Archean Eon

The early Earth populated by microscopic life

The Archean Eon occurs after the Hadean Eon, and before the Proterozoic Era.

Geologic Age

  • 4,000–2,500 million years ago


What happened during this time?


  • Beginning of Archean dated to the oldest rocks found on Earth

  • Recent discoveries have pushed back the earliest dated rocks to about 4.0 billion years old

  • Earth moderately warm

    • Sun 30% cooler than today,

    • Earth's geological activity was much greater, creating warmer climate

  • Earth was hotter than it is today, its mantle may have contained less water because mantle minerals hold onto less water at higher temperatures (Dong et al. 2021)

    • Assuming that the mantle currently has more than 0.3-0.8 times the mass of the ocean, a larger surface ocean might have existed during the early Archean.

    • At that time, the mantle was about 1,900-3,000 degrees Kelvin (2,960-4,940 degrees Fahrenheit), compared to 1,600-2,600 degrees Kelvin (2,420-4,220 degrees Fahrenheit) today.

  • If early Earth had a larger ocean than today, that could have altered the composition of the early atmosphere and reduced how much sunlight was reflected back into space (Dong et al. 2021)

    • These factors would have affected the climate and the habitat that supported the first life on Earth.

  • Formation of proto-continents

    • When continents appear is under debate, but there is evidence of continental drift

    • These original proto-continents no longer exist; remnants found in cratons

      • Cratons are areas of ancient rock that survive on some continents

      • Appear when the overlying rock (mostly volcanic igneous rock) is buried deep, but not deep enough to be re-melted.

      • Heat and pressure converts it into metamorphic rock.

      • These are areas where the crust has thickened, with fresh igneous rock on top and metamorphic rock beneath (though folding of the crust can obscure this relationship).

    • Complexity of life in the ocean hints at a greater continental mass during this time

      • "...if there was life in the ocean, you need some amount of continental weathering taking place to deliver phosphorus so the organisms can live." (Satkoski 2016)

  • Earth has a reducing atmosphere

  • The Great Oxidation Crisis event

    • Oxygen produced by bacteria (see below) oxidized rocks and iron in the oceans,

    • Increase in atmospheric oxygen for a very long time. Atmospheric oxygen did not begin to rise significantly until billions of years after photosynthesis first began.


  • Origin of prokaryotic organisms, probably bacteria

    • Oldest direct evidence found in Western Australia (3.43 Ga)

    • First organisms were likely non-photosynthetic, utilizing methane, ammonia or sulfates for their energy needs

  • The beginning of bioenergetic processes, such as photosynthesis, (2.7 Ga, possibly as early as 3.5 Ga)

    • Photosynthesis became common with the cyanobacteria, perhaps as early as 3.5 billion years ago.

    • The oxygen produced by these bacteria went into oxidizing rocks on the Earth and the iron in the oceans,

    • Increase in atmospheric oxygen occur during Siderian (2.3-2.5 Ga)

  • Origin of the Archaea, and then toward the end of the eon, their decline.

  • Colonial stromatolites appear and proliferate