Snowball Earth

The Snowball Earth is a geohistorical hypothesis that proposes during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen with no liquid oceanic or surface water exposed to the atmosphere. The most academically referred period of such global glaciation is believed to have occurred sometime before 650 mya during the Cryogenian period. 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. A number of 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 radiation of multicellular bioforms known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Long before the idea of a global glaciation was first proposed, a series of discoveries occurred that accumulated evidence for ancient Precambrian glaciations. The first of these discoveries was published in 1871 by J. Thomson, who found ancient glacier-reworked material (tillite) in Islay, Scotland. Similar findings followed in Australia (1884) and India (1887). A fourth and very illustrative finding, which came to be known as "Reusch's Moraine," was reported by Hans Reusch in northern Norway in 1891. Many other findings followed, but their understanding was hampered by the rejection (at the time) of continental drift. Douglas Mawson (1882–1958), an Australian geologist and Antarctic explorer, spent much of his career studying the Neoproterozoic stratigraphy of South Australia, where he identified thick and extensive glacial sediments.