The Cryogenian (from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from . The Sturtian and Marinoan glaciations, which are the greatest ice ages known to have occurred on Earth and may have covered the entire planet, occurred during this period. It was the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran. These so-called 'snowball earth' events are the subject of much scientific controversy. The main debate involves whether these glaciations are truly global events or whether they are localised glaciations and not a worldwide event.

The period has not yet undergone the international ratification that all geological time periods undergo (the most recent being the Ediacaran Period which was ratified in 2004.) The period is defined only on the dates of the rocks and no observable and documented systematic global event. This is problematic as estimates of rock dates are variable and are subject to laboratory error. For instance, the Cambrian Period is marked not by rock younger than a given age ( million years), but by the appearance of the world wide Phycodes pedum trace fossil assemblage. This means that rocks can be recognised as Cambrian when you are looking at them in the field and do not require extensive testing to be performed in a lab to find a date. As yet, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian period, and its base is only loosely set to 850 Ma.

The name refers to the very cold global climate of the Cryogenian: characteristic glacial deposits indicate that Earth suffered the most severe ice ages in its history during this period. Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.^[1] It is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 750 million years ago to 700 Ma, and the Marinoan/Varanger glaciation terminated at circa 635 Ma. The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply-frozen planetary oceans called "Snowball Earth".^[2]

During the Cryogenian, the supercontinent Rodinia broke up, and the supercontinent Pannotia began to form.

References

1. ↑ Dave Lawrence, "Microfossil lineages support sloshy snowball Earth" in Geotimes, April 2003. http://www.agiweb.org/geotimes/apr03/WebExtra041803.html, accessed Sept 18, 2007.
2. ↑ Hoffman, P.F. 2001. Snowball Earth theory, accessed 15/Jun/2007, http://www.snowballearth.org

Further reading

• The Cryogenian Period

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