Snowball Earth
Proterozoic snowball periods | ||||||
−750 — – −725 — – −700 — – −675 — – −650 — – −625 — – −600 — – −575 — – −550 — – −525 — | ||||||
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The Snowball Earth is a geohistorical hypothesis proposing that during one or more of Earth's icehouse climates, the planet's surface was nearly entirely frozen, with little or no liquid water exposed. This global glaciation hypothesis is most commonly associated with the Cryogenian Period, which included two major glacial episodes: the Sturtian (c. 717–660 Ma) and Marinoan (c. 650–635 Ma).
Proponents of the hypothesis argue that it best explains sedimentary deposits that are generally believed to be of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean, and they emphasize the difficulty of escaping an all-frozen condition. Several unanswered questions remain, including whether Earth was a full "snowball" or a "slushball" with a thin equatorial band of open (or seasonally open) water. The Snowball Earth episodes are proposed to have occurred before the sudden radiations of multicellular bioforms known as the Avalon and Cambrian explosions; the most recent Snowball episode may have triggered the evolution of multicellularity.