Cryogenian

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The Cryogenian Period (from Greek cryos "ice" and genesis "birth") is the second geologic period of the Neoproterozoic Era, followed by the Ediacaran Period. The Cryogenian includes the Sturtian and Marinoan (formerly considered together as the Varanger) glaciations, and lasted from 800 Ma (million years ago, boundary defined chronometrically) to approximately 635 Ma.

The name is derived from the glacial deposits characteristic of the period, indicating that at this time, the Earth suffered the most severe ice ages in its history, with glaciers extending to the equator, in a series of rhythmical pulses. These glaciations are represented by tillite deposits in Congo, Sahara, and Oman, in Australia and in China, and in North America, Ireland, Scotland, and Norway, and many other places around the world. It is generally considered to be divisible into at least two (Sturtian around 750 to 700 Ma and Marinoan/Varanger which terminated at ca. 635 Ma) major worldwide glaciations. The tillite deposits occur also in places which were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of the deeply-frozen planetary oceans called "Snowball Earth".

The population of acritarchs crashed during this glaciation and it is claimed that oxygen levels in the atmosphere increased after the glaciation. There are a number of enigmatic features about this glaciation, including indications of glaciation at very low latitudes and the presence of limestones -- sediments which are normally warm water above and below and intermixed with glacial deposits.

Paleomagnetism seems to show very high continental drift rates, leading some geologists to question whether some of the phenomena might be due to magnetic pole wandering (inertial interchange true polar wander: see the work of J. Kirschvink (Caltech news release, link below) rather than plate motion and low latitude glaciation. Basically, a very lopsided distribution of continental crust causes the Earth's daily rotation to cause centrifugal force which would cause the Earth to rotate (while its daily rotation axis stays in the same direction) until the aggregation of continents is on the equator; this causes apparent continental drift that is much faster than average rates.

Other known world-wide glaciations include the Huronian from 2400 Ma to 2100 Ma, Andean-Saharan from 450 Ma to 420 Ma, the Karoo glaciation from 360 Ma to 260 Ma, and the Cenozoic glaciation which started 30 Ma in Antarctica and is ongoing.

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