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 Map of the supercontinent Kenorland 2.5 billion years ago[citation needed] | |
| Historical continent | |
|---|---|
| Formed | 2.72 Ga |
| Type | Supercontinent |
| Today part of | [1] |
Kenorland is a hypotheticalNeoarcheansupercontinent. If it existed, it would have been one of the earliest known supercontinents onEarth. It is thought to have formed during the NeoarchaeanEra c. 2.72 billion years ago (2.72Ga) by theaccretion of Neoarchaeancratons and the formation of newcontinental crust. It comprised what later becameLaurentia (the core of today's North America and Greenland),Baltica (today's Scandinavia and Baltic),Western Australia andKalaharia.[1]
Swarms of volcanicdikes and theirpaleomagnetic orientation as well as the existence of similar stratigraphic sequences permit thisreconstruction. The core of Kenorland, theBaltic/Fennoscandian Shield, traces its origins back to over 3.1 Ga. TheYilgarn craton (present-dayWestern Australia) containszircon crystals in its crust that date back to 4.4 Ga.
Kenorland was named after theKenoran orogeny (also called the Algoman orogeny),[2] which in turn was named after the town ofKenora,Ontario.[3]
Kenorland was formed around 2.72 billion years ago (2.72 Ga) as a result of a series of accretion events and the formation of new continental crust.[4]
The accretion events are recorded in thegreenstone belts of theYilgarn craton as metamorphosed basalt belts and granitic domes accreted around the high grademetamorphic core of the Western Gneiss terrane, which includes elements of up to 3.2 Ga in age and some older portions, for example theNarryer Gneiss terrane.
Paleomagnetic studies show Kenorland was in generally lowlatitudes until tectonicmagma-plumerifting 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 both theKola andKarelia cratons.[5] The protracted breakup of Kenorland during the LateNeoarchaean and earlyPaleoproterozoic Era 2.48 to 2.10 Gya, during theSiderian andRhyacian periods, is manifested bymafic dikes andsedimentary rift-basins and rift-margins on many continents.[1] On early Earth, this type of bimodal deepmantle 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 deep-mantle-plume method of continent formation in theHadean to EarlyArchean (before the final formation of the Earth's innercore) to the subsequent two-layer core-mantleplate tectonics convection theory. However, the findings of an earlier continent,Ur, and asupercontinent of around 3.1 Gya,Vaalbara, indicate this transition period may have occurred much earlier.
The Kola and Karelia cratons began to drift apart around 2.45 Gya, and by 2.4 Gya the Kola craton was at about 30 degrees south latitude and the Karelia craton was at about 15 degrees south latitude. Paleomagnetic evidence shows that at 2.45 Gya theYilgarn craton (now the bulk of Western Australia) was not connected to Fennoscandia-Laurentia and was at about ~5 degrees south latitude.[citation needed]
This implies that at 2.515 Gya an ocean existed between the Kola and Karelia cratons, and that by 2.45 Gya there was no longer a supercontinent. Also, there is speculation based on the rift margin spatial arrangements of Laurentia, that at some time during the breakup, theSlave andSuperior cratons were not part of the supercontinent Kenorland, but, by then may have been two differentNeoarchaean 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 theCanadian Shield, respectively.
The breakup of Kenorland was contemporary with theHuronianglaciation which persisted for up to 60 million years. Thebanded 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 thegreenhouse gasmethane (oxidized intocarbon 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 runawaySnowball Earth scenario, where average temperatures planet-wide plummeted to below freezing. Despite theanoxia indicated by the BIF,photosynthesis continued, stabilizing climates at new levels during the second part of theProterozoicEra.
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