Eoarchean

EtymologyName formalityFormalAlternate spelling(s)EoarchaeanUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitEraStratigraphic unitErathemTime span formalityFormalLower boundary definitionTen oldest U-Pb zircon agesLower boundary GSSAAlong the Acasta River, Northwest Territories, Canada
65°10′26″N115°33′14″W / 65.1738°N 115.5538°W / 65.1738; -115.5538Lower GSSA ratified2023Upper boundary definitionDefined ChronometricallyUpper GSSA ratifiedNot formally defined

The Eoarchean (IPA: ; also spelled Eoarchaean) is the first era of the Archean Eon of the geologic record. It spans 431 million years, from the end of the Hadean Eon 4031 Mya to the start of the Paleoarchean Era 3600 Mya. Some estimates place the beginnings of life on Earth in this era, while others place it earlier. Evidence of archaea and cyanobacteria date to 3500 Mya, which is shortly after the Eoarchean. At that time, the atmosphere was without oxygen and the pressure values ranged from 10 to 100 bar (around 10 to 100 times the atmospheric pressure today).