Cryogenian

The Cryogenian (pronunciation: /kraɪoʊˈdʒɛniən/, from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from 720 to 635 million years ago. 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 during the Cryogenian period, 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').

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 until 2015, when it was changed to 720 million years.

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