User:Milton Beychok/Sandbox

Earth was formed 4.54 billion years ago (within an uncertainty of 1%) with a primordial gaseous atmosphere surrounding a very dense, molten core. About 4.4 billion year ago, as the Earth began to cool and form a crust, the primordial atmosphere was stripped away by a combination of heat from that molten crust, periods of intense solar activity, and the solar wind.

As the crust formed, volcanic activity became incessant. The outgassing from the volcanoes replaced the primordial atmosphere with what is referred to as the second atmosphere that most probably consisted of water vapor (steam), nitrogen, methane, ammonia, carbon monoxide, carbon dioxide, hydrogen sulfide, sulfur dioxide, and other gases — a mixture much like that emitted from volcanoes today. The dominant gases of the secondary atmosphere were water vapor, carbon dioxide and nitrogen. There was very little free oxygen (if any) in that secondary atmosphere and it would have been poisonous for almost all modern life forms.

A6t about 4.0 billion years ago, cooling of the Earth and its atmosphere caused precipitation of the atmospheric water vapor as rainfall and subsequently the development of the Earth's oceans. Most of the atmospheric carbon dioxide was dissolved in the oceans and then precipitated out as solid carbonates. By about 3.5 billion years ago, life emerged in the oceans in the form of single-celled microorganisms (referred to as archaea).

By about 2.7 billion year ago, the archaea were joined by microorganisms called cyanobacteria which were the first organisms to produce free gaseous oxygen. It took a long time for the cyanobacteria to get started but between 2.2 and 2.7 billion years ago, the Earth's atmosphere had been converted from an oxygen-lacking (anoxic) atmosphere to an oxygen-containing (oxic) atmosphere. This is often referred to as the Great Oxidation and it resulted in the mass extinction of any life forms that may have existed during the era of the anoxic atmosphere.

The evolution of the Earth's modern oxygen-containing atmosphere (referred to as the third atmosphere) led to the formation of the ozone layer which protects life on earth by blocking the the harmful incoming ultraviolet solar radiation.

References

• G.B. Dalrymple (1991). The Age of the Earth. California: Stanford University Press. ISBN 0-8047-1569-6.
• G.B. Dalrymple (2001). <a href="http://sp.lyellcollection.org/content/190/1/205.abstract">"The age of the Earth in the twentieth century: a problem (mostly) solved"</a>. Geological Society, London, Special Publications Vol. 190. Issue(1), pp. 205–221.
• William L. Newman, (2007). <a href="http://pubs.usgs.gov/gip/geotime/age.html>"Age of the Earth"</a>. Publications Services, U.S. Geological Survey (USGS).
• Note: 1 billon years ago is 10⁹ years ago. It may be denoted as 1 Bya. It is also very often denoted as Gya or Ga, meaning giga years ago which is the SI equivalent and probably more commonly used worldwide.