Dictionary:
Pre·cam·bri·an (prē-kăm'brē-ən)  |
Of or belonging to the geologic time period between Hadean Time and the Cambrian Period, often subdivided into the Archean and Proterozoic eras, comprising most of the earth's history and marked by the appearance of primitive forms of life.
n.The Precambrian Eon.
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A major interval of geologic time between about 540 million years (Ma) and 3.8 billion years (Ga) ago, comprising the Archean and Proterozoic eons and encompassing most of Earth history. The Earth probably formed around 4.6 Ga and was then subjected to a period of intense bombardment by meteorites so that there are few surviving rocks older than about 3.8 billion years. Ancient rocks are preserved exclusively in continental areas. All existing oceanic crust is younger than about 200 million years, for it is constantly being recycled by the processes of sea-floor spreading and subduction. Development of techniques for accurate determination of the ages of rocks and minerals that are billions of years old has revolutionized the understanding of the early history of the Earth. See also Dating methods; Geochronometry; Geologic time scale; Rock age determination.
Detailed sedimentological and geochemical investigations of Precambrian sedimentary rocks and the study of organic remains have facilitated understanding of conditions on the ancient Earth. Microorganisms are known to have been abundant in the early part of Earth history. The metabolic activities of such organisms played a critical role in the evolution of the atmosphere and oceans. There have been attempts to apply the concepts of plate tectonics to Precambrian rocks. These diverse lines of investigation have led to a great leap in understanding the early history of the planet. See also Plate tectonics.
Rocks of the Archean Eon (2.5–3.8 Ga) are preserved as scattered small “nuclei” in shield areas on various continents. The Canadian shield contains perhaps the biggest region of Archean rocks in the world, comprising the Superior province. Much of the Archean crust is typified by greenstone belts, which are elongate masses of volcanic and sedimentary rocks that are separated and intruded by greater areas of granitic rocks. The greenstones are generally slightly metamorphosed volcanic rocks, commonly extruded under water, as indicated by their characteristic pillow structures. These structures develop when lava is extruded under water and small sac-like bodies form as the lava surface cools and they are expanded by pressure from lava within. Such structures are common in Archean greenstone assemblages in many parts of the world. See also Archean; Metamorphic rocks.
The Proterozoic Eon extends from 2.5 Ga until 540 Ma, the beginning of the Cambrian Period and Phanerozoic Eon. Proterozoic successions include new kinds of sedimentary rocks, display proliferation of primitive life forms such as stromatolites, and contain the first remains of complex organisms, including metazoans (the Ediacaran fauna). Sedimentary rocks of the Proterozoic Eon contain evidence of gradual oxidation of the atmosphere. Abundant and widespread chemical deposits known as banded iron formations (BIF) make their appearance in Paleo-Proterozoic sedimentary basins. See also Banded iron formation; Proterozoic.
| Geography Dictionary: Precambrian |
The oldest era in earth's history dating from about 4600 million years bp.
| Columbia Encyclopedia: Precambrian |
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.
| Wikipedia: Precambrian |
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The Precambrian (Pre-Cambrian) is an informal name for the span of time before the current Phanerozoic Eon, and is divided into several eons of the geologic timescale. It spans from the formation of Earth around 4500 Ma (million years ago) to the beginning of the Cambrian Period, when macroscopic hard-shelled animals first appeared in abundance about 542 Ma. The Precambrian is so named because it precedes the Cambrian, the first period of the Phanerozoic Eon, which is named after the Roman name for Wales, Cambria, where rocks from this age were first studied.
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Very little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what little is known has largely been discovered in the past four or five decades. The Precambrian fossil record is poor, and those fossils present (e.g. stromatolites) are of limited biostratigraphic use.[1] Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.[2][3]
It is thought that the Earth itself coalesced from material in orbit around the Sun roughly 4500 Ma and may have been struck by a very large (Mars-sized) planetesimal shortly after it formed, splitting off material that came together to form the Moon (see Giant impact theory). A stable crust was apparently in place by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma.[4]
The term Precambrian is somewhat out-moded, but is still in common use among geologists and paleontologists. It was briefly also called the Cryptozoic eon. It seems likely that it will eventually be replaced by the preferred terms Proterozoic, Archaean, and Hadean, and become a deprecated term. (See geologic time scale.)
It is not known when life originated, but carbon in 3.8 billion year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia.[5] Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.
Excepting a few contested reports of much older forms from USA and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan.
A very diverse collection of forms appeared around 544 Ma, starting in the latest Precambrian with a poorly understood small shelly fauna and ending in the very early Cambrian with a very diverse, and quite modern Burgess fauna, the rapid radiation of forms called the Cambrian explosion of life.
Details of plate motions and other tectonic functions are only hazily known in the Precambrian. It is generally believed that small proto-continents existed prior to 3000 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1000 Ma. The supercontinent, known as Rodinia, broke up around 600 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2200 Ma. The best studied is the Sturtian-Varangian glaciation, around 600 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".
The atmosphere of the early Earth is poorly known, but it is thought to have been smothered in reducing gases, containing very little free oxygen.
When evolving life forms developed photosynthesis, molecular oxygen began to be produced in large quantities, causing an ecological crisis sometimes called the oxygen catastrophe. The oxygen was immediately tied up in chemical reactions, primarily with iron, until the supply of oxidizable surfaces ran out. After that the modern high-oxygen atmosphere developed. Older rocks contain massive banded iron formations that were apparently laid down as iron and oxygen first combined.
An established terminology has evolved covering the early years of the Earth's existence, as radiometric dating allows plausible real dates to be assigned to specific formations and features.[6] The Precambrian Supereon is divided into three Precambrian eons: the Hadean (4500-3950 Ma), Archean (3950-2500 Ma), and Proterozoic (2500-542 Ma). See Timetable of the Precambrian.
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows.[8]
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| Precambrian | Phanerozoic | |||
|---|---|---|---|---|
| (Hadean) | Archean | Proterozoic | ||
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved. Read more |
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| Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Precambrian". Read more |
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