Snowball Earth

The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes, and other otherwise enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation, the geophysical feasibility of an ice- or slush-covered ocean, and the difficulty of escaping an all-frozen condition. There are a number of unanswered questions, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.

The geological time frames under consideration come before the sudden multiplication of life forms on Earth known as the Cambrian explosion, and the most recent snowball episode may have triggered the evolution of multi-cellular life on Earth. Another, much earlier and longer, snowball episode, the Huronian glaciation, which occurred 2400 to 2100 Ma may have been triggered by the oxygen catastrophe.