The Snowball Earth hypothesis as it currently stands proposes that the Earth was entirely covered by ice in part of the Cryogenian period of the Proterozoic eon, and perhaps at other times in the history of Earth. It was developed to explain sedimentary glacial deposits at tropical latitudes during the Cryogenian period (850 to 630 million years ago) and other enigmatic features of the Cryogenian geological record. After the last big freeze ended, multicellular evolution began to accelerate. 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 beginning of a Snowball Earth event could be facilitated by an equatorial continental distribution, which allows rapid, unchecked weathering of continental rocks, absorbing vast quantities of carbon dioxide from the atmosphere. The depletion of this greenhouse gas causes ice accumulation, which further cools the planet by reflecting solar energy back to space. The runaway system would lead a new ice-covered equilibrium with equatorial temperatures similar to modern-day Antarctica.
To break out of the frozen condition, huge quantities of greenhouse gases such as carbon dioxide and methane, emitted primarily by volcanic activity, would have to accumulate over millions of years. Once melting began, however, it would be quick, perhaps only 1000 years.
Weathering of glacial sediments, by reacting with carbon dioxide, and fertilising oceanic photosynthesisers, may have eventually drawn down enough of the greenhouse gas to instigate another Snowball Earth.
Sedimentary features usually formed by glaciers, found in what may have been equatorial locations at the time of deposition, have been taken as evidence implying global ice cover. Many other features of the sedimentary record are easily explained by extensive glacial cover. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacial times, which is consistent with a freezing of the surface oceans.
Whilst the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to prefer a "slushball" to a "snowball". In a slushball scenario a band of ice-free, or ice-thin, waters remains around the equator, allowing for a continued hydrologic cycle. This appeals to scientists who believe that certain features of the sedimentary record can only be explained by rapidly moving ice, which would require somewhere ice free to move to, or that observed sedimentary structures could only form below open water. Attempts to construct computer models of a Snowball Earth have also struggled to accommodate global ice cover, without fundamental changes in the laws and constants which govern the planet. Attempts have been made to explain equatorial ice-deposits by claiming Earth's spin axis or magnetic field changed dramatically. Recent research using observed geochemical cyclicity in clastic rocks suggests that the "Snowball" periods were punctuated by warm spells, similar to ice age cycle in recent Earth history.
Snowball Earth has profound implications on the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place directly at the end of the Cryogenian period.