Ice Age

Throughout Earth's history, there have been times when conditions have been alternately warmer and colder than they are today. There have also been several periods, called ice ages, during which large portions of the planet's surface have been covered with ice. Ice ages, or glacial periods, have occurred at irregular intervals for more than 2.3 billion years.

The Precambrian period (from 4.6 billion years ago to 570 million years ago) had four ice ages. The first occurred somewhere between 2.7 billion and 2.3 billion years ago. Then the Earth warmed up and was free of ice for almost a billion years. The second ice age occurred between 950 and 890 million years ago; the third between 820 and 730 million years ago; and the fourth between 640 and 580 million years ago. In each case, some area of the Earth was iced over for about 100 million years.