Precambrian/Archean eon

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The Archean Eon does not have an initial time boundary, however, by evidence, it is a popular belief that the Archean Eon began around 3.8 billion years ago and terminates at 2.5 billion years ago.The Archean Eon contains about 45% of earth's history. The oldest fossils are found in the Archean Eon. The age of the oldest preserved rocks on Earth's surface has been formally proposed as a definition for the base of the Archaean.

Some examples of these old preserved rocks are Apex Chert (3.465 billion years old) and stromatolites (3.45 billion years old) from Australia, and the Swaziland microfossils from Africa (also about 3.45 billion years old)

During the Archean Eon the earth acquired its basic configuration with the mantel and crust surrounding the core. This general period, from about 3.0 to 2.5 billion years ago, was the period of maximum continent formation. 70% of continental landmasses date from this period (Thus, most of the continents are extremely ancient). Modern Earth sciences recognize that the present continents are built around cores of extremely ancient rock, called "shields". A large part of Australia is a "shield", as is much of Canada, India, Siberia, and Scandinavia.

None of the Archean eon eras have been further subdivided into periods.


Greenstone Belt

Eoarchean (era) The Eoarechean era is bound at 3.8 to 3.6 billion years ago.It is best known through the Isua Greenstone Belt, which is the oldest known rock formation (3.8 - 3.7 billion years old). It is located in Greenland. The Greenstone belt contains metamorphosed volcanic (mafic) and sedimentary rocks.Crust formation continued throughout this era.



Stromatolites

Paleoarchean (era) The Paleoarchean era is bound at 3.6 to 3.2 billion years ago. Both archaea and eubacteria evolved during the paleoarchean, implying that the last universal common ancestor (LUCA) of all life of earth existed during this era. The oldest stromatolites date back to about 3.5 billion years, within the Paleoarchean.Vast numbers of cyanobacteria were instrumental in changing the geology of earth and the evolution of life through the production of oxygen. The carbon isotopes in rocks point to photosynthesis. Continent formation continued throughout the Paleoarchean.




Banded Iron Formations

Mesoarchean (era) The Mesoarchean era is bound at 3.2 to 2.8 billion years ago. Plate tectonics forced the separation of the Kaapvaal and Pilbara cratons, and the separation of these ancient parts of South Africa and Australia was complete by the end of the Mesoarchean, around 2.8 billion years ago. Another super continent that may have originated during the mesoarchean was Ur. The first incontrovertible fossils appear from this time. All life from this period was consequently anaerobic. The oldest banded iron formations (BIFs) are dated to this period.



Neoarchean (era) The Neoarchean era is bound at 2.5 to 2.8 billion years ago. Cyanobacteria started producing significant amounts of oxygen in this period.Lead to the Oxygen Catastrophe during the early proterozoic, in which rising levels of oxygen poisoned much of the life that existed at the time. Large continents first appeared on earth, with modern plate tectonics (with subduction zones, continental plates sliding over each other and the upwelling of lava to produce new crust where continental plates tore apart). This is when the first large continent shields were formed.




References[edit]

http://essayweb.net/geology/timeline/archean.shtml

http://www.palaeos.com/Archean/Archean.htm

http://www.chriscon.org/timeline.pdf