Mesoarchean
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The Mesoarchean is one of four divisions of the Archean Eon, part of Earth's early history. It spans approximately 3.2 billion to 2.8 billion years ago. The name derives from the Greek words mesos (middle) and arkhaios (ancient).
Like other Archean divisions, the Mesoarchean boundary is defined by an assigned numerical age (chronometric definition) rather than by distinctive changes in rock type or fossils visible in the geological record. This represents the main difference between Archean divisions and the later divisions of the Phanerozoic Eon, which are typically defined by biostratigraphic markers.
Position in the geological time scale
The Archean is divided into four parts: Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean. The Mesoarchean occupies the third position. Before the Archean lies the Hadean; after it comes the Proterozoic. These divisions are established by the International Commission on Stratigraphy (ICS), which maintains the geological time scale.
Geological features
During the Mesoarchean, Earth's crust continued to solidify, and stable continental cores (cratons) that would form the foundations of present-day continents grew larger. Rocks from this period are found in regions including Canada, Australia, South Africa, and Greenland. The climate and atmospheric composition remain uncertain; the atmosphere contained little oxygen.
Life
Organisms during this time were single-celled and lacked a nucleus (bacteria and archaea). Stromatolites—layered rock structures built by bacterial communities living in shallow water—are thought to have existed during this interval. Cyanobacteria, which perform oxygen-producing photosynthesis, may have been present, but oxygen had not yet accumulated significantly in the atmosphere.