The Precambrian (Pre-Cambrian) is an informal name for the span of time before the current Phanerozoic The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared. Its name is derived from the Greek words φαίνω and ζωή, meaning make life appear, since it was once Eon, and is divided into several eons The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates of the geologic time scale The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates. It spans from the formation of Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7] around 4500 Ma A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic (million years ago) to the beginning of the Cambrian The Cambrian is the first geological period of the Paleozoic Era, lasting from 542 ± 0.3 million years ago to 488.3 ± 1.7 million years ago(ICS, 2004,; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the classical name for 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 The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates of the Phanerozoic Eon, which is named after the Roman name for Wales Wales ( /ˈweɪlz/ Welsh: Cymru; pronounced [ˈkəmrɨ] (help·info)) is a country that is part of the United Kingdom, bordered by England to its east, and the Atlantic Ocean and Irish Sea to its west. Wales has a population estimated at three million and is officially bilingual; Welsh and English have equal status, and bilingual signs are the, Cambria According to Geoffrey of Monmouth in the first part of his pseudohistory Historia Regum Britanniae, the Trojan Brutus had three sons among whom he divided his lands after landing in Britain. His elder son, Locrinus, received the land between the rivers Humber and Severn, which he called Loegria (a Latinization of the Welsh name Lloegr, ", where rocks from this age were first studied. The Precambrian period accounts for 87% of geologic time.

Overview

Very little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.6 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of, and what little is known has largely been discovered in the past fifty years. The Precambrian fossil record is poor, and those fossils present (e.g. stromatolites Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). They include some of the most ancient records of life on Earth) are of limited biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at some other section use.^[1] Many Precambrian rocks are heavily metamorphosed Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared. Its name is derived from the Greek words φαίνω and ζωή, meaning make life appear, since it was once 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 Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact-sized) planetesimal A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously shortly after it formed, splitting off material that came together to form the Moon The Moon is Earth's only natural satellite[nb 4] and is the fifth largest satellite in the Solar System. It is the largest natural satellite in the Solar System relative to the size of its planet, a quarter the diameter of Earth and 1/81 its mass, and is the second densest satellite after Io. It is in synchronous rotation with Earth, always (see Giant impact theory). A stable crust was apparently in place by 4400 Ma, since zircon Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is Zr crystals from Western Australia have been dated Radiometric dating is a technique used to date materials, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth at 4404 Ma.^[4]

The term Precambrian is somewhat out-moded, but is still in common use among geologists A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using physics, chemistry and biology as well as other and paleontologists Paleontology [note 1] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments (their paleoecology). As a "historical science" it tries to explain causes rather than conduct experiments to observe effects. Paleontological observations have been documented as far back as the 5. It was briefly also called the Cryptozoic eon. It seems likely that it will eventually be replaced by the preferred terms Proterozoic The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life". The Proterozoic Eon extended from 2500 Ma to 542.0 ± 1.0 Ma (million years ago), and is the most recent part of the old, informally named ‘Precambrian’ time, Archaean The Archean (pronounced /ɑrˈkiːən/, also spelled Archaean, formerly called the Archaeozoic , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga (billion years ago, or 2,500 Ma). Instead of being based on stratigraphy, this date is defined chronometrically. The lower, and Hadean The Hadean is the geologic eon before the Archean. It started at Earth's formation about 4.6 billion years ago (4,600 Ma), and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the conditions on Earth at, and become a deprecated term In computer software or authoring programs standards and documentation, the term deprecation is applied to software features that are superseded and should be avoided. Although deprecated features remain in the current version, their use may raise warning messages recommending alternative practices, and deprecation may indicate that the feature. (See geologic time scale The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates.)

Life before the Cambrian

It is not known when life originated, but carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life of in 3.8 billion year old rocks from islands off western Greenland b. ^ Greenland, the Faeroes and Iceland were formally Norwegian possessions until 1814 despite 400 years of Danish monarchy beforehand may be of organic origin. Well-preserved bacteria The bacteria ( [bækˈtɪəriə] ; singular: bacterium)[α] are a large group of single-celled, prokaryote microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, older than 3.46 billion years have been found in Western Australia Western Australia is a state of Australia, occupying the entire western third of the Australian continent. Australia's largest state and the second largest subnational entity in the world, it has 2.2 million inhabitants , 85% of whom live in the south-west corner of the state.^[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 The Ediacara biota consisted of enigmatic tubular and frond-shaped sessile (stationary) organisms which lived during the Ediacaran Period (ca. 635-542 Ma). Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.[note 1] The Ediacara biota first appeared as the Earth thawed from. 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 Burgess Shale Formation — located in the Canadian Rockies of British Columbia — is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils. It is 505 million years old, one of the earliest soft-parts fossil beds, the rapid radiation of forms called the Cambrian explosion The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, over a period of many million years, of most major groups of complex animals around 530 million years ago, as found in the fossil record. This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes. Before of life.

Planetary environment and the oxygen catastrophe

Details of plate motions Plate tectonics is a scientific theory which describes the large scale motions of Earth's lithosphere. The theory builds on the older concepts of continental drift, developed during the first decades of the 20th century by Alfred Wegener, and seafloor spreading, developed in the 1960s and other tectonic Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth (or other planets) and particularly with the forces and movements that have operated in a region to create these structures 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 In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today around 1000 Ma. The supercontinent, known as Rodinia In contrast with Pangaea, the last supercontinent about 300 million years ago, little is known yet about the exact configuration and geodynamic history of Rodinia. Paleomagnetic evidence provides some clues to the paleolatitude of individual pieces of the Earth's crust, but not to their longitude, which geologists have pieced together by comparing, broke up around 600 Ma. A number of glacial periods A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate within an ice age. The last glacial period ended about 15,000 years ago ; the current Holocene epoch is the interglacial we are presently in have been identified going as far back as the Huronian epoch, roughly 2200 Ma. The best studied^{[citation needed]} 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 Snowball Earth refers to the hypothesis that the Earth's surface became nearly or entirely frozen at least once, before 650 million years ago. The geological community generally accepts this hypothesis because it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes and other enigmatic features in".

The atmosphere The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21% of the early Earth is poorly known, but it is thought to have been smothered in reducing Redox describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the gases, containing very little free oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly. The oxygen free early atmosphere has been disputed with evidence supporting an oxygenic atmosphere since the early Archean.^[6]

When evolving life forms developed photosynthesis Photosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria,, molecular A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly began to be produced in large quantities, causing an ecological crisis sometimes called the oxygen catastrophe The Great Oxygenation Event , also called the oxygen catastrophe or oxygen crisis or Great Oxidation, was the appearance of free oxygen (O2) in Earth's atmosphere. This major environmental change happened around 2,400 million years ago. 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 Banded iron formations are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed over 3,700 million years that were apparently laid down as iron and oxygen first combined.

Subdivisions

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.^[7] 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.

• Proterozoic: this eon refers to the time from the lower Cambrian boundary, 542 Ma, back through 2500 Ma. The boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary. The Proterozoic eon is divided into three eras: the Neoproterozoic, Mesoproterozoic, and Paleoproterozoic.
  • Neoproterozoic: The upper (i.e., youngest) geologic era of the Proterozoic eon, from the Cambrian period lower boundary (542 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
    • Ediacaran: The upper (youngest) geologic period within the Neoproerozoic Era. The "2009 GSA Geologic Time Scale" dates it from 630-542 Ma. (542 Ma is the beginning of the Cambrian period, the earliest period of the Paleozoic Era.) In this period the Ediacaran fauna appeared.
    • Cryogenian: The middle period in the Neoproterozoic Era: 950-630 Ma. =)
    • Tonian: the earliest period of the Neoproterozoic Era: 1000-950 Ma.
  • Mesoproterozoic: the middle era of the Proterozoic Eon, 1000-1600 Ma. Corresponds to "Precambrian Y" rocks of older North American geology.
  • Paleoproterozoic: oldest era of the Proterozoic Eon, 1600-2500 Ma. Corresponds to "Precambrian X" rocks of older North American geology.
• Archaean Eon: 2500-3800 Ma.
• Hadean Eon: 3950-4500 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the moon and meteorites.^[8]}

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.^[9]

1. Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
2. Hadean: dominated by heavy bombardment from about 4.51, (possibly including a Cool Early Earth period) to the end of the Late Heavy Bombardment period.
3. Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.
4. Transition: a period of continued iron banded formation until the first continental red beds.
5. Proterozoic: a period of modern plate tectonics until the first animals.

References

1. ^ James Monroe and Reed Wicander, The Changing Earth, 2nd ed, (Belmont: Wadsworth Publishing Company, 1997), p. 492.
2. ^ Monroe and Wicander, p. 492.
3. ^ "Pamela J.W. Gore, "The Precambrian". Retrieved on 12/6/06.". http://gpc.edu/~pgore/geology/geo102/precamb.htm.
4. ^ Zircons are Forever "Zircons are Forever". http://www.geology.wisc.edu/zircon/zircon_home.html Zircons are Forever. Retrieved 2007-04-28.
5. ^ Brun, Yves and Lawrence J. Shimkets, Prokaryotic development, ASM Press, Jan. 2000, p. 114 ISBN 978-1555811587
6. ^ Clemmey, Harry and Nick Badham, Oxygen in the Precambrian Atmosphere Geology; March 1982; v. 10; no. 3; p. 141-146
7. ^ Geological Society of America's "2009 GSA Geologic Time Scale."
8. ^ www.uwsp.edu/geo/faculty/hefferan/Geol106/.../hadean.htm
9. ^ Bleeker, W. (2004) [2004]. "Toward a "natural" Precambrian time scale". in Felix M. Gradstein, James G. Ogg, and Alan G. Smith. A Geologic Time Scale 2004. Cambridge University Press. ISBN 0-521-78673-8. also available at Stratigraphy.org: Precambrian subcommission

Further reading

• Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison Wgeology.wisc.edu – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
• Wilde, S. A.; Valley, J. W.; Peck, W. H.; Graham, C. M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature 409 (6817): 175–178. doi:10.1038/35051550. PMID 11196637.
• Wyche, S.; Nelson, D. R.; Riganti, A. (2004). "4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton". Australian Journal of Earth Sciences 51 (1): 31–45. doi:10.1046/j.1400-0952.2003.01042.x.

See also

• Timeline of the Precambrian

External links

• Late Precambrian Supercontinent and Ice House World from the Paleomap Project

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