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The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes, and other otherwise enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation, the geophysical feasibility of an ice- or slush-covered ocean, and the difficulty of escaping an all-frozen condition. A number of unanswered questions exist, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water. The geological time frames under consideration come before the sudden appearance of multicellular life forms on Earth known as the Cambrian explosion, and the most recent snowball episode may have triggered the evolution of multi-cellular life on Earth. Another, much earlier and longer, snowball episode, the Huronian glaciation, which occurred 2400 to 2100 Mya may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event." ==History== Sir 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 and late in his career speculated about the possibility of global glaciation. Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and that of other continents where low-latitude glacial deposits are found, has remained constant through time. With the advancement of the continental drift hypothesis, and eventually plate tectonic theory, came an easier explanation for the glaciogenic sediments—they were deposited at a point in time when the continents were at higher latitudes. In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial tillites in Svalbard and Greenland were deposited at tropical latitudes. From this palaeomagnetic data, and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued for an ice age that was so extreme that it resulted in the deposition of marine glacial rocks in the tropics. In the 1960s, Mikhail Budyko, a Russian climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a feedback loop ensued where the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. While Budyko's model showed that this ice-albedo ''stability'' could happen, he concluded that it had never happened, because his model offered no way to escape from such a scenario. In 1971, Aron Faegre, an American physicist, showed that a similar simple energy balance model predicted three stable global climates, one of which was snowball earth. This model introduced Edward Norton Lorenz concept of intransitivity indicating that there could be a major jump from one climate to another, including to snowball earth. The term "snowball Earth" was coined by Joseph Kirschvink in a short paper published in 1992 within a lengthy volume concerning the biology of the Proterozoic eon.〔 The major contributions from this work were: (1) the recognition that the presence of banded iron formations is consistent with such a glacial episode and (2) the introduction of a mechanism with which to escape from an ice-covered Earth—the accumulation of from volcanic outgassing leading to an ultra-greenhouse effect. Franklyn Van Houten's discovery of a consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle." His studies of phosphorus deposits and banded iron formations in sedimentary rocks made him an early adherent of the "snowball Earth" hypothesis postulating that the planet's surface froze more than 650 million years ago.〔(Princeton University - Franklyn Van Houten, expert on sedimentary rocks, dies at 96 )〕 Interest in the snowball Earth increased dramatically after Paul F. Hoffman applied Kirschvink's ideas to a succession of Neoproterozoic sedimentary rocks in Namibia and elaborated upon the hypothesis by incorporating such observations as the occurrence of cap carbonates in the journal ''Science'' in 1998. In 2010, Francis MacDonald reported evidence that Pangaea was at equatorial latitude during the Cryogenian with glacial ice at or below sea level and the associated Sturtian glaciation was global.〔MacDonald, Francis, ''Calibrating the Cryogenian,'' Science, 5 March 2010: Vol. 327 no. 5970 pp. 1241-1243 5 March 2010 (Abstract )〕〔(Snowball Earth: New Evidence Hints at Global Glaciation 716.5 Million Years Ago )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Snowball Earth」の詳細全文を読む スポンサード リンク
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