Archean
The Archean (also Archäikum ) is a eon of Earth's history. It extends from about 4,000 to about 2,500 million years ago. The obsolete term for this era is Azoikum.
Archean heads the Hadean, and it is from the Proterozoic or from its first unit, the Siderium, detached.
The subdivision of the Archean:
- Eon: Phanerozoic ( 541-0 mya )
- Aeon: Proterozoic ( 2500-541 mya )
- Aeon: Archean ( 4000-2500 mya ) Era: Neoarchaikum ( 2800-2500 mya )
- Era: Mesoarchaikum ( 3200-2800 mya )
- Era: Paläoarchaikum ( 3600-3200 mya )
- Era: Eoarchaikum ( 4000-3600 mya )
In the beginning of the Isua Archean gneiss from the area of Nuuk is dated in Greenland. Only since 1999 are known rocks, which were therefore formed before this age in the Hadean. The oldest known securely dated rock is the Acasta - gneiss with 4030 mya from the western Canadian Shield. 2008 more dating from the Hadean have become known, which originate from the Nuvvuagittuq greenstone belt in the eastern Canadian Shield. The first - hypothetical - Large continent Ur may have formed 3 billion years ago from these first hadaischen mainland Islands ( cratons ).
Although a long time it was assumed that Archean no fossils have been preserved, it has been found that within a very short time after the formation of stable crust ingredients evolutionary processes - have begun - for now the Chemical Evolution from 3800 mya: macromolecules were able, through to increase attachment of other molecules and to reproduce itself. In the early Archean time this can be taken as the beginning of life on Earth.
The oldest fossils found so far chemotherapy, ie fossil traces of living beings, are microscopic threads " in rocks that, first discovered in South Africa, as remains of 3.5 billion year old cyanobacteria - or blue-green algae - could apply. Later follow clearly biogenic fossil remains in the form of stromatolites, which date from the Archean.
Naturally the amount of findable fossils and rocks from this age minimal, since most of the land masses of this period, which could bear such traces are completely eroded to fine sand, metamorphoses, were the basis of sedimentary rocks or aufschmolzen in the mantle. Only in the oldest, unchanged since their formation cratons - the Archons - there is a chance by deep drilling in several thousand meters to make such findings. Such Archone include the Kola Peninsula, Zimbabwe, the Canadian Shield, parts of China or Western Australia.
The atmosphere in the early Archean still contained no free oxygen. The photosynthetic prokaryotes the first time being the oxidized minerals of the primeval ocean and only towards the end of the Archean - 2500 mya - oxygen to the atmosphere have been made.