The Snowball Earthhypothesis as it was originally proposed suggests that the Earth was entirely covered by ice during parts of the Cryogenian period, from 790 to 630 million years ago. It was developed to explain sedimentary deposits generally regarded as of glacial origin at seemingly tropicallatitudes, and other enigmatic features of the Cryogenian geological record. The existence of a Snowball Earth remains controversial, and is contested by various scientists who dispute the geophysical feasibility of a completely frozen ocean, or the geological evidence on which the hypothesis is based.
The initiation of a Snowball Earth event would involve some initial cooling mechanism, followed by runaway cooling due to increasing ice accumulation. The initial cooling could be facilitated by an equatorial continental distribution, which would increase the Earth's albedo near the equator, where most solar radiation is incident.