earth's past

Introduction to the Archaean

3.8 to 2.5 billion years ago

If you were able to travel back to visit the Earth during the Archaean, you would likely not recognize it is the same planet we inhabit today. The atmosphere was very different from what we breathe today; at that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also during this time, the Earth's crust cooled enough that rocks and continental plates began to form.
It was early in the Archaean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archaean was bacterial. At right is an artist's depiction of what an Archaean coast might have looked like 3.5 billion years ago. The mounds in the foreground are stromatolites, colonies of photosynthetic bacteria which have been found as fossils in Early Archaean rocks of South Africa and Western Australia. Stromatolites increased in abundance throughout the Archaean, but began to decline during the Proterozoic. They are not common today.

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Subdivisions of the Archaean: The chart at left shows the three major subdivisions of the Archaean. These divisions are somewhat arbitrary, and we have not attempted to prepare exhibits on each of them. Dates and divisions follow those of Schopf (1983). The Archaean occurs between the Hadean and the Proterozoic.

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Read about Tectospheric keels and plate accretion of the Archaean and Proterozoic, as studied by Paul Stoddard at Northern Illinois University.