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2800-2500Ma

Neoarchean Era

The Neoarchean Era is the fourth, and final, era of the Archean Eon. The Neoarchean, like the Mesoarchean and the eras of the Hadean, is not associated with a specific rock layer, but is simply defined chronometrically. The supercontinent of Kenorland formed during this era, at approximately 2700 MYA, 100 MYA after the supercontinent of Vaalbara broke up. Kenorland was one of the earliest supercontinents on Earth. It is believed to have formed during the Neoarchaean Era ~2.7 billion years ago (2.7 Ga) by the accretion of Neoarchaean cratons and the formation of new continental crust. Kenorland comprised what later became Laurentia (the core of today's North America and Greenland), Baltica (today's Scandinavia and Baltic), Western Australia and Kalaharia.

The present continents are built around extremely ancient rock cores called shields. A large portion of Australia, Canada, India, Scandinavia and Siberia are made up of shield rock.

Oxygenic photosynthesis first evolved in this era and was accountable for the oxygen catastrophe which was to happen later in the paleo proterozoic 2.4 from a poisonous buildup of oxygen in the atmosphere, produced by these oxygen producing photoautotrophs, which evolved earlier in the neoarchean.

In the upper atmosphere things were different. Research on oxides in 2.7 billion year old micrometeorites found in Australia revealed that t the upper atmosphere of the time was about 20 percent oxygen, in line with modern levels.

Certain bacteria first became photoautotrophic when they evolved to use the sun's energy to survive, the by product being this oxygen. Some of the oxygen would interract with the iron particles which were rich in the world's oceans. The combination of iron, oxygen and water creates iron oxides which we commonly call rust. Rusted iron deposits built up on the ocean bed and would later become the rich sources of iron and steel which were exploited in abundance during the Industrial Revolution of the 18th and 19th Centuries. During the following Siderian Period, after all of the iron in the ocean had become oxidised, the photoautotrophically produced oxygen became free oxygen which was the catalyst for the Great Oxygenation Event.

The Great Oxygenation Event (GOE)

The Great Oxygenation Event, also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen (O2) in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.

Photosynthesis was producing oxygen both before and after the GOE. The difference was that before the GOE, organic matter and dissolved iron chemically captured any free oxygen. The GOE was the point when these minerals became saturated and could not capture any more oxygen. The excess free oxygen started to accumulate in the atmosphere.

The rising oxygen levels may have wiped out a huge portion of the Earth's anaerobic inhabitants at the time. Cyanobacteria, by producing oxygen that was toxic to anaerobic organisms, were essentially responsible for what was likely the largest extinction event in Earth's history. Additionally the free oxygen reacted with the atmospheric methane, a greenhouse gas, triggering the Huronian glaciation, possibly the longest snowball Earth episode ever. Free oxygen has been an important constituent of the atmosphere ever since.

Formation of Kenorland

Kenorland was formed around 2.7 billion years ago (2.7 Ga) as a result of a series of accretion events and the formation of new continental crust (Halla, 2005).

According to an in-depth analyses by Barley and others (2005), 2.78 billion years ago submarine magmatism culminated with the eruption of extensive suites of mantle plume derived komatiites at 2.72 to 2.70 Ga. Extensive hydrothermal activity produced volcanic massive sulfide mineralization and banded iron formation (BIF) deposition in anoxic arc-related basins. Arc and plume magmatism were followed by orogenic deformation, granitoid emplacement (by 2.68 Ga), stabilization of continental lithosphere, and collision with the other cratons to form the Kenorland continent.

The formation of Kenorland and possible collision of the Zimbabwe and Kaapvaal cratons at 2.6 Ga provides evidence that Late Archean cratons started to aggregate into larger continents at that time. Importantly granitoid–greenstone terranes and high-grade gneiss belts in the Gawler Craton, Antarctica, India, and China provide evidence for a second cycle of convergent margin tectonics and collision of cratons between 2.6 and 2.42 Ga.

The Gawler Craton contains 2.56 to 2.5 Ga ultr-amafic to felsic volcanic rocks (including 2.51 Ga plume-derived komatiites), metasedimentary rocks, and granitoids with compositions that are typical of Archean granitoid–greenstone terranes interpreted to have formed at convergent continental margins.

Central India and possibly eastern North China have similar histories from 2.6 Ga culminating with orogeny between 2.5 and 2.42 Ga corresponding to the aggregation and stabilization of Indian cratons within a larger continent. The Pilbara and Kaapvaal cratons are the only cratons with relatively complete and well-dated 2.6 to 2.4 Ga supracrustal rock records.

The accretion events are recorded in the greenstone belts of the Yilgarn Craton as metamorphosed basalt belts and granitic domes accreted around the high grade metamorphic core of the Western Gneiss Terrane, which includes elements of up to 3.2 Ga in age and some older portions, for example the Narryer Gneiss Terrane. Breakup of Kenorland

Paleomagnetic studies show Kenorland was in generally low latitudes until tectonic magma-plume rifting began to occur between 2.48 Ga and 2.45 Ga. At 2.45 Ga the Baltic Shield was over the equator and was joined to Laurentia (the Canadian Shield), and formed a unity with both the Kola and Karelia craton. The protracted breakup of Kenorland during the Late Neoarchaean and early Paleoproterozoic Era 2.48 to 2.10 Ga, during the Siderian and Rhyacian periods, is manifested by mafic dikes 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 surrounding the breakup of Kenorland is thought by many geologists to be the beginning of the transition point from the Hadean to Early Archean deep-mantle-plume method of continent formation (before the final formation of the Earth's inner core), to the subsequent two-layer core-mantle plate tectonics convection theory. However, with the findings of the earlier continent Ur and the ca. 3.1 Ga supercontinent Vaalbara, this transition period may have occurred much earlier.

The Kola and Karelia cratons began to drift apart ~2.45 Ga, and by 2.4 Ga the Kola craton was located at ~15 degrees latitude and the Karelia craton was located at ~30 degrees latitude. Paleomagnetic evidence shows 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 implies that at 2.45 Ga there was no longer a supercontinent and by 2.4 Ga an ocean existed between the Kola and Karelia cratons. Also, there is speculation based on the rift margin spatial arrangements of Laurentia, that at some time during the breakup, the Slave and Superior cratons were not part of the supercontinent Kenorland, but, by then may have been two different Neoarchaean landmasses (supercratons) on opposite ends of a very large Kenorland. This is based on 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 northwest and southeast portions of the Canadian Shield, respectively.

The breakup of Kenorland was contemporary with the Huronian glaciation which persisted for up to 60 million years. The banded iron formations (BIF) show their greatest extent at this period, thus indicating a massive increase in oxygen build-up from an estimated 0.1% of the atmosphere to 1%. The rise in oxygen levels caused the virtual disappearance of the greenhouse gas methane (oxidized into carbon dioxide and water).

The simultaneous breakup of Kenorland generally increased continental rainfall everywhere, thus increasing erosion and further reducing the other greenhouse gas carbon dioxide. With the reduction in greenhouse gases, and with solar output being less than 85% its current power, this led to a runaway Snowball Earth scenario, where 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.

Eruptions

Mistassini dike swarm and Matachewan dike swarm form 2,500 million years ago.

Sources

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