Archaean

The Archaean, or Archean Eon extends from 3.8 billion years ago until 2.5 billion years ago.

The Eon was formerly called the Archaeozoic, and is subdivided into Eoarchean (3.8-3.6 Ga), Paleoarchean (3.6-3.2 Ga), Mesoarchean (3.2-2.8 Ga), and Neoarchean (2.8-2.5 Ga) Eras or into early (to 3.3Ga), middle (to 2.9 Ga), and late Archaean.

Life arose about 4 billion years ago, at the close of the Hadean Eon, and Cyanobacteria comprise the earliest known microfossils. The Cyanobacteria built large stromatolite reefs and dominated life for more than 2 billion years (~3.5 Ga to ~1 Ga). The atmosphere remained reducing for much of the Achaean, though Cyanobacteria began to generate oxygen after approximately 2.8 to 2.7 Ga.

Earth's heat flux during the Archaean was much higher than current levels: early in the Archaean the flux was almost triple current levels, while heat flows fell to twice current levels by the end of the Archaean. As a result of the high heat flow, the Earth had numerous hot spots, very active volcanoes, and rift valleys. Earth continued to experience bombardment by extrasolar debris plus the late heavy bombardment until 3.5 Ga. The majority of Archean rocks still in existence are crystalline cratonic remnants, which include unusual lavas (e.g., komatiite), intrusive igneous rocks such as great melt sheets, and voluminous plutonic masses of granite, diorite, ultramafic to mafic layered intrusions, anorthosites and monzonites known as sanukitoids.

Scientists continue to actively debate the contribution of plate tectonic activity in the formation of Archaean crustal configurations. Large continents did not form until late in the Archean, and the felsic protocontinents probably formed at hotspots over mantle plumes rather than at subduction zones. Protocontinents probably resulted from:
● igneous differentiation of mafic rocks to produce intermediate and felsic rocks
● mafic magma melting of more felsic rocks
● granitization of intermediate rocks
● partial melting of mafic rock
● metamorphic alteration of felsic sedimentary rocks.
Much continental material may have been lost if rocks were not sufficiently buoyant or were consumed at energetic subduction zones.[2]

The oldest rocks so far discovered on Earth are:
1) Jack Hills, Western Australia, a 4.4 Ga detrital zircon (sample W74) in the Jack Hills metaconglomerate, Eranondoo Hill. More at Earliest Piece of Earth [news article]
2) The Acasta Gneisses near Canada’s Great Slave Lake (4.03 Ga)
3) The Isua Supracrustal rocks of West Greenland (3.7 to 3.8 Ga)
4) Northern Michigan (3.5-3.7 Ga)
5) Swaziland (3.4-3.5 Ga)

Archaean Supercontinents and continents:
Vaalbara is Earth's theorized first supercontinent, which, according to radiometric data, existed by 3.3 billion years ago (3.3 Ga) and possibly even as far back as 3.6 Ga. Geochronological and palaeomagnetic evidence suggests Valbaara began to break up ~2.8 Ga.

Ur was the first known continent, which probably formed 3 billion years ago in the early Archean Eon.

By about 2.7 Ga, Neoarchean sanukitoid cratons plus new continental crust accreted to form another of Earth's earliest supercontinents, Kenorland.

Two contrasting mechanisms have been proposed for Archean tectonics: “vertical tectonism” and “horizontal tectonism”. Vertical tectonism is attributed to density inversions inherent between denser volcanic sequences (greenstones) and underlying less dense sialic material (granitoids), which result in the buoyant rising of granitoids (diapirism) and sinking of greenstones (sagduction). Horizontal tectonism in the Archean is assumed to be similar (though probably not identical) to present-day plate tectonics, so is characterized by regional scale horizontal motion (drift) and resulting interactions of plates or microplates. Vertical tectonism can lead to local horizontal movement, and horizontal tectonism commonly results in vertical movement, as in collisional zones. However, regional scale horizontal displacements and regional scale strike-slip faults (or shear zones) cannot be explained by vertical tectonism and most likely results from horizontal tectonism. Vertical and horizontal tectonism were not necessarily mutually exclusive.

While Archean geologists now accept that processes similar to present-day plate tectonics existed in some form in the Archean, particularly in the Neoarchean, studies continue to document evidence for vertical tectonism. Recognition of synchronous vertical and horizontal tectonism proves challenging because it is often difficult to differentiate structures formed by synchronous processes from those formed by one of the two processes followed by the other. It has been suggested that the granite-greenstone patterns in the Dharwar craton in India resulted from the interplay of diapirism and bulk horizontal inhomogeneous contraction. It has also been suggested that the kinematics of a high strain zone in the northwestern Superior craton resulted from synchronous horizontal and vertical tectonism. [Lin]

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Archaean supercontinents

The Archaean, or Archean Eon extends from 3.8 billion years ago until 2.5 billion years ago.

(image at left – current approximate distribution of most Archean crustal rocks (the projection greatly exaggerates the area of polar landmasses – compare with image of location of cratons and platforms)

The Archean mantle was hotter than the Earth's current interior, resulting in heavy mantle convection and crustal turbulence. Archean Earth was very active tectonically, with higher rates of volcanic activity and plate boundary movement than is occurring today. Active tectonics during the Archean created numerous, mobile protocontinental landmasses that floated on the turbulent mantle. Toward the end of the Archean, the proto-continents began to coalesce, and by about 3.3 Ga the more tectonically stable supercontinent Vaalbara is hypothesized to have formed by accretion of smaller landmasses.

The oldest Archaean rocks so far discovered on Earth are:
1) The Isua Supracrustal rocks of West Greenland (3.7 to 3.8 Ga)
2) Northern Michigan (3.5-3.7 Ga)
3) Swaziland (3.4-3.5 Ga)

Archaean Eon:
70% of modern continental shields date from 3.0 to 2.5 billion years ago, which was the period of maximum continent formation. Most of the earliest rocks have been greatly altered through regional metamorphic processes, but later rocks (3.2-2.5 Ga) are mostly pillow-basalts that formed beneath the vast oceans. Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.

The oldest fossilized signs of life date from the Archean – Cyanobacteria that created 3.45 Ga old stromatolite fossils found in the Apex Chert at Marble Bar, Western Australia. Microfossils from Swaziland (South Africa) have a similar age.[image]

Vaalbara is Earth's theorized first supercontinent, which, according to radiometric data, existed by 3.3 billion years ago (3.3 Ga) and possibly even as far back as 3.6 Ga. Evidence includes geochronological and palaeomagnetic studies between the two Archaean cratons (protocontinents) called the Kaapvaal craton (the Kaapvaal province of South Africa) and the Pilbara craton (the Pilbara province of Western Australia).

Further evidence is the structural sequence similarities of the greenstone belts and gneiss belts of the Kaapvaal and Pilbara cratons. These same Archaean greenstone belts are now spread out across the margins of the Superior craton of Canada and across the cratons of the former Gondwana and Laurasia continents. The subsequent drift paths of the Kaapvaal and Pilbara cratons after 2.8 Ga gives further evidence that they were once connected.

It is not certain when Vaalbara began to break up, but geochronological and palaeomagnetic evidence show that the two cratons had a rotational 30 degree latitudinal separation at 2.78 to 2.77 Ga, implying that they were no longer contiguous after ~2.8 Ga.

Ur was the first known continent, which probably formed 3 billion years ago in the early Archean Eon. Ur joined with the continents Nena and Atlantica about one billion years ago to form the supercontinent Rodinia. Ur survived for a long time, until it was torn apart when the supercontinent Pangaea broke apart about 208 million years ago into Laurasia and Gondwanaland. Ur now forms parts of Africa, Australia, India, and Madagascar. In the early period of its existence, it was probably the only continent on Earth, and is so considered by some to be a supercontinent, even though it was probably smaller than Australia is now.

By about 2.7 Ga, Neoarchean sanukitoid cratons plus new continental crust accreted to form another of Earth's earliest supercontinents, Kenorland, which comprised the Laurentia, Baltica, Australia, and Kalahari cratons. The Baltic/Fennoscandian Shield formed the core of Kenorland, with origins that trace back more than 3.1 Ga. The Yilgarn Craton (present-day Western Australia) contains 4.4 Ga detrital zircon elements.

Protracted tectonic magna plume rifting occurred between 2.48 to 2.10 Ga, contributing to the break-up of Kenorland during the Late Neoarchaean and early Paleoproterozoic Era (2.48 to 2.10 Ga) during the Siderian and Rhyacian periods. The rifting is manifested by mafic dykes and sedimentary rift-basins and rift-margins on many continents. On early Earth, this type of bimodal deep mantle plume rifting was common in Archaean and Neoarchaean crust and continent formation.

The geological time period remains conjectural at which Earth made the transition from deep-mantle-plume mechanism of continent formation to the current two-layer core-mantle plate tectonics convection mechanism that forms continents by subduction, convergence and accretion. However, with the findings of the earlier continent Ur and the ca. 3.1 Ga supercontinent Vaalbara, this transition period may have occurred much than the time of Kenorland.

Paleomagnetic studies indicate that Kenorland lay in low latitudes until the rifting breakup commenced around 2.48 Ga. Paleomagnetic evidence further indicates that at 2.45 Ga the Baltic/Fennoscandian Shield, astride the equator, was attached to Laurentia (the Canadian Shield), forming a unity with both the Kola and Karelia cratons, which began to drift apart ~2.45 Ga. By 2.4 Ga, the Kola craton was located at ~15 degrees latitude and the Karelia craton had drifted to ~30 degrees latitude. Paleomagnetic evidence indicates that at 2.45 Ga the Yilgarn craton (now the bulk of Western Australia) was not connected to Fennoscandia-Laurentia and was located at ~70 degrees latitude. This suggests that Kenorland was no longer a supercontinent by 2.45 Ga. By 2.4 Ga, an ocean separated the formerly joined Kola and Karelia cratons.

Based on the rift margin spatial arrangements of Laurentia, it is hypothesized that the Slave and Superior cratons were not part of the rifting supercontinent Kenorland, but may instead have been two different Neoarchaean landmasses (supercratons) located at opposite poles of a very large Kenorland. This speculation is based on the mechanics of how drifting assemblies of various constituent pieces should flow reasonably together toward the amalgamation of the new subsequent continent. The Slave and Superior cratons now constitute the NW and SE portions of the Canadian Shield, respectively.

The breakup of Kenorland was contemporary with the 60 million year long Huronian glaciation. The banded iron formations (BIF) show their greatest extent at this period, indicating massive increase in oxygen build-up (estimated 0.1% to 1% of atmosphere). Rising oxygen levels caused virtual disappearance of the greenhouse gas methane, which was oxidized into carbon dioxide and water. The simultaneous breakup of Kenorland increased continental rainfall, increasing erosion and further reducing the other greenhouse gas carbon dioxide. This reduction in greenhouse gases coupled with solar output still less than 85% its current levels precipitated a runaway Snowball Earth scenario, when average temperatures planet-wide plummeted to below freezing. Despite the anoxia indicated by the BIF, photosynthesis continued, stabilizing climates at new levels during the second part of the Proterozoic Era.

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More: GeoWhen Database : Tectospheric keels : 2004 time scale endorsed by the International Commission on Stratigraphy

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