Archean Eon ( /ɑːrˈkiːən/, also spelled Archaean) is a
geologic eon, 4,000 to 2,500 million years ago (4 to 2.5
billion years), that followed the
Hadean Eon and preceded the
Proterozoic Eon. During the Archean, the Earth's crust had cooled
enough to allow the formation of continents.
1 Etymology and changes in classification
Earth at the beginning of the Archean
4 Early life in the Archean
5 See also
7 External links
Etymology and changes in classification
Archean (or Archaean) comes from the ancient Greek Αρχή (Arkhē),
meaning "beginning, origin". Its earliest use is from 1872, when it
meant "of the earliest geological age." In earlier literature the
Hadean Eon was included as part of the Archean.
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
Gya (4 billion years ago) is officially recognized
by the International Commission on Stratigraphy.
Earth at the beginning of the Archean
Artist's impression of an
Archean is one of the four principal eons of
Earth history. When
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 million years
ago). The extra heat was the result of a mix of remnant heat from
planetary accretion, from the formation of the Earth's core, and
produced by radioactive elements.
Archean rocks are metamorphic or igneous. Volcanic
activity was considerably higher than today, with numerous lava
eruptions, including unusual types such as komatiite.
predominate throughout the crystalline remnants of the surviving
Archean crust. Examples include great melt sheets and voluminous
plutonic masses of granite, diorite, layered intrusions, anorthosites
and monzonites known as sanukitoids.
The evolution of Earth's radiogenic heat flow over time
Earth of the early
Archean may have supported a tectonic regime
unlike that of the present. Some scientists[who?] argue that, because
Earth was much hotter, tectonic activity was more vigorous than it
is today, resulting in a much faster rate of recycling of crustal
material. This may have prevented cratonisation and continent
formation until the mantle cooled and convection slowed. Others[who?]
argue that the oceanic lithosphere was too buoyant to subduct, and
that the rarity of
Archean rocks is a function of erosion by
subsequent tectonic events. The question of whether plate tectonic
activity existed in the
Archean is an active area of modern
There are two schools of thought concerning the amount of continental
crust that was present in the Archean. One school maintains that no
large continents existed until late in the Archean: small
protocontinents were common, prevented from coalescing into larger
units by the high rate of geologic activity. The
other school follows that of Richard Armstrong, who argued that the
continents grew to their present volume in the first 500 million years
Earth history and have maintained a near-constant ever since:
throughout most of
Earth history, recycling of continental material
crust back to the mantle in subduction or collision zones balances
crustal growth.
Opinion is also divided about the mechanism of continental crustal
growth. Those scientists who doubt that plate tectonics operated in
Archean argue that the felsic protocontinents formed at hotspots
rather than subduction zones. Through a process called "sagduction",
which refers to partial melting in downward-directed diapirs, a
variety of mafic magmas produce intermediate and felsic
rocks. Others accept that granite formation in island
arcs and convergent margins was part of the plate tectonic process,
which has operated since at least the start of the Archean.[citation
The lack of rocks older than 3800 Ma (million years ago) might be
explained by the efficiency of the processes that either cycled those
rocks back into the mantle or removed any isotopic record of their
age. All rocks in the continental crust are subject to metamorphism,
partial melting and tectonic erosion during multiple orogenic events,
and the chance of survival at the surface decreases with increasing
age. In addition, a period of intense meteorite bombardment at
4.0–3.8 Ga may have pulverized all rocks at the Earth's surface. The
similar age of the oldest surviving rocks and the Late Heavy
Bombardment may not be coincidental.
Archean atmosphere is thought to have nearly lacked free oxygen.
Astronomers think that the Sun had about 70–75 percent of the
present luminosity, yet temperatures on
Earth appear to have been near
modern levels after only 500 Ma of 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.
By the end of the Archaean c. 2500 Ma, plate tectonic activity may
have been similar to that of the modern Earth. 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. Liquid water was
prevalent, and deep oceanic basins are known to have existed attested
by the presence of banded iron formations, chert beds, chemical
sediments and pillow basalts.
Although a few mineral grains are known to be Hadean, the oldest rock
formations exposed on the surface of the
Archean or slightly
Archean rocks are found in Greenland, Siberia, the Canadian
Wyoming (exposed parts of the
Wyoming Craton), the
Baltic Shield, Scotland, India, Brazil, western Australia, and
southern Africa. Although the first continents formed during this eon,
rock of this age makes up only 7% of the present world's cratons; even
allowing for erosion and destruction of past formations, evidence
suggests that continental crust equivalent to only 5–40% of the
amount formed during the Archean.
In contrast to
Proterozoic Eon rocks,
Archean Eon rocks are often
heavily metamorphized deep-water sediments, such as graywackes,
mudstones, volcanic sediments, and banded iron formations. Carbonate
rocks are rare, indicating that the oceans were more acidic due to
dissolved carbon dioxide than during the Proterozoic. Greenstone
belts are typical
Archean formations, consisting of alternating units
of metamorphosed mafic igneous and sedimentary 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, being both types of metamorphosed rock, represent
sutures between the protocontinents.
Early life in the Archean
view • discuss • edit
Earliest sexual reproduction
Axis scale: million years
Orange labels: ice ages.
Human timeline and
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. Biogenic carbon has been detected in zircons dated to 4.1
billion years ago, but this evidence is preliminary and needs
validation. More solid indirect evidence of life comes from banded
iron formations in greenstones that date to 3.7 billion years. The
formation of banded iron deposits is thought to require oxygen, and
the only known source of molecular oxygen in the
Archean Eon was
photosynthesis, which implies life. The earliest identifiable fossils
consist of stromatolites—accretionary structures formed in shallow
water by micro-organisms—dated to 3.5 billion years ago.
Hadean atmosphere was dominated by carbon dioxide and nitrogen (in
much the same ratio as in the present day atmospheres of Venus and
Mars) but with some NO, CO, P4O10, SO2 and native sulfur. These gases
could have accumulated in the atmosphere because volcanic eruptions
were between 10 and 100 times more prolific in the
today. Thus, the
Hadean Ocean was a reservoir of the inorganic
elements that may have been the earliest catalysts of organic
reactions and, ultimately, of enzymes. The presence of an ocean, first
dating from the late Hadean, would suggest the start of life in the
Archean Eon rather than in the
Hadean Eon depended on the
presence of an ocean.
Water bodies on dry land, the atmosphere, beaches, sea ice, the sea
surface micro-layer, marine sediments, oceanic crusts and hydrothermal
systems all contributing to the
Hadean micro-environment, would have a
drastic impact on the origin of life in the Archaean. Miller and
Urey’s 1953 experiments demonstrated the production of biologically
important organic compounds (including amino acids) induced by passing
electric charge through a mixture of gases which were at the time
considered to be the components of Earth's early, reducing atmosphere
(H2O, CH4, H2 and NH3).
Hadean atmosphere could also have hosted particulate matter with
catalytic surfaces. On the modern Earth, natural dust particles are
largely derived from continental erosion. Dehydration of amino acids
during atmospheric transport has been suggested as a mechanism for
activation and polymerization. Additionally, amphiphiles (organic
molecules with both hydrophilic and lipophilic properties) including
stearic and oleic acids have been shown to form exterior films on
marine aerosols that could have served as proto-membranes in prebiotic
Another important role of the modern atmosphere is to protect life in
surface environments from solar UV radiation. In the Hadean, the Sun's
output in the extreme UV range was stronger and the
Earth lacked a
protective ozone layer. Hence, UV radiation at the surface was much
more intense. It is possible that a hydrocarbon haze might have acted
as a UV shield but was transparent to visible light. But in the
absence of a UV shield, solar UV radiation could have had both
positive and negative impacts on prebiotic chemical reactions in the
lower atmosphere and in surface exposed settings, by either activating
or destroying prebiotic molecules.
Life in the Archaean may have been either very developed as to what we
might have expected, or might be a little less so. The production of
life has to do with the geological structures present at the time that
it was being formed including the relative abundances of each of the
elements in the surroundings. This conclusion comes from the Archaean
landscape, which at that time consisted of volcanic and tectonic plate
activities that formed the greenstone belts found today on the
mainland of Greenland. One such example is that of MORB, a primitive
Archaean volcanic sediment found in the greenstone belt, which led to
the emissions of CO2 and O2 due to the volcanic eruptions at the
Prerequisites for the origin of life—such as energy, catalysis, the
synthesis of organic carbon compounds, and their concentration—can
all be seen in both the Late Hadean, as well as Early Archean
environments, at different levels and different places on the
landscape. This leads to a multi-regional origin of life hypothesis.
The microbial life that might have been formed at the time would have
been so small that it would have been very easy for it to travel long
distances on the Early Earth. These prerequisites allow the last
universal common ancestor of life to have its origin placed in this
The earliest evidence for life on
Earth are graphite of biogenic
origin found in 3.7-billion-year-old metasedimentary rocks discovered
in Western Greenland and microbial mat fossils found in
3.48-billion-year-old sandstone discovered in Western
Pyrite found in 3.47-billon-year-old baryte, in the
Warrawoona Group of Western Australia, shows sulfur fractionation of
as much as 21.1%, because sulfate-reducing bacteria metabolize
sulfur-32 more readily than sulfur-34. More recently, in 2015,
"remains of biotic life" were found in 4.1-billion-year-old rocks in
Western Australia. According to one of the researchers, "If
life arose relatively quickly on
Earth … then it could be common in
Fossils of cyanobacterial mats (stromatolites, which were instrumental
in creating free oxygen in the atmosphere) are found throughout
the Archean, becoming especially common late in the eon, while a
few probable bacterial fossils are known from chert beds. In
addition to the domain
Bacteria (once known as Eubacteria),
microfossils of the domain
Archaea have also been identified.
The Archaean Eon fossils might have formed as agglutination bubbles in
rock that include, but are not limited to, stromatolites.
Stromatolites are solid structures created by single-celled microbes
called cyanobacteria. They are both micro as well as macro examples of
life from the Archaean Eon.
It is difficult to determine whether a rock may be just that, or a
stromatolite. They are found in Zimbabwe, Australia, Canada and South
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. It can, however, be said
that the origins of life could have occurred earlier, while the
conditions necessary to sustain life could only have been possible in
Life was probably present throughout the Archean, but may have been
limited to simple single-celled organisms (lacking nuclei), called
Prokaryota (formerly known as Monera). There are no known eukaryotic
fossils from the earliest Archean, though they might have evolved
Archean without leaving any. No fossil evidence has
been discovered for ultramicroscopic intracellular replicators such as
Earliest known life forms
Geologic time scale
History of Earth
Timeline of natural history
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Wikisource has the text of the 1911 Encyclopædia Britannica article
When Did Plate Tectonics Begin?
Geologic history of Earth
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Miocene (5.333–23.03 Mya)
Paleogene (23.03–66.0 Mya)
Oligocene (23.03–33.9 Mya)
Eocene (33.9–56.0 Mya)
Paleocene (56.0–66.0 Mya)
Cretaceous (66.0–145.0 Mya)
Late (66.0–100.5 Mya)
Early (100.5–145.0 Mya)
Jurassic (145.0–201.3 Mya)
Late (145.0–163.5 Mya)
Middle (163.5–174.1 Mya)
Early (174.1–201.3 Mya)
Triassic (201.3–251.902 Mya)
Late (201.3–237 Mya)
Middle (237–247.2 Mya)
Early (247.2–251.902 Mya)
Permian (251.902–298.9 Mya)
Lopingian (251.902–259.8 Mya)
Guadalupian (259.8–272.3 Mya)
Cisuralian (272.3–298.9 Mya)
Carboniferous (298.9–358.9 Mya)
Pennsylvanian (298.9–323.2 Mya)
Mississippian (323.2–358.9 Mya)
Devonian (358.9–419.2 Mya)
Late (358.9–382.7 Mya)
Middle (382.7–393.3 Mya)
Early (393.3–419.2 Mya)
Silurian (419.2–443.8 Mya)
Pridoli (419.2–423.0 Mya)
Ludlow (423.0–427.4 Mya)
Wenlock (427.4–433.4 Mya)
Llandovery (433.4–443.8 Mya)
Ordovician (443.8–485.4 Mya)
Late (443.8–458.4 Mya)
Middle (458.4–470.0 Mya)
Early (470.0–485.4 Mya)
Cambrian (485.4–541.0 Mya)
Furongian (485.4–497 Mya)
Series 3 (497–509 Mya)
Series 2 (509–521 Mya)
Terreneuvian (521–541.0 Mya)
(541.0 Mya–2.5 Gya)
Neoproterozoic era (541.0 Mya–1 Gya)
Ediacaran (541.0-~635 Mya)
Cryogenian (~635-~720 Mya)
Tonian (~720 Mya-1 Gya)
Mesoproterozoic era (1–1.6 Gya)
Stenian (1-1.2 Gya)
Ectasian (1.2-1.4 Gya)
Calymmian (1.4-1.6 Gya)
Paleoproterozoic era (1.6–2.5 Gya)
Statherian (1.6-1.8 Gya)
Orosirian (1.8-2.05 Gya)
Rhyacian (2.05-2.3 Gya)
Siderian (2.3-2.5 Gya)
Archean eon² (2.5–4 Gya)
Neoarchean (2.5–2.8 Gya)
Mesoarchean (2.8–3.2 Gya)
Paleoarchean (3.2–3.6 Gya)
Eoarchean (3.6–4 Gya)
Hadean eon² (4–4.6 Gya)
kya = thousands years ago. Mya = millions years ago.
Gya = billions
years ago.¹ =
Phanerozoic eon. ² =
Source: (2017/02). International Commission on Stratigraphy. Retrieved
13 July 2015. Divisions of Geologic Time—Major Chronostratigraphic
and Geochronologic Units USGS Retrieved 10 March 2013.