Archean

Archean
Name	Archean
Color	Archean
Top bar	all time
Time start	4000
Time end	2500
Timeline	Paleoproterozoic
Caption	Artist's conception of an Archean landscape
Celestial body	earth
Usage	Global (ICS)
Chrono unit	Eon
Strat unit	Eonothem
Timespan	from 4 Ga to 2.5 Ga
Lower def	Defined chronometrically
Lower gssp acceptance	N/A
Upper def	Defined chronometrically
Upper gssp acceptance	N/A
Previous	Hadean
Next	Proterozoic

Archean. The Archean Eon represents a foundational chapter in Earth's history, spanning from roughly 4 billion to 2.5 billion years ago. This immense interval witnessed the transition from a molten, hostile world to a planet with stable continents, liquid oceans, and the first stirrings of life. The geological record from this deep time is sparse and heavily metamorphosed, preserved primarily within ancient cratonic cores like the Canadian Shield and the Pilbara Craton.

Formation and Definition

The Archean Eon is formally defined chronometrically, beginning at the end of the preceding Hadean and concluding at the start of the Proterozoic. Its temporal boundaries are not marked by a specific Global Boundary Stratotype Section and Point but are instead based on radiometric dating of the oldest preserved rocks. The onset is generally associated with the formation of the earliest stable continental crust, as evidenced by detrital zircon crystals found in the Jack Hills of Western Australia. The conclusion is marked by a major shift in atmospheric chemistry and the onset of widespread Paleoproterozoic glaciations. Key figures in defining and studying this eon include early geologists like James Hutton and modern researchers such as Stephen Moorbath.

Geologic Events

The Archean was a period of intense planetary differentiation and crustal formation. Major events included the late heavy bombardment, a period of intense meteorite impacts that likely tapered off early in the eon. Widespread magma ocean crystallization gave way to the formation of the first substantial continental nuclei, known as protocontinents. Significant episodes of crustal growth and stabilization occurred, recorded in the Isua Greenstone Belt in Greenland and the Barberton Greenstone Belt in South Africa. The latter part of the eon saw the assembly of larger cratonic blocks, precursors to modern continents like Laurentia.

Atmosphere and Oceans

The early Archean atmosphere was largely anoxic, dominated by gases like carbon dioxide, methane, and ammonia, with negligible free oxygen. This reducing environment was maintained by volcanic outgassing and the absence of oxygenic photosynthesis. The first oceans condensed from water vapor released by volcanism and delivered by comets and asteroids, becoming vast reservoirs for dissolved iron and other elements. The profound transition to an oxidizing atmosphere, the Great Oxidation Event, began at the very end of the Archean, driven by the metabolic activity of cyanobacteria.

Life and Biosphere

Life emerged during the Archean, leaving behind subtle but compelling evidence in the rock record. The oldest putative microfossils are found in the Apex Chert of the Pilbara Craton. Stromatolites, layered structures formed by microbial mats, are preserved in locations like the Dresser Formation. These early ecosystems were dominated by prokaryotes, including archaea and bacteria, with metabolisms based on chemosynthesis and anoxygenic photosynthesis. The evolution of oxygenic photosynthesis, likely within cyanobacteria, was the eon's most transformative biological innovation, setting the stage for the Great Oxidation Event.

Geologic Features and Tectonics

The Archean crust was dominated by distinctive features such as greenstone belts, which are linear belts of metamorphosed volcanic and sedimentary rocks, and granite-gneiss complexes that form the cores of cratons. Tectonic processes were likely more vigorous and different from modern plate tectonics, possibly involving a form of "stagnant lid" or episodic overturn of the crust. Widespread komatiite volcanism, characterized by extremely high-temperature lava flows, indicates a much hotter mantle. These features are exceptionally well-preserved in the Superior Craton and the Yilgarn Craton.

Paleomagnetism and Plate Tectonics

Evidence for the onset of modern-style plate tectonics during the Archean is a subject of intense debate. Studies of paleomagnetism in ancient rocks from the Kaapvaal Craton and the Slave Craton suggest some form of lateral crustal motion may have been active. However, the interpretation of apparent polar wander paths from this deep time remains challenging due to metamorphic overprinting. The presence of ophiolite sequences, though rare and controversial in Archean terrains, is sought as a key indicator of ancient seafloor spreading and subduction processes.

Category:Geological history of Earth Category:Archean