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The Paleoarchean ( ), also spelled Palaeoarchaean (formerly known as the early Archean), is a geologic era within the Archean Eon. The name derives from Greek "Palaios" ''ancient''. It spans the period of time . The era is defined chronometrically and is not referenced to a specific level of a rock section on
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
. The earliest confirmed evidence of life comes from this era, and Vaalbara, one of Earth's earliest supercontinents, may have formed during this era.


Early life

The geological record from the Paleoarchean era is very limited. Due to deformation and
metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...
, most rocks from the Paleoarchean era cannot provide any useful information. There are only two locations in the world containing rock formations that are intact enough to preserve evidence of early life: the Kaapvaal craton in
Southern Africa Southern Africa is the southernmost region of Africa. No definition is agreed upon, but some groupings include the United Nations geoscheme for Africa, United Nations geoscheme, the intergovernmental Southern African Development Community, and ...
and the Pilbara Craton in
Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...
. The Dresser Formation is located in the Pilbara Craton, and contains
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
from the Paleoarchean Era. It is estimated to be 3.48 billion years old. The Dresser Formation includes a great variety of structures caused by ancient life including stromatolites and MISS once formed by microbial mats. Such microbial mats belong to the oldest ascertained life form and may include fossilized bacteria. The Strelley Pool Chert, also located in the Pilbara Craton, contains stromatolites that may have been created by bacteria 3.4 billion years ago. However, it is possible that these stromatolies are abiogenic and were actually formed through evaporitic precipitation then deposited on the sea floor. The Barberton Greenstone Belt, located in the Kaapvaal craton, also contains evidence of life. It was created around 3.26 Ga when a large asteroid, about wide, collided with the Earth. The Buck Reef chert and the Josefsdal chert, two rock formations in the Barberton Greenstone Belt, both contain microbial mats with fossilized bacteria from the Paleoarchean era. The Kromberg Formation, near the top of the Onverwacht Group which itself is a part of the Barberton Greenstone Belt, dates back to approximately 3.416–3.334 Ga and contains evidence of microbial life reproducing via multiple fission and binary fission.


Continental development

Similarities between the Barberton Greenstone Belt in the Kaapvaal craton and the eastern part of the Pilbara Craton indicate that the two formations were once joined as part of the supercontinent Vaalbara, one of Earth's earliest supercontinents. Both cratons formed at the beginning of the Paleoarchean era. While some paleomagnetic data suggests that they were connected during the Paleoarchean era, it is possible that Vaalbara did not form until the Mesoarchean or Neoarchean eras. It is also unclear whether there was any exposed land during the Paleoarchean era. Although several Paleoarchean formations such as the Dresser Formation, the Josefsdal Chert, and the Mendon Formation show some evidence of being above the surface, over 90 percent of
Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...
continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...
has been destroyed, making the existence of exposed land practically impossible to confirm or deny. It is likely that during the Paleoarchean era, there was a large amount of continental crust, but it was still underwater and would not emerge until later in the Archean era. Hotspot islands may have been the only exposed land at the time. Due to a much hotter mantle and an elevated oceanic geothermal gradient compared to the present day, plate tectonics in its modern form did not exist during the Paleoarchean. Instead, a model of "flake tectonics" has been proposed for this era of geologic time. According to this model, instead of normal subduction of oceanic plates, extensively silicified upper oceanic crust delaminated from lower oceanic crust and was deposited in a manner similar to ophiolites from the later
Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...
and
Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...
eons.


Impacts

The Pilbara craton contains the earliest known surviving remnant of an
impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
, the North Pole Dome, estimated to be 3.47 billion years old and the original crater estimated to be over in diameter. Other inferred Paleoarchean impacts are recorded in spherule layers at various locations in South Africa and Australia (including the Barberton Greenstone Belt) at some distance from the impact sites. The impactors that generated these events are thought to have been much larger than those that created the largest known still existing craters/impact structures on Earth ( Vredefort and Chicxulub), with the impactors having an estimated diameter of ~, with the craters generated by these impacts having an estimated diameter of . The most powerful of the impacts such as the S2 impact, which dates to 3.26 billion years ago, probably caused extremely strong earthquakes worldwide,
megatsunamis A megatsunami is an incredibly large wave created by a substantial and sudden displacement of material into a body of water. Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater plate tectonics, ...
thousands of meters high, as well as boiling the surface of the oceans, resulting in tens of meters of global ocean evaporation. Earth would have also been plunged into darkness lasting years to decades.


References


External links


Paleoarchean (chronostratigraphy scale)
{{Geological history, c *02 Geological eras Precambrian geochronology