Archean
The
Archean (
Archaean; previously known as the
Archaeozoic or
Archeozoic) is a
geologic eon that refers to the time before the
Proterozoic, 2500
Ma (million years ago). Instead of being based on
stratigraphy, this date is defined chronometrically. The lower boundary has not been officially recognized by the
International Commission on Stratigraphy, but it is usually set to 3800
mya at the end of the
Hadean eon.
At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. Thus,
tectonic and
volcanic activity were considerably more active than they are today; the Earth's crust was not only thinner than is today, but probably broken up into many more
plates, with numerous
hot spots,
rift valleys, and
transform faults.
[Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0716728826 p. 297-301]There were no large continents until late in the Archean; small
protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. These
felsic protocontinents probably formed at hot spots rather than
subduction zones, from a variety of sources:
mafic magma melting more felsic rocks,
partial melting of mafic rock, and from the
metamorphic alteration of felsic sedimentary rocks.
[Stanley, pp. 297-301] The Archean atmosphere apparently lacked free
oxygen. Temperatures appear to have been near modern levels, although astronomers think that the sun was about one-third dimmer. This is thought to reflect larger amounts of greenhouse gases than later in the
Earth's history.
Although a few stones are known that are older, the oldest rock formations exposed on the surface of the
Earth are Archean or slightly older. Archean rocks are known from
Greenland, the
Canadian Shield, western
Australia, and southern
Africa. Although the first
continents formed during this eon, rock of this age makes up only 7% of the world's current
cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5-40% of the present continental
crust formed during the Archean.
[Stanley, pp. 301-2]In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as
graywackes,
mudstones, volcanic sediments, and
banded iron formations.
Greenstone belts are typical Archean formations, consisting of alternating high and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic
island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a
forearc basin. In short, greenstone belts represent sutured protocontinents.
[Stanley, pp. 302-3]Fossils of cyanobacterial mats (
stromatolites) are found throughout the Archeanwhile a few probable
bacterial fossils are known from
chert beds.
[Stanley, 307] In addition to the domain
Bacteria (once known as
Eubacteria), microfossils of the extremophilic domain
Archaea have also been identified.
Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called
Prokaryota (and formerly known as Monera); there are no known
eukaryotic fossils, though they might have evolved during the Archean and simply not left any fossils.
[Stanley, pp. 306, 323] However, no fossil evidence yet exists for ultramicroscopic intracellular organisms such as
viruses.
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GeoWhen Database
