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Precambrian

Precambrian

In geologic time , Precambrian time encompasses the time from Earth's formation, approximately 4.5 billion years ago, until the start of the Cambrian approximately 540 million years ago (mya). Because the Precambrian is not a true geologic eon, era, period, or epoch, geologists often refer to it as Precambrian time (or simply, Precambrian). Precambrian time represents the vast bulk of Earth's geologic history and covers nearly 90% of Earth's history.

Although scientists do not yet know all the exact steps by which the earth formed, cooled, and took on its approximate shape and physical characteristics, a good deal of reliable evidence can be inferred from studies that concentrate on the formation of landmass, oceans , and atmosphere. Astrophysical dataand theories of physics that explain the evolution of physical law and nucelosynthesismake these studies of Earth's formation both possible and reliable because the same laws of physics and chemistry that exist now operated during the formation of Earth's solar system .

Radiological dating provides overwhelming evidence that dates known terrestrial (Earth origin) rock specimens to more than 3.6 billion years old. Earth and lunar meteorites date to 4.5 billion years.

Precambrian time is subdivided into Hadean time (4.5 billion years ago to 3.8 billion years ago); Archean time (3.8 billion years ago to 2.5 billion years ago; Paleoproterozoic time (2.5 billion years ago to 1.6 billion years ago); Mesoproterozoic time (1.6 billion years ago to 900 million years ago); and Neoproterozoic time (900 million years ago to 540 mya).

Hadean time represents the time during which the solar system formed. During the subsequent course of Precambrian time, Earth's lithospheric plates formed and the mechanisms of geologic change described by modern plate tectonic theory began to occur. During Precambrian time, life arose on Earth. The oldest known fossil evidence (fig tree group fossils in what is now Africa ) dates to early in Archaean time. During the Paleoproterozoic, Earth's primitive atmosphere made a transition to an oxygen rich atmosphere. Soon thereafter in geologic time, i.e. within a few hundred million years, there is evidence of the earliest appearance of eukaryotes (organisms with a true nucleus containing DNA). Evidence of the oldest fossilized animal remains dates to the end of Neoproterozic time.

The extensive debris field that existed in the early solar system assured frequent bombardment of Earth's primitive atmosphere by asteroids and comets . Despite the consuming effects of geological weathering and erosion , evidence of Precambrian time impacts dating almost 2.0 billion years ago have been found in what are now South Africa and Canada.

See also Cambrian Period; Cenozoic Era; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miller-Urey experiment; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Tertiary Period; Triassic Period

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"Precambrian." World of Earth Science. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

"Precambrian." World of Earth Science. . Encyclopedia.com. (August 16, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/precambrian

"Precambrian." World of Earth Science. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/precambrian

Precambrian

Precambrian, name of a major division of geologic time (see Geologic Timescale, table), from c.5 billion to 570 million years ago. It is now usually divided into the Archean and Proterozoic eons. Precambrian time includes 80% of the earth's history.

Precambrian rocks are mostly covered by rock systems of more recent origin, but where visible they commonly display evidence of having been altered by intense metamorphism. Precambrian rocks often occur in shields, which are large areas of relatively low elevation that form parts of continental masses. One of the largest exposed areas of early Precambrian rocks is the Canadian Shield, where geologist Sir William Logan did his pioneer work. It covers most of Greenland, extends over more than half of Canada, and reaches into the United States as the Superior Highlands and the Adirondack Mts.

The rocks of this region, and of the early Precambrian as a whole, are generally granite, schist, or gneiss. The most notable formations are the Keewatin and Coutchiching of Minnesota and the adjoining part of Canada; the Grenville of Ontario, which, however, may be late Precambrian; and the widely distributed Laurentian. The Keewatin series of rocks is composed chiefly of metamorphosed lava, with some sediments; the Coutchiching series is chiefly of sedimentary gneisses and schists. The Grenville limestone, marble, gneiss, and quartzite are predominantly metamorphosed sediments; the Laurentian gneiss and granite are probably younger than the other series, having been forced up through the Grenville as igneous rock. After the appearance of the Laurentian, the Temiskaming, or Sudburian, sediments were deposited, and a second series of gneisses and granites, the Algoman, was formed.

Elsewhere in North America, early Precambrian rocks are exposed in the Grand Canyon of Arizona and in the Teton Range of Wyoming. Among the other shield areas composed of early Precambrian rocks are the Angara Shield in Siberia, the Australian Shield, the Baltic Shield in Europe, the Antarctic Shield, and the African Shield comprising most of the African continent. In South America, the Amazon River basin separates the Guiana and the Brazilian shields. Fossils have been reported from this era, but few have been found in strata universally acknowledged to be early Precambrian. Evidence such as bacteria and algallike spheroids, supports the belief that rudimentary life existed. During the early Precambrian, radioactive heat from the new planet may have been so great that little permanent crust could survive.

By the latter Precambrian, heat dissipated enough to allow the continental crust to form; crustal rifting, mountain building, and volcanic activity then dominated, as did sedimentation. The life of the late Precambrian is poorly represented by fossils, but a few invertebrates including creatures resembling jellyfish and worms have been discovered. The best evidence that there probably were numerous forms of life is the variety and complexity which suddenly appears in Cambrian fauna. Mineral deposits associated with Precambrian rocks have yielded most of the world's gold and nickel in addition to large quantities of copper, silver, radium, and uranium.

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"Precambrian." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

"Precambrian." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (August 16, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/precambrian

"Precambrian." The Columbia Encyclopedia, 6th ed.. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/precambrian

Precambrian

Precambrian A name that is now used only informally to describe the longest period of geological time, which began with the consolidation of the Earth's crust and ended with the beginning of the Cambrian Period 590 Ma ago. The Precambrian lasted approximately 4000 Ma; the rocks of this period of geological time are usually altered and few fossils with hard parts or skeletons have been found within them, although Precambrian limestone rocks in Australia, Siberia, and parts of the USA contain stromatolites, which are believed to have been formed by cyanobacteria; frond-like impressions of a supposed plant (Charnia) have also been found. The Ediacaran fauna is late Precambrian. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea. In modern usage the Precambrian is subdivided into the Proterozoic, Archaean, and Priscoan.

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"Precambrian." A Dictionary of Ecology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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"Precambrian." A Dictionary of Ecology. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/precambrian-0

Precambrian

Precambrian A name that is now used only informally to describe the longest period of geological time, which began with the consolidation of the Earth's crust and ended with the beginning of the Cambrian Period 590 Ma ago. The Precambrian lasted approximately 4000 Ma; the rocks of this period of geological time are usually altered, and few fossils with hard parts or skeletons have been found within them, although Precambrian limestone rocks in Australia, Siberia, and parts of the USA contain stromatolites, which are believed to have been formed by cyanobacteria; frond-like impressions of a supposed plant (Charnia) have also been found. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea. In modern usage the Precambrian is subdivided into the Proterozoic, Archaean, and Priscoan.

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"Precambrian." A Dictionary of Plant Sciences. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Precambrian

Precambrian Describing the time from the formation of the earth, nearly 5 billion years ago, to the start of the Cambrian period, around 570 million years ago. The term `Precambrian' is no longer used for a specific geological time interval, but remains as a general adjective. Precambrian time is now divided into three eons: Hadean, Archaean, and Proterozoic, the latter extending to the start of the present (Phanerozoic) eon. Fossils are rare, although stromatolites indicate that there were flourishing populations of cyanobacteria and other bacteria. However, subsequent metamorphism of Precambrian rocks makes correlation of rocks and events extremely difficult. The largest areas of exposed Precambrian rocks are the shield areas, such as the Canadian (Laurentian) Shield and the Baltic Shield.

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"Precambrian." A Dictionary of Biology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Precambrian

Precambrian A name that is now used only informally to describe the longest period of geological time, which began with the consolidation of the Earth's crust and ended with the beginning of the Cambrian Period 542 Ma ago. The Precambrian lasted approximately 4000 Ma; the rocks of this period of geological time are usually altered and few fossils with hard parts or skeletons have been found within them. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea. In modern usage the Precambrian is subdivided into the Hadian, Archaean, and Proterozoic.

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"Precambrian." A Dictionary of Zoology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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"Precambrian." A Dictionary of Zoology. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/precambrian-2

Precambrian

Precambrian A name now used only informally to describe the Priscoan, Archaean, and Proterozoic, which together comprise the longest period of geologic time that began with the consolidation of the Earth's crust and ended approximately 4000 million years later with the beginning of the Cambrian Period around 570 Ma ago. The rocks of this period of geologic time are usually altered and few fossils with hard parts or skeletons have been found within them. Precambrian rocks outcrop extensively in shield areas such as northern Canada and the Baltic Sea.

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"Precambrian." A Dictionary of Earth Sciences. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Precambrian

Precambrian Oldest and longest era of Earth's history, lasting from the formation of the Earth c.4.6 billion years ago to the beginning of a good fossil record c.590 million years ago. Precambrian fossils are extremely rare, probably because the earliest life forms did not have hard parts suitable for preservation. Also, Precambrian rocks have been greatly deformed. Primitive bacteria and cyanobacteria have been identified in deposits that are more than 3 billion years old.

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"Precambrian." World Encyclopedia. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Precambrian

Pre·cam·bri·an / prēˈkambrēən; -kām-/ • adj. Geol. of, relating to, or denoting the earliest eon, preceding the Cambrian period and the Phanerozoic eon. Compare with Cryptozoic. ∎  [as n.] (the Precambrian) the Precambrian eon or the system of rocks deposited during it.

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"Precambrian." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

"Precambrian." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (August 16, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/precambrian

"Precambrian." The Oxford Pocket Dictionary of Current English. . Retrieved August 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/precambrian