The Archean Eon encompasses events from earliest Earth history to 2.5 Byr. ago. The rock record begins at approximately 3.8 Byr. ago and shows that the Archean Earth was strikingly different from that of the present day. For example, the atmosphere contained no free oxygen and the biosphere consisted solely of the simplest bacteria and blue-green algae. Continents existed during the Archean, but they probably did not consist, as do modern continents, of technically stable cratonic regions surrounded by technically active mobile belts; rather, Archean continents were permobile (i.e., they featured no stable areas at all but rather were everywhere susceptible to tectonism). Indeed, it is this tectonic permobility which distinguishes the Archean Eon as a separate interval of geological time, and it was the advent of stable cratons that marked the end of the Archean.
KeywordsGeothermal Gradient Greenstone Belt Band Iron Formation Archean Rock Garnet Granulite
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