The history of the earth's atmosphere has been divided into three stages. The first covers the earliest part of earth history, prior to the formation of the core; during this time native iron was probably present in the upper part of the mantle. Volcanic gases ejected during the first stage probably contained a large amount of hydrogen, and the atmosphere was highly reduced. Carbon was probably present in the form of methane, nitrogen may have been present as ammonia, and free oxygen was absent in all but nonequilibrium quantities.

The second stage of atmospheric history began when iron was...