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The Archean
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
Hadean
Eon and preceded the Proterozoic
Proterozoic
Eon. During the Archean, the Earth's crust had cooled enough to allow the formation of continents.

Contents

1 Etymology and changes in classification 2 Earth
Earth
at the beginning of the Archean

2.1 Palaeoenvironment

3 Geology 4 Early life in the Archean 5 See also 6 References 7 External links

Etymology and changes in classification[edit] Archean
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."[1] In earlier literature the Hadean
Hadean
Eon was included as part of the Archean.[citation needed] Instead of being based on stratigraphy, the beginning and end of the Archean
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.[2] Earth
Earth
at the beginning of the Archean[edit]

Artist's impression of an Archean
Archean
landscape

The Archean
Archean
is one of the four principal eons of Earth
Earth
history. When the Archean
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
Archean
to the Proterozoic
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. Most surviving Archean
Archean
rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Granitic
Granitic
rocks predominate throughout the crystalline remnants of the surviving Archean
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

The Earth
Earth
of the early Archean
Archean
may have supported a tectonic regime unlike that of the present. Some scientists[who?] argue that, because the Earth
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
Archean
rocks is a function of erosion by subsequent tectonic events. The question of whether plate tectonic activity existed in the Archean
Archean
is an active area of modern research.[3] 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.[citation needed] The other school follows that of Richard Armstrong, who argued that the continents grew to their present volume in the first 500 million years of Earth
Earth
history and have maintained a near-constant ever since: throughout most of Earth
Earth
history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth.[citation needed] Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics operated in the Archean
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.[citation needed] 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 needed] 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.[citation needed] Palaeoenvironment[edit] The Archean
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
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.[4][5] Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.[6] 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. Geology[edit] Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth
Earth
are Archean
Archean
or slightly older. Archean
Archean
rocks are found in Greenland, Siberia, the Canadian Shield, Montana
Montana
and Wyoming
Wyoming
(exposed parts of the Wyoming
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.[7] In contrast to Proterozoic
Proterozoic
Eon rocks, Archean
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.[8] Greenstone belts are typical Archean
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.[9] Early life in the Archean[edit]

Life
Life
timeline

view • discuss • edit

-4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 —

water

Single-celled life

photosynthesis

Eukaryotes

Multicellular life

Land life

Dinosaurs    

Mammals

Flowers

 

Earliest Earth
Earth
(−4540)

Earliest water

Earliest life

LHB meteorites

Earliest oxygen

Atmospheric oxygen

Oxygen
Oxygen
crisis

Earliest sexual reproduction

Ediacara biota

Cambrian
Cambrian
explosion

Earliest humans

P h a n e r o z o i c

P r o t e r o z o i c

A r c h e a n

H a d e a n

Pongola

Huronian

Cryogenian

Andean

Karoo

Quaternary

Axis scale: million years Orange labels: ice ages. Also see: Human
Human
timeline and Nature
Nature
timeline

The processes that gave rise to life on Earth
Earth
are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean
Hadean
Eon or early in the Archean
Archean
Eon. Biogenic carbon has been detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation.[10] 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
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.[11] The Hadean
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 Hadean
Hadean
than today.[12] Thus, the Hadean
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 following Archean
Archean
Eon rather than in the Hadean
Hadean
Eon depended on the presence of an ocean.[citation needed] Water
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
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).[13] The Hadean
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 chemistry.[14][15] 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
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
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 time.[16] 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 timeframe.[17] The earliest evidence for life on Earth
Earth
are graphite of biogenic origin found in 3.7-billion-year-old metasedimentary rocks discovered in Western Greenland[18] and microbial mat fossils found in 3.48-billion-year-old sandstone discovered in Western Australia.[19][20] Pyrite
Pyrite
found in 3.47-billon-year-old baryte, in the Warrawoona Group
Warrawoona Group
of Western Australia, shows sulfur fractionation of as much as 21.1%,[21] because sulfate-reducing bacteria metabolize sulfur-32 more readily than sulfur-34.[22] More recently, in 2015, "remains of biotic life" were found in 4.1-billion-year-old rocks in Western Australia.[23][24] According to one of the researchers, "If life arose relatively quickly on Earth
Earth
… then it could be common in the universe."[23] Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating free oxygen in the atmosphere[25]) are found throughout the Archean,[26] becoming especially common late in the eon, while a few probable bacterial fossils are known from chert beds.[27] In addition to the domain Bacteria
Bacteria
(once known as Eubacteria), microfossils of the domain Archaea
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 Africa. Earth
Earth
was very hostile to life before 4.2–4.3 Ga and the conclusion is that before the Archean
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 the Archean
Archean
Eon.[28] Life
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 during the Archean
Archean
without leaving any.[29] No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses. See also[edit]

Abiogenesis Cosmic Calendar Earliest known life forms Geologic time scale History of Earth Precambrian Timeline of natural history

References[edit]

^ Harper, Douglas. "Archaean". Online Etymology Dictionary.  ^ "International Chronostratigraphic Chart v.2013/01" (PDF). International Commission on Stratigraphy. January 2013. Retrieved April 6, 2013.  ^ Stanley, Steven M. (1999). Earth
Earth
System History. New York: W.H. Freeman and Company. pp. 297–301. ISBN 0-7167-2882-6.  ^ Walker, James C. G. (June 1985). " Carbon dioxide
Carbon dioxide
on the early earth" (PDF). Origins of Life
Life
and Evolution of the Biosphere. 16 (2): 117–27. Bibcode:1985OLEB...16..117W. doi:10.1007/BF01809466. Retrieved 2010-01-30.  ^ Pavlov, Alexander A.; Kasting, James F.; Brown, Lisa L.; Rages, Kathy A.; Freedman, Richard (May 2000). "Greenhouse warming by CH4 in the atmosphere of early Earth". Journal of Geophysical Research. 105 (E5): 11981–90. Bibcode:2000JGR...10511981P. doi:10.1029/1999JE001134.  ^ Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; Bjerrum, Christian J. (April 1, 2010). "No climate paradox under the faint early Sun". Nature. 464 (7289): 744–47. Bibcode:2010Natur.464..744R. doi:10.1038/nature08955. PMID 20360739.  ^ Stanley, pp. 301–02 ^ Cooper, John D.; Miller, Richard H.; Patterson, Jacqueline (1986). A Trip Through Time: Principles of Historical Geology. Columbus: Merrill Publishing Company. p. 180. ISBN 0675201403.  ^ Stanley, pp. 302–03 ^ Bell EA, Boehnke P, Harrison TM, Mao WL (2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon". Proc. Natl. Acad. Sci. U.S.A. 112: 14518–21. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. PMC 4664351 . PMID 26483481.  ^ Noffke N, Christian D, Wacey D, Hazen RM (2013). "Microbially induced sedimentary structures recording an ancient ecosystem in the ca. 3.48 billion-year-old Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12): 1103–24. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916 . PMID 24205812.  ^ Martin RS; Mather TA & Pyle DM (2007). " Volcanic
Volcanic
emissions and the early Earth
Earth
atmosphere". Geochimica et Cosmochimica Acta. 71: 3673–85. Bibcode:2007GeCoA..71.3673M. doi:10.1016/j.gca.2007.04.035.  ^ Miller SL (1953). "A production of amino acids under possible primitive Earth
Earth
conditions". Science. 117: 528–29. Bibcode:1953Sci...117..528M. doi:10.1126/science.117.3046.528. PMID 13056598.  ^ Tervahattu H; Juhanoja J & Kupianinen K (2002). "Identification of an organic coating on marine aerosol particles by TOF-SIMS". Journal of Geophysical Research. 107. Bibcode:2002JGRD..107.4319T. doi:10.1029/2001jd001403.  ^ Donaldson DJ; Tervahattu H; Tuck AF & Vaida V (2004). "Organic aerosols and the origin of life: a hypothesis". Origins of Life
Life
and Evolution of Biospheres. 34: 57–67. Bibcode:2004OLEB...34...57D. doi:10.1023/b:orig.0000009828.40846.b3.  ^ Polat, Ali (2013). "Geochemical Variations in Archaeon Volcanic Rocks, Southwestern Greenland: Traces of Diverse Tectonic Settings in the Early Earth". Geology. 41: 379–80. Bibcode:2013Geo....41..379P. doi:10.1130/focus0320131.1.  ^ Stüeken, E. E.; R. E. Anderson; J. S. Bowman; W. J. Brazelton; J. Colangelo-Lillis; A. D. Goldman; et al. (2013). "Did Life
Life
Originate from a Global Chemical Reactor?". Geobiology. 11: 101–26. doi:10.1111/gbi.12025.  ^ Yoko Ohtomo; Takeshi Kakegawa; Akizumi Ishida; Toshiro Nagase; Minik T. Rosing (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature
Nature
Geoscience. Bibcode:2014NatGe...7...25O. doi:10.1038/ngeo2025. Retrieved 9 Dec 2013.  ^ Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013.  ^ Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12): 1103–24. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916 . PMID 24205812. Retrieved 15 November 2013.  ^ Shen, Y.; Buick, R.; Canfield, D. E. (March 2001) "Isotopic evidence for microbial sulfate in the early Archaean era" Nature
Nature
410 (6824): 77–81. doi:10.1038/35065017 . ^ R. R. Seal, II (2006), "Sulfur Isotope Geochemistry of Sulfide Minerals", Reviews in Mineralogy and Geochemistry 61 (1): 633–77. doi:10.2138/rmg.2006.12 . ^ a b Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Retrieved 2015-10-20.  ^ Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark; et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon" (PDF). Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 112: 14518–21. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. ISSN 1091-6490. PMC 4664351 . PMID 26483481. Retrieved 2015-10-20.  Early edition, published online before print. ^ "Early life: Oxygen
Oxygen
enters the atmosphere". BBC. Retrieved September 20, 2012.  ^ Garwood, Russell J. (2012). "Patterns In Palaeontology: The first 3 billion years of evolution". Palaeontology Online. 2 (11): 1–14. Retrieved June 25, 2015.  ^ Stanley, p. 307 ^ Nisbet, Euan (1980). "Archaeon Stromatolites and the Search for the Earliest Life". Nature. 284: 395–96. Bibcode:1980Natur.284..395N. doi:10.1038/284395a0.  ^ Stanley, pp. 306, 323

External links[edit]

Wikimedia Commons has media related to Archean.

Wikisource
Wikisource
has the text of the 1911 Encyclopædia Britannica article Archean
Archean
System.

GeoWhen Database When Did Plate Tectonics Begin?

v t e

Archean
Archean
Eon

Eoarchean Paleoarchean Mesoarchean Neoarchean

v t e

Geologic history of Earth

Cenozoic
Cenozoic
era¹ (present–66.0 Mya)

Quaternary
Quaternary
(present–2.588 Mya)

Holocene
Holocene
(present–11.784 kya) Pleistocene
Pleistocene
(11.784 kya–2.588 Mya)

Neogene
Neogene
(2.588–23.03 Mya)

Pliocene
Pliocene
(2.588–5.333 Mya) Miocene
Miocene
(5.333–23.03 Mya)

Paleogene (23.03–66.0 Mya)

Oligocene
Oligocene
(23.03–33.9 Mya) Eocene
Eocene
(33.9–56.0 Mya) Paleocene
Paleocene
(56.0–66.0 Mya)

Mesozoic
Mesozoic
era¹ (66.0–251.902 Mya)

Cretaceous
Cretaceous
(66.0–145.0 Mya)

Late (66.0–100.5 Mya) Early (100.5–145.0 Mya)

Jurassic
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
Triassic
(201.3–251.902 Mya)

Late (201.3–237 Mya) Middle (237–247.2 Mya) Early (247.2–251.902 Mya)

Paleozoic
Paleozoic
era¹ (251.902–541.0 Mya)

Permian
Permian
(251.902–298.9 Mya)

Lopingian
Lopingian
(251.902–259.8 Mya) Guadalupian
Guadalupian
(259.8–272.3 Mya) Cisuralian
Cisuralian
(272.3–298.9 Mya)

Carboniferous
Carboniferous
(298.9–358.9 Mya)

Pennsylvanian (298.9–323.2 Mya) Mississippian (323.2–358.9 Mya)

Devonian
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
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
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
Cambrian
(485.4–541.0 Mya)

Furongian (485.4–497 Mya) Series 3 (497–509 Mya) Series 2 (509–521 Mya) Terreneuvian
Terreneuvian
(521–541.0 Mya)

Proterozoic
Proterozoic
eon² (541.0 Mya–2.5 Gya)

Neoproterozoic era (541.0 Mya–1 Gya)

Ediacaran
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
Orosirian
(1.8-2.05 Gya) Rhyacian (2.05-2.3 Gya) Siderian
Siderian
(2.3-2.5 Gya)

Archean
Archean
eon² (2.5–4 Gya)

Eras

Neoarchean (2.5–2.8 Gya) Mesoarchean (2.8–3.2 Gya) Paleoarchean
Paleoarchean
(3.2–3.6 Gya) Eoarchean
Eoarchean
(3.6–4 Gya)

Hadean
Hadean
eon² (4–4.6 Gya)

 

 

kya = thousands years ago. Mya = millions years ago. Gya = billions years ago.¹ = Phanerozoic
Phanerozoic
eon. ² = Precambrian
Precambrian
supereon. 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.

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