Snowball Earth

The Snowball Earth hypothesis proposes that Earth's surface became entirely or nearly entirely covered by ice during one or more severe glaciations in its deep past, most notably in the Cryogenian period roughly 720 to 635 million years ago. British geologist W. Brian Harland first noted in 1964 that glacial deposits of that age occur at what were then tropical latitudes, and geobiologist Joseph Kirschvink coined the term Snowball Earth in 1992 while proposing a mechanism by which a fully frozen ocean could still eventually thaw through volcanic carbon dioxide buildup. Paul Hoffman's 1998 paper brought the hypothesis wide scientific attention by linking it to the geochemical signature of cap carbonate rocks that abruptly overlie the glacial deposits. Whether the freeze was a complete hard snowball or a partial slushball with an open equatorial band of water remains a genuinely debated question among geologists, and the episode is widely discussed in connection with the diversification of complex life that followed in the subsequent Ediacaran period.