Kenorland
Encyclopedia
Kenorland was one of the earliest supercontinent
s 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 craton
s and the formation of new continent
al 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 Kalahari
.
Swarms of volcanic dikes
and their paleomagnetic orientation as well as the existence of similar stratigraphic sequences permit this reconstruction. The core of Kenorland, the Baltic/Fennoscandian Shield
, traces its origins back to over 3.1 Ga. The Yilgarn Craton
(present-day Western Australia
) contains zircon
crystals in its crust that date back to 4.4 Ga.
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 komatiite
s 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 craton
s 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 ultramafic 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 belt
s 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
.
s until tectonic magma
-plume rift
ing 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 craton
s 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
.
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
s on Earth. It is believed to have formed during the Neoarchaean Era ~2.7 billion years ago (2.7 Ga) by the accretion
Accretion (geology)
Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.-Description:...
of Neoarchaean craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s and the formation of new continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
al crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
. Kenorland comprised what later became Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
(the core of today's North America and Greenland), Baltica
Baltica
Baltica is a name applied by geologists to a late-Proterozoic, early-Palaeozoic continent that now includes the East European craton of northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 billion years ago. Before this time, the three segments/continents that now comprise...
(today's Scandinavia and Baltic), Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
and Kalahari
Kalahari craton
The Kalahari craton is a craton, an old and stable part of the continental lithosphere with thick crust and deep lithospheric roots extending up to a few hundred kilometers into the Earth's mantle, that occupies a large portion of South Africa and consists of the Kaapvaal, the Zimbabwe craton, the...
.
Swarms of volcanic dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
and their paleomagnetic orientation as well as the existence of similar stratigraphic sequences permit this reconstruction. The core of Kenorland, the Baltic/Fennoscandian Shield
Baltic Shield
The Baltic Shield is located in Fennoscandia , northwest Russia and under the Baltic Sea. The Baltic Shield is defined as the exposed Precambrian northwest segment of the East European Craton...
, traces its origins back to over 3.1 Ga. The Yilgarn Craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
(present-day Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
) contains zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
crystals in its crust that date back to 4.4 Ga.
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 komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
s at 2.72 to 2.70 Ga. Extensive hydrothermal activity produced volcanic massive sulfide mineralization and banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
(BIF) deposition in anoxic arc-related basins. Arc and plume magmatism were followed by orogenic
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
deformation, granitoid
Granitoid
A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...
emplacement (by 2.68 Ga), stabilization of continental lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
, and collision with the other cratons to form the Kenorland continent.
The formation of Kenorland and possible collision of the Zimbabwe
Zimbabwe craton
The Zimbabwe craton is an area in Southern Africa of ancient continental crust and an example of Early Archaean lithology dating back to 3.5 billion years ago in the southern African nation of Zimbabwe. Late Archean metamorphism joined the Southern Marginal Zone of the Kaapvaal craton to the...
and Kaapvaal craton
Kaapvaal craton
The Kaapvaal craton , along with the Pilbara craton of Western Australia, are the only remaining areas of pristine 3.6-2.5 Ga crust on Earth...
s 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
Gawler craton
The Gawler Craton covers approximately 440,000 square kilometres of central South Australia. Its Precambrian crystalline basement crustal block was cratonised ca. 1550-1450 Ma...
, Antarctica, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
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 ultramafic 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
Pilbara craton
The Pilbara craton , along with the Kaapvaal craton are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth...
and Kaapvaal cratons are the only cratons with relatively complete and well-dated 2.6 to 2.4 Ga supracrustal rock
Supracrustal rock
Supracrustal rocks are rocks that were deposited on the existing basement rocks of the crust, hence the name. They may be further metamorphosed from both sedimentary and volcanic rocks....
records.
The accretion events are recorded in the greenstone belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
s of the Yilgarn Craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
as metamorphosed basalt belts and granitic domes accreted around the high grade metamorphic
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
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
Narryer Gneiss Terrane
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6...
.
Breakup of Kenorland
Paleomagnetic studies show Kenorland was in generally low latitudeLatitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s until tectonic magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
-plume rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing 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
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
Era 2.48 to 2.10 Ga, during the Siderian
Siderian
The Siderian is the first geologic period in the Paleoproterozoic Era and lasted from 2500 Mya to 2300 Mya . Instead of being based on stratigraphy, these dates are defined chronometrically....
and Rhyacian periods, is manifested by mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
dikes and sedimentary rift-basins and rift-margins on many continents. On early Earth, this type of bimodal deep mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
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
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
to Early Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
deep-mantle-plume method of continent formation (before the final formation of the Earth's inner core), to the subsequent two-layer core-mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
convection theory. However, with the findings of the earlier continent Ur
Ur (continent)
Ur was a supercontinent that formed in the early Archean eon; the oldest continent on Earth, half a billion years older than Arctica. Ur joined with the continents Nena and Atlantica about to form the supercontinent Rodinia...
and the ca. 3.1 Ga supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
Vaalbara
Vaalbara
Vaalbara is theorized to be Earth's first supercontinent, beginning its formation about , completing its formation by about and breaking up by . The name Vaalbara is derived from the South African Kaapvaal craton and the West Australian Pilbara craton...
, 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
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
(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
Slave craton
The Slave craton is a Canadian geological formation located in the Northwest Territories and Nunavut. This craton is approximately in size and forms part of the Canadian Shield. It is dominated by ca. 2.73-2.63 Ga greenstones and turbidite sequences and ca. 2.72-2.58 Ga plutonic rock, with large...
and Superior craton
Superior craton
The Superior craton forms the core of the Canadian Shield at the heart of the North American continent. It extends from Quebec in the east to eastern Manitoba in the west...
s 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
Canadian Shield
The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...
, respectively.
The breakup of Kenorland was contemporary with the Huronian
Huronian
The Huronian glaciation extended from 2400 Mya to 2100 Mya, during the Siderian and Rhyacian periods of the Paleoproterozoic era, triggered by the oxygen catastrophe, which oxidised the atmospheric methane...
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
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
(oxidized into carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
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
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
scenario, where average temperatures planet-wide plummeted to below freezing. Despite the anoxia
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
indicated by the BIF, photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
continued, stabilizing climates at new levels during the second part of the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
Era
Era
An era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in...
.