The word Archean is derived from the Greek word arkhē (αρχή), meaning 'beginning, origin'.[3] The
Pre-Cambrian had been believed to be without life (azoic); however, fossils were found in deposits that were judged to belong to the Azoic age. 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
stratigraphy, the beginning and end of the Archean Eon are defined
chronometrically. The eon's lower boundary or starting point of 4,031±3 million years ago is officially recognized by the
International Commission on Stratigraphy,[1] which is the age of the oldest known intact rock formations on Earth. Evidence of rocks from the preceding Hadean Eon are therefore restricted by definition to non-rock and non-terrestrial sources such as individual mineral grains and lunar samples.
Geology
When the Archean began, the Earth's
heat flow was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was partly remnant heat from
planetary accretion, from the formation of the
metallic core, and partly arose from the decay of
radioactive elements. As a result, the Earth's mantle was significantly hotter than today.[4]
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
Greenland,
Siberia, the
Canadian Shield,
Montana,
Wyoming (exposed parts of the
Wyoming Craton),
Minnesota (Minnesota River Valley), the
Baltic Shield, the
Rhodope Massif,
Scotland,
India,
Brazil, western
Australia, and southern
Africa.[citation needed]Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminous
plutonic masses of
granite,
diorite,
layered intrusions,
anorthosites and
monzonites known as
sanukitoids. Archean rocks are often heavily metamorphized deep-water sediments, such as
graywackes,
mudstones, volcanic sediments, and
banded iron formations.
Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as
komatiite.[5]Carbonate rocks are rare, indicating that the oceans were more acidic, due to dissolved
carbon dioxide, than during the Proterozoic.[6]Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed
mafic igneous and sedimentary rocks, including
Archean felsic volcanic rocks. The metamorphosed igneous rocks were derived from volcanic
island arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a
forearc basin. Greenstone belts, which include both types of metamorphosed rock, represent
sutures between the protocontinents.[7]: 302–303
Plate tectonics likely started vigorously in the
Hadean, but slowed down in the Archean.[8][9] The slowing of plate tectonics was probably due to an increase in the viscosity of the
mantle due to outgassing of its water.[8] Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely.[10] Only at the end of the Archean did the continents likely emerge from the ocean.[11] The emergence of continents towards the end of the Archaean initiated continental weathering that left its mark on the oxygen isotope record by enriching seawater with isotopically light oxygen.[12]
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.[13] Another hypothesis, which conflicts with the first, is that rocks from western Australia and southern Africa were assembled in a continent called
Vaalbara as far back as 3,600 Ma.[14] Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled.[8]
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.[15][16] 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.[17] Evidence from
spherule layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of the
Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.[18]
Environment
The Archean atmosphere is thought to have almost completely lacked
free oxygen; oxygen levels were less than 0.001% of their present atmospheric level,[20][21] with some analyses suggesting they were as low as 0.00001% of modern levels.[22] However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980–2,960 Ma,[23] 2,700 Ma,[24] and 2,501 Ma.[25][26] The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of the
Great Oxygenation Event,[24][27] which most scholars consider to have begun in the
Palaeoproterozoic.[28][29][30] Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean.[31] The ocean was broadly
reducing and lacked any persistent
redoxcline, a water layer between oxygenated and anoxic layers with a strong
redox gradient, which would become a feature in later, more oxic oceans.[32] Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present.[33] Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur.[34] The Archean ocean was enriched in heavier oxygen isotopes relative to the modern ocean, though
δ18O values decreased to levels comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering.[35]
Astronomers think that the Sun had about 75–80 percent of its present luminosity,[36] 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
sedimentaryprotoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history.[37][38][39] Extensive abiotic denitrification took place on the Archean Earth, pumping the greenhouse gas nitrous oxide into the atmosphere.[40] Alternatively, Earth's
albedo may have been lower at the time, due to less land area and cloud cover.[41]
For details on how life got started, see
Abiogenesis.
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 identifiable fossils consist of
stromatolites, which are
microbial mats formed in shallow water by
cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old
sandstone discovered in
Western Australia.[43][44] Stromatolites are found throughout the Archean[45] and become common late in the Archean.[7]: 307 Cyanobacteria were instrumental in creating free oxygen in the atmosphere.[citation needed]
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.[48][49]
Earth was very hostile to life before 4,300 to 4,200 Ma, 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.[50]
Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called
prokaryotes. In addition to the domain
Bacteria, microfossils of the domain
Archaea have also been identified. There are no known
eukaryotic fossils from the earliest Archean, though they might have evolved during the Archean without leaving any.[7]: 306, 323 Fossil
steranes, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter.[51] No fossil evidence has been discovered for
ultramicroscopicintracellular replicators such as
viruses.
Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.[52][53]
^Cheney ES (1996). "Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia". Precambrian Research. 79 (1–2): 3–24.
Bibcode:
1996PreR...79....3C.
doi:
10.1016/0301-9268(95)00085-2.