What is Archaean?
[[File:Karijini8.jpg|thumb|right|300px|Weano Gorge in Karijini National Park: showing the bright-red strata of the iron-bearing Archaean rocks of the [[Pilbara]].]] The '''Archaean''' (or Archean) [[eon]] is the [[Geology|geological]] period after the [[Hadean]] and before the [[Proterozoic]]. It is one of the four main time periods ([[eon]]s) of [[Earth]] history. The Archaean lasted from 4,000 million years ago (mya) to 2,500 mya. It contains the first [[sedimentary rock]]s, and the first [[fossil]] life forms, which were [[cyanobacteria]],<ref>Knoll, Andrew H. 2004. ''Life on a young planet: the first three billion years of evolution on Earth''. Princeton, N.J. {{ISBN|0-691-12029-3}}</ref> and [[acritarch]]s.<ref>Buick R. 2010. Early life: ancient acritarchs. ''Nature'' '''463''': 885–886. doi:10.1038/463885a. PMID 20164911.</ref> Most of the rocks which survive are [[volcano|volcanic]] ([[igneous]]) and [[Metamorphic rock|metamorphic]] in origin. Volcanic activity was everywhere. When the Archaean began, the Earth's heat flow was nearly three times higher than it is today. It was still twice the current level when the Archaean ended, 2,500 mya. This high level of heat flow may have made [[plate tectonics]] more vigorous than today. The question of when plate tectonics began is a major research area.<ref>Stanley, Steven M. 1999. ''Earth system history''. New York: Freeman. p297-301 {{ISBN|0-7167-2882-6}}</ref> The large [[supercontinent]] [[Vaalbara]] formed in the Archaean. There were oceans already, before the Archaean. The [[atmosphere]] almost entirely lacked free [[oxygen]], and instead, it was mostly [[methane]] and [[Carbon dioxide|CO<sub>2</sub>]]. Fossil [[bacteria]]l mats, called [[stromatolite]]s, are found throughout the Archaean after about 3,500mya. These were formed by [[cyanobacteria]], which used [[photosynthesis]] and gave off [[oxygen]] as a by-product.<ref>Schopf J.W. 1992. Geology and paleobiology of the Archaean Earth, in Schopf J.W., and Klein C. ''The Proterozoic biosphere: a multidi