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The Cryogenian (pronounced , from Greek ''cryos'' "cold" and ''genesis'' "birth") is a geologic period that lasted from . It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran. The Sturtian and Marinoan glaciations,〔These events were formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.〕 which are the greatest ice ages known to have occurred on Earth, occurred during this period. These events are the subject of much scientific controversy. The main debate contests whether these glaciations covered the entire planet (the so-called 'Snowball Earth') or if a band of open sea survived near the equator (termed 'slushball Earth'). ==Ratification== The Cryogenian period was ratified in 1990 by the International Commission on Stratigraphy. In contrast to most other time periods the beginning of the Cryogenian is not linked to a globally observable and documented event. Instead the base of the period is defined by a fixed rock age, that was set at 850 million years〔(【引用サイトリンク】url=https://engineering.purdue.edu/Stratigraphy/gssp/index.php?parentid=181 )〕 until 2015, when it was changed to 720 million years.〔(【引用サイトリンク】 Chart )〕 This is problematic as estimates of rock ages are variable and are subject to laboratory error. For instance, the time scale of the Cambrian Period is not reckoned by rock younger than a given age ( million years), but by the appearance of the worldwide ''Treptichnus pedum'' diagnostic trace fossil assemblages. This means that rocks can be recognized as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date. Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but a global glaciation would be a likely candidate.〔 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「Cryogenian」の詳細全文を読む スポンサード リンク
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