Archean Eon

Archean Eon, also spelled Archaean Eon, the earlier of the two formal divisions of Precambrian time (about 4.6 billion to 541 million years ago) and the period when life first formed on Earth. The Archean Eon began about 4 billion years ago with the formation of Earth’s crust and extended to the start of the Proterozoic Eon 2.5 billion years ago; the latter is the second formal division of Precambrian time. The Archean Eon was preceded by the Hadean Eon, an informal division of geologic time spanning from about 4.6 billion to 4 billion years ago and characterized by Earth’s initial formation. Records of Earth’s primitive atmosphere and oceans emerge in the earliest Archean (Eoarchean Era). Fossil evidence of the earliest primitive life-forms—prokaryotic microbes from the domain called Archaea and bacteria—appears in rocks about 3.5–3.7 billion years old; however, the presence of ancient fragments of graphite (which may have been produced by microbes) suggest that life could have emerged sometime before 3.95 billion years ago. Archean greenstone-granite belts contain many economic mineral deposits, including gold and silver.

The start of the Archean Eon is only defined by the isotopic age of the earliest rocks. Prior to the Archean Eon, Earth was in the astronomical (Hadean) stage of planetary accretion that began about 4.6 billion years ago; no rocks are preserved from this stage. The earliest terrestrial materials are not rocks but minerals. In Western Australia some sedimentary conglomerates, dated to 3.3 billion years ago, contain relict detrital zircon grains that have isotopic ages between 4.2 and 4.4 billion years. These grains must have been transported by rivers from a source area, the location of which has never been found; it was possibly destroyed by meteorite impacts—quite frequent on both Earth and the Moon before 4 billion years ago.

It is thought that the oxygen content in today’s atmosphere must have slowly accumulated through time starting with an atmosphere that was anoxic during Archean times. Although volcanoes exhale much water vapour (H₂O) and carbon dioxide (CO₂), the amount of free oxygen (O₂) emitted is very small. The inorganic breakdown (photodissociation) of volcanic-derived water vapour and carbon dioxide in the atmosphere would have produced only a small amount of free oxygen. The bulk of the free oxygen in the Archean atmosphere was derived from organic photosynthesis of carbon dioxide (CO₂) and water (H₂O) by anaerobic cyanobacteria (blue-green algae), a process that releases oxygen as a by-product. These organisms were prokaryotes, a group of unicellular organisms with rudimentary internal organization that began to appear near the end of the Archean Eon. Although oxygen did not accumulate in any appreciable amount in the atmosphere until early Proterozoic time, processes occurring in Earth’s oceans toward the end of Archean helped set the stage for the increase in atmospheric oxygen.

Archean oceans were likely created by the condensation of water derived from the outgassing of abundant volcanoes. Iron was released then (as today) into the oceans from submarine volcanoes in oceanic ridges and during the creation of thick oceanic plateaus. This ferrous iron (Fe²⁺) combined with oxygen and was precipitated as ferric iron in hematite (Fe₂O₃), which produced banded-iron formations on the flanks of the volcanoes. The transfer of biologically produced oxygen from the atmosphere to the sediments was beneficial to the photosynthetic organisms, because at the time free oxygen was toxic to them. When banded-iron formations were being deposited, oxygen-mediating enzymes had not yet developed. Therefore, this removal of oxygen allowed early anaerobes (life-forms not requiring oxygen for respiration) to develop in the early oceans of Earth.

Carbon dioxide emissions are abundant from modern volcanoes, and it is assumed that the intense volcanism during the Archean Eon caused this gas to be highly concentrated in the atmosphere. This high concentration most likely gave rise to an atmospheric greenhouse effect that warmed Earth’s surface sufficiently to prevent the development of glaciations, for which there is no evidence in Archean rocks. The CO₂ content in the atmosphere has decreased over geological time, because much of the oxygen formerly bound in CO₂ has been released to provide increasing amounts of O₂ to the atmosphere. In contrast, carbon has been removed from the atmosphere via the burial of organic sediments.

Throughout the Archean, oceanic and island arc crust was produced semi-continuously for 1.5 billion years; thus, most Archean rocks are igneous. The oldest known rocks on Earth, estimated at 4.28 billion years old, are the faux amphibolite volcanic deposits of the Nuvvuagittuq greenstone belt in Quebec, Canada. The second oldest rocks are the 4-billion-year-old Acasta granitic gneisses in northwestern Canada, and a single relict zircon grain dated to 4.2 billion years ago was found within these gneisses. Other ancient sediments and lavas occur in the 3.85-billion-year-old Isua belt of western Greenland (which is similar to an accretionary wedge in the trench of a modern subduction zone) and the 3.5-billion-year-old Barberton Complex in South Africa, which is probably a slice of oceanic crust. A huge pulse in the formation of island arcs and oceanic plateaus took place worldwide from 2.9 to 2.7 billion years ago. By the time of the Archean-Proterozoic boundary, about 2.5 billion years ago, many small cratons (stable interior portions of continents) dominated by island arcs had coalesced into one large landmass, or supercontinent, that some scholars refer to as Kenorland.

Archean rocks mostly occur in large blocks hundreds to thousands of kilometres across, such as in the Superior and Slave provinces in Canada; the Pilbara and Yilgarn blocks in Australia; the Kaapvaal craton in southern Africa; the Dharwar craton in India; the Baltic, Anabar, and Aldan shields in Russia; and the North China craton. Smaller relicts of Archean rocks in various stages of obliteration occur in many younger Proterozoic and Phanerozoic orogenic (mountain) belts. Some Archean rocks that occur in greenstone-granite belts (zones rich in volcanic rocks that are primitive types of oceanic crust and island arcs) formed on or near the surface of Earth and thus preserve evidence of the early atmosphere, oceans, and life-forms. Other rocks that occur in granulite-gneiss belts (zones of rocks that were metamorphosed in the Archean mid-lower crust) are exhumed remnants of the lower parts of the Archean continents and thus preserve evidence of deep crustal processes operating at the time.

In greenstone-granite belts there are many oceanic lavas, island arcs, and oceanic plateaus; therefore, they commonly contain rock types such as basalts, andesites, rhyolites, granitic plutons, oceanic cherts, and ultramafic komatiites (lavas enriched in magnesium, a special product of the melting of the hot Archean mantle). These igneous rocks are host to multitudes of economic mineral deposits of gold, silver, chromium, nickel, copper, and zinc, which are important components of the economies of Canada, Australia, and Zimbabwe.

In granulite-gneiss belts the roots of many Andean-type active continental margins are exposed, the rocks being highly deformed and recrystallized during metamorphism in the deep crust. Common rocks are tonalites (a granitic-type rock rich in plagioclase feldspar) transformed into tonalitic gneisses, amphibolite dikes, and amphibolites derived from volcanic activity. Few mineral deposits occur in granulite-gneiss belts, in common with the deep crust of younger orogenic belts, which are relatively barren of ore concentrations.