|Alternate spelling(s)||Archaean, Archæan|
|Time span formality||Formal|
|Lower boundary definition||Defined Chronometrically|
|Lower GSSA ratified||1991|
|Upper boundary definition||Defined Chronometrically|
|Upper GSSA ratified||1991|
The Archean Eon (
The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under an ocean deeper than today's ocean. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The earliest known life started in the Archean. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen.
Etymology and changes in classification
The word Archean comes from the Greek word arkhē (αρχή), meaning 'beginning, origin'. It was first used in 1872, when it meant 'of the earliest geological age'.[a] Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540 million years ago until 2,500 million years ago.
Instead of being based on
When the Archean began, the Earth's
Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in
Plate tectonics likely started vigorously in the Hadean, but slowed down in the Archean. The slowing of plate tectonics was probably due to an increase in the viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean.
Due to recycling and metamorphosis of the Archean crust, there is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma. Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled.
By the Neoarchean, plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of slab detachment resulting from a hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed.
Asteroid impacts were frequent in the early Archean.
The Archean atmosphere is thought to have nearly lacked
Astronomers think that the Sun had about 75–80 percent of the present luminosity, yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the faint young Sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.
The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon.
The earliest evidence for life on Earth is graphite of biogenic origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland.
The earliest identifiable fossils consist of
Further evidence for early life is found in 3.47 billion-year-old
Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation.
Earth was very hostile to life before 4.2–4.3 Ga and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before the Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon.
Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called
Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.
- Abiogenesis – Natural process by which life arises from non-living matter
- Cosmic Calendar – Method to visualize the chronology of the universe
- Earliest known life forms – Putative fossilized microorganisms found near hydrothermal vents
- Geologic time scale – System that relates geologic strata to time
- Geological history of oxygen – Timeline of the development of free oxygen in the Earth's seas and atmosphere
- History of Earth – Development of planet Earth from its formation to the present day
- Precambrian – History of Earth 4600–539 million years ago
- Timeline of natural history
- Archean felsic volcanic rocks – Felsic volcanic rocks formed in the Archean Eon
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