.3 million years ago.
In the past million years, the Earth experienced a major ice age about every 100,000 years.
There are several theories concerning this phenomenon, including: astronomical factors (Milanković cycles), marine iron concentrations, reduction of carbon dioxide in the atmosphere, uplift of continental blocks.