Hadean Eon- 4.6-3.9 Billion years ago (Ga)
The Hadean Eon represents the time from which the Earth first formed (4.6 Ga) to approximately the oldest dated rocks (3.8-4.0 Ga) on Earth, located in Northwest Canada, Montana,Greenland and Australia. Essentially, during the Archean the Earth cooled from a molten state, differentiated into discrete layers and lithified as a layered Earth. Remember that meteorites, moon rocks and even isolated zircon minerals --4.4 Ga discovered in 200 from Earth have been dated between 4.5 and 4.0 Ga; however, little is known of this "infancy" time in Earth's history. Given that 4.4 Ga zircons exist and 4.2 Ga detrital zircons have been discovered in 3.5 Ga Warrawoona Group rocks of Western Australia, the Earth did have a solid crustal component within a few hundred million years.
It is presumed that the Archean Earth was depleted in atmospheric oxygen, but enriched in noxious gases such as carbon dioxide, ammonia, methane, sulfur oxides, nitrous oxides, etc. It is also thought that little to no continental crust had evolved; instead the Earth was essentially an oceanic soup with a few microcontinents. No evidence of life exists from this time. Note that some geologists do not recognize the Hadean at all (first proposed by Preston Cloud); others consider the Hadean to have ended anywhere from 3.8-3.96 Ga. Science is not simple!
Great Scientific Discoveries in 2001!!!
Caption: Microscopic view of a zircon crystal determined to be 4.4 billion years old making it the world's oldest known terrestrial material. Zircon is a mineral commonly used to determine the geological age of rocks. Chemical analysis of this grain suggests that the Earth was cool enough to have water, a hydrosphere and, possibly, life much earlier than previously thought. (http://www.nsf.gov/od/lpa/news/press/01/pr0102.htm)
Photo credit: courtesy John W. Valley, UW-Madison professor of geology and geophysics. http://www.news.wisc.edu/newsphotos/oldrock.html
Valley et al. (Geology, V. 30, No. 4, p. 351-354, 2002) suggest that the Early Earth cooled sufficiently such that by 4.4 Ga liquid water existed. These authors speculate that bacterial life may have developed at that time.
For additional information refer to the April 2002 issue of Geology, available online at: http://www.gsajournals.org/gsaonline/?request=get-document&issn=0091-7613&volume=030&issue=04&page=0351
oldest rocks: http://www.geology.ufl.edu/manuscripts/396zirc.abst.html
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