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. 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
thumb|left|upright 1.2| A [[stromatolite formed by Paleoarchean microbial mats, preserved as a fossil, from Pilbara craton, Western Australia.]]
The geological record from the Paleoarchean era is very limited. Due to deformation and metamorphism, 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 and the Pilbara Craton in Western Australia.
The Dresser Formation is located in the Pilbara Craton, and contains sedimentary rock from the Paleoarchean Era. It is estimated to be 3.48 billion years old.
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.
Continental development
thumb|left|A map of the [[Barberton Greenstone Belt in southern Africa.]]
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. 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.
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 and Phanerozoic eons.
Impacts
The Pilbara craton contains the earliest known surviving remnant of an impact crater, 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,