Archean
The Archean eon is the geological period after the Hadean and before the Proterozoic. It lasted from 3,800 million years ago to 2,500mya. It contains the first sedimentary rocks, and the first fossil life forms, which were cyanobacteria,[1] and acritarchs.[2]
Most of the rocks which survive are volcanic (igneous) and metamorphic in origin. Volcanic activity was everywhere; the heat flow from the Earth was at least twice as much as today. The question of when plate tectonics began is a major research area.[3] Small protocontinents formed. There were oceans already, early in the Archaean.
The atmosphere almost entirely lacked free oxygen, and instead, CO2 was a major constituent. Fossil bacterial mats, called stromatolites, are found throughout the Archaean after about 3,500mya. These were formed by cyanobacteria, who used photosynthesis and gave off oxygen as a by-product.[4]
[change] Archean Earth
The Archean is one of the four main time periods of Earth history. When the Archean began, the Earth's heat flow was nearly three times higher than it is today. It was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma).
Most of the surviving Archean rocks are metamorphic or igneous. Volcanic activity was much higher than today, with numerous hot spots, rift valleys, and lava eruptions.
The Earth of the early Archean may have supported a tectonic condition different from that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster rate of recycling of crustal material. This may have prevented continents formating until the mantle cooled and convection slowed down. Others argue that the subcontinental lithospheric mantle was too buoyant to subduct, and that the rarity of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether or not plate tectonic activity existed in the Archean is an active area of modern geoscientific research.[5]
An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the large amount of rocky and icy debris present in the early Solar System. After the eight planets formed, large numbers of leftover protoplanets, asteroids, and comets pursued eccentric orbits throughout our system, bombarding the early Earth and the other planets and moons until approximately 3800 Ma. This activity may have prevented any large crustal fragments from forming by literally shattering the early protocontinents. A barrage of particularly large impactors known as the Late Heavy Bombardment represents the climax of this violent era.
[change] References
- ↑ 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
- ↑ Buick R. 2010. Early life: ancient acritarchs. Nature 463: 885–886. doi:10.1038/463885a. PMID 20164911.
- ↑ Stanley, Steven M. 1999. Earth system history. New York: Freeman. p297-301 ISBN 0-7167-2882-6
- ↑ Schopf J.W. 1992. Geology and paleobiology of the Archean Earth, in Schopf J.W., and Klein C. The Proterozoic biosphere: a multidisciplinary study, Cambridge University Press. ISBN 0-521-36615-1
- ↑ Stanley, Steven M. 1999. Earth system history. Freeman, New York. p297-301 ISBN 0-7167-2882-6
