The Paleoarchean (), also spelled Palaeoarchaean (formerly known as early Archean), is a
geologic era
The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...
within the
Archaean 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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
. 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 (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
, 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 subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number o ...
and the
Pilbara Craton in
Western Australia
Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...
.
The
Dresser Formation, located in the
Pilbara Craton, contains samples of
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
from the Paleoarchean Era.
Inside the rocks, there are
microbial mats containing the
oldest ascertained life form,
fossilized bacteria estimated to be 3.48 billion years old.
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
Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.''
Certain magnetic minerals in rock ...
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 Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth during the Arc ...
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic 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 '' sial'' be ...
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
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
from the later
Proterozoic
The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided i ...
and
Phanerozoic
The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anim ...
eons.
References
External links
Paleoarchean (chronostratigraphy scale)
{{Geological history, c
*02
Geological eras
Precambrian geochronology