Neoarchean

The Neoarchean is a geochronological eon that forms part of the larger Archean eon. It spans approximately 2,800 to 2,500 million years ago. It is the final of the four subdivisions of the Archean, following the Mesoarchean and preceding the Paleoprotozoic, which marks the beginning of the Proterozoic eon.

Archean time intervals are not defined by distinctive rock formations or characteristic fossils. Instead, they are defined by chronometric boundaries established by the International Commission on Stratigraphy (ICS). Consequently, the 2,500 million year boundary is a chronometric boundary set by agreement, not one based on a specific rock type observed at a particular location.

Crustal and Continental Growth

Most of the continental crust that exists today formed or stabilized during the Neoarchean. This time is characterized by structures called granite–greenstone belts, alternating zones of granitic rock and foliated metamorphic rocks with a greenish appearance. Many scientists view this period as one in which plate tectonics as understood today intensified, though the mechanics may have differed from modern plate tectonics.

Life and Atmosphere

Life during this time consisted entirely of unicellular prokaryotes: bacteria and archaea. Cyanobacteria were particularly significant, as they performed oxygen-producing photosynthesis. Neoarchean rocks contain stromatolites, layered rock structures built by bacterial communities. Geochemical signatures indicate that oxygen accumulated in certain locales before spreading throughout the atmosphere.

The major rise in atmospheric oxygen, known as the Great Oxidation Event, is generally dated to the beginning of the Paleoprotozoic, which follows the Neoarchean. The Neoarchean is thus viewed as a transitional period: the atmosphere still contained little oxygen, but oxygen-producing systems were becoming established.

Rock Record and Research

Neoarchean rocks are found in large cratons such as the Kaapvaal (southern Africa), Pilbara (Australia), Superior (Canada), and Baltica. Information is obtained through radiometric dating, particularly the uranium–lead method applied to zircon crystals, which provides reliable age determinations.