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850-635Ma

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Cryogenian Period

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 850 Ma (as defined by the ICS based on radiometric chronometry) 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, small organic fossils, 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 capstones of limestone -- which are normally warm water sediments -- above and below and intermixed with glacial deposits. The reappearance of banded iron formations associated with the glacial periods suggests low and fluctuating oxygen levels, not seen since the Paleoproterozoic period, also temporarily returned.

Paleomagnetism studies appear to show very high continental drift rates, leading some geologists to question whether some of the phenomena might be due to magnetic pole wandering, 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 or Ordovician glaciation from 450 Ma to 420 Ma, the Karoo or Carboniferous-Permian glaciation from 360 Ma to 260 Ma, and the Cenozoic Quarternary glaciation which started 30 Ma in Antarctica and is ongoing.

Eruptions

723 million year old Franklin Large Igneous Province

780 million year old Hottah gabbro sheets

Pole Shift

There is geological evidence that a pole shift took place around 800 million years ago. The event was not sudden, and therefore not catastrophic. It is believed that the pole shift event took a million years to complete.

Sources

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