The Archean Eon, spanning from approximately 4 billion years ago to 2.5 billion years ago, is marked by the development of the first continents, the emergence of early life forms, and the evolution of the early atmosphere and oceans.
Formation of Continents
During the Archean, the first stable continents began to form through the accumulation of volcanic and sedimentary rocks. These early landmasses were smaller and less distinct than modern continents.
Volcanic Activity and Tectonic Processes
Volcanic activity that began in the Hadean was still a significant geological process during the Archean. It contributed to the growth of the continents and played a role in the cycling of elements between the crust and the mantle. It continued to add gasses to the atmosphere, including water vapor that would become part of the growing oceans.
Emergence of Life
The Archean is associated with the emergence of the earliest life forms. Simple prokaryotic cells, including bacteria and archaea, evolved and adapted to a variety of environments, including hydrothermal vents and shallow seas. This included the cyanobacteria and the earliest stromatolite structures.
Rise of Stromatolites
Stromatolites are layered structures formed by the trapping and binding of sediment by microbial communities. They are formed as a film of microbes that traps sand and other particles, and the microbes build another layer on top of the trapped material, which traps more material.
Stromatolites became common during the Archean. They are some of the earliest fossil evidence of life on Earth. They can still be found in places around the world. One of the most famous is in Shark Bay, Western Australia, where the stromatolites can be seen without having to dive.
Great Oxygenation Event (GOE)
Around 2.4 billion years ago, the Great Oxygenation Event marked a significant shift in Earth’s atmosphere. Cyanobacteria, through photosynthesis, began releasing oxygen into the atmosphere, leading to the oxygenation of oceans and atmosphere. Before that, oxygen was scarce in the atmosphere. Whenever it was present, it reacted to iron and other elements in the atmosphere and rocks.
Formation of Banded Iron Formations (BIFs)
Banded iron formations are sedimentary rocks that formed during the Archean, primarily between 3.8 billion and 1.8 billion years ago. They resulted from the interaction of iron-rich ocean water and oxygen produced by cyanobacteria, leading to the deposition of iron oxide layers. It was not until all the free iron in the rocks had reacted with the oxygen from the cyanobacteria, that oxygen could build up in the atmosphere.
Early Oceans and Climate
The early oceans of the Archean likely contained higher levels of dissolved minerals and were influenced by the volcanic activity on the seafloor. The climate was likely warmer due to higher levels of greenhouse gases, released by volcanic activity, in the early atmosphere.