Precambrian

The Precambrian (or Pre-Cambrian, sometimes abbreviated , or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

The Precambrian (colored green in the timeline figure) is an informal unit of geologic time,[1] subdivided into three eons (Hadean, Archean, Proterozoic) of the geologic time scale. It spans from the formation of Earth about 4.6 billion years ago (Ga) to the beginning of the Cambrian Period, about 541 million years ago (Ma), when hard-shelled creatures first appeared in abundance.

Precambrian Eon
4600–541 million years ago
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Overview

Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what is known has largely been discovered from the 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding Phanerozoic, and fossils from the Precambrian (e.g. stromatolites) are of limited biostratigraphic use.[2] This is because many Precambrian rocks have been heavily metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.[2][3][4]

It is thought that the Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by a very large (Mars-sized) planetesimal shortly after it formed, splitting off material that formed the Moon (see Giant impact hypothesis). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma.[5]

The term "Precambrian" is recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time; it is so called because it includes the Hadean (~4.6—4 billion), Archean (4—2.5 billion), and Proterozoic (2.5 billion—541 million) eons. (There is only one other eon: the Phanerozoic, 541 million-present.)[6] "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. As of 2010, the United States Geological Survey considers the term informal, lacking a stratigraphic rank.[7]

Life forms

Positions of ancient continents, 550 million years ago
Landmass positions near the end of the Precambrian

A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks (Archean eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia.[8] Probable fossils 100 million years older have been found in the same area. However, there is evidence that life could have evolved over 4.280 billion years ago.[9][10][11][12] There is a fairly solid record of bacterial life throughout the remainder (Proterozoic eon) of the Precambrian.

Excluding a few contested reports of much older forms from North America and India, the first complex multicellular life forms seem to have appeared at roughly 1500 Ma, in the Mesoproterozoic era of the Proterozoic eon. Fossil evidence from the later Ediacaran period of such complex life comes from the Lantian formation, at least 580 million years ago. A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic eon. By the middle of the following Cambrian period, a very diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life.[13][14]

While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas.[15]

Tracks from an animal with leg like appendages have been found in what was mud 551 million years ago.[16]

Planetary environment and the oxygen catastrophe

Temagami greenstone belt pillow lava
Weathered Precambrian pillow lava in the Temagami Greenstone Belt of the Canadian Shield

Evidence of the details of plate motions and other tectonic activity in the Precambrian has been poorly preserved. It is generally believed that small proto-continents existed prior to 4280 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia, broke up around 750 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 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 not well understood. Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and was lacking in free oxygen. There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean.[17]

At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism. This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe. At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides.

Subdivisions

A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed real dates to be assigned to specific formations and features.[18] The Precambrian is divided into three eons: the Hadean (4600–4000 Ma), Archean (4000-2500 Ma) and Proterozoic (2500-541 Ma). See Timetable of the Precambrian.

  • Proterozoic: this eon refers to the time from the lower Cambrian boundary, 541 Ma, back through 2500 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 youngest geologic era of the Proterozoic Eon, from the Cambrian Period lower boundary (541 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
    • 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.
  • Archean Eon: 2500-4000 Ma.
  • Hadean Eon: 4000–4600 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.[19]

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:[20]

  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 Ga (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.

Precambrian supercontinents

The movement of Earth's plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent containing most or all of the landmass. The earliest known supercontinent was Vaalbara. It formed from proto-continents and was a supercontinent 3.636 billion years ago. Vaalbara broke up c. 2.845–2.803 Ga ago. The supercontinent Kenorland was formed c. 2.72 Ga ago and then broke sometime after 2.45–2.1 Ga into the proto-continent cratons called Laurentia, Baltica, Yilgarn craton, and Kalahari. The supercontinent Columbia or Nuna formed 2.06–1.82 billion years ago and broke up about 1.5–1.35 billion years ago.[21][22] The supercontinent Rodinia is thought to have formed about 1.13–1.071 billion years ago, to have embodied most or all of Earth's continents and to have broken up into eight continents around 750–600 million years ago.

See also

  • Phanerozoic – Fourth and current eon of the geological timescale
    • Paleozoic – First era of the Phanerozoic Eon
    • Mesozoic – Second era of the Phanerozoic Eon, also known as Age of Reptiles
    • Cenozoic – Third and current era of the Phanerozoic Eon

References

  1. ^ Gradstein, F.M.; Ogg, J.G.; Schmitz, M.D.; Ogg, G.M. (editors) (2012). The Geologic Timescale 2012 (volume 1). Elsevier. p. 301. ISBN 978-0-44-459390-0.CS1 maint: Multiple names: authors list (link) CS1 maint: Extra text: authors list (link)
  2. ^ a b Monroe, James S.; Wicander, Reed (1997). The Changing Earth: Exploring Geology and Evolution (2nd ed.). Belmont: Wadsworth Publishing Company. p. 492. ISBN 9781285981383.
  3. ^ Levin, Harold L. (25 October 2005). "The Earliest Earth: 2,100,000,000 years of the Archean Eon". In Gore, Pamela J.W. The Earth Through Time. p. 1.
  4. ^ Davis, C.M. (1964). "The Precambrian Era". Readings in the Geography of Michigan. Michigan State University.
  5. ^ "Zircons are Forever". Department of Geoscience. 2005. Retrieved 28 April 2007.
  6. ^ Fan, Junxuan; Hou, Xudong (February 2017). "Chart". International Commission on Stratigraphy. International Chronostratigraphic Chart. Retrieved 10 May 2018.
  7. ^ U.S. Geological Survey Geologic Names Committee (2010), "Divisions of geologic time—major chronostratigraphic and geochronologic units", U.S. Geological Survey Fact Sheet 2010–3059, United States Geological Survey, p. 2, retrieved 20 June 2018
  8. ^ Brun, Yves; Shimkets, Lawrence J. (January 2000). Prokaryotic development. ASM Press. p. 114. ISBN 978-1-55581-158-7.
  9. ^ Dodd, Matthew S.; Papineau, Dominic; Grenne, Tor; slack, John F.; Rittner, Martin; Pirajno, Franco; O'Neil, Jonathan; Little, Crispin T. S. (2 March 2017). "Evidence for early life in Earth's oldest hydrothermal vent precipitates". Nature. 543 (7643): 60–64. Bibcode:2017Natur.543...60D. doi:10.1038/nature21377. PMID 28252057. Archived from the original on 10 February 2017. Retrieved 2 March 2017.
  10. ^ Zimmer, Carl (1 March 2017). "Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest". The New York Times. Retrieved 2 March 2017.
  11. ^ Ghosh, Pallab (1 March 2017). "Earliest evidence of life on Earth 'found'". BBC News. Retrieved 2 March 2017.
  12. ^ Dunham, Will (1 March 2017). "Canadian bacteria-like fossils called oldest evidence of life". Reuters. Retrieved 1 March 2017.
  13. ^ Fedonkin, Mikhail A.; Gehling, James G.; Grey, Kathleen; Narbonne, Guy M.; Vickers-Rich, Patricia (2007). The Rise of Animals: Evolution and Diversification of the Kingdom Animalia. JHU Press. p. 326. doi:10.1086/598305. ISBN 9780801886799.
  14. ^ Dawkins, Richard; Wong, Yan (2005). The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution. Houghton Mifflin Harcourt. p. 673. ISBN 9780618619160.
  15. ^ Selden, Paul A. (2005). "Terrestrialization (Precambrian-Devonian)" (PDF). Terrestrialization (Precambrian–Devonian). Encyclopedia of Life Sciences. John Wiley & Sons, Ltd. doi:10.1038/npg.els.0004145. ISBN 978-0470016176.
  16. ^ Scientists discover 'oldest footprints on Earth' in southern China dating back 550 million years The Independent
  17. ^ Clemmey, Harry; Badham, Nick (1982). "Oxygen in the Precambrian Atmosphere". Geology. 10 (3): 141–146. Bibcode:1982Geo....10..141C. doi:10.1130/0091-7613(1982)10<141:OITPAA>2.0.CO;2.
  18. ^ Geological Society of America's "2009 GSA Geologic Time Scale."
  19. ^ "Archived copy". Archived from the original on 2012-05-10. Retrieved 2011-03-27.CS1 maint: Archived copy as title (link)
  20. ^ Bleeker, W. (2004) [2004]. "Toward a "natural" Precambrian time scale". In Felix M. Gradstein; James G. Ogg; Alan G. Smith. A Geologic Time Scale 2004. Cambridge University Press. ISBN 978-0-521-78673-7. also available at Stratigraphy.org: Precambrian subcommission
  21. ^ Zhao, Guochun; Cawood, Peter A.; Wilde, Simon A.; Sun, M. (2002). "Review of global 2.1–1.8 Ga orogens: implications for a pre-Rodinia super-continent". Earth-Science Reviews. 59 (1): 125–162. Bibcode:2002ESRv...59..125Z. doi:10.1016/S0012-8252(02)00073-9.
  22. ^ Zhao, Guochun; Sun, M.; Wilde, Simon A.; Li, S.Z. (2004). "A Paleo-Mesoproterozoic super-continent: assembly, growth and breakup". Earth-Science Reviews (Submitted manuscript). 67 (1): 91–123. Bibcode:2004ESRv...67...91Z. doi:10.1016/j.earscirev.2004.02.003.

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.eduEvidence 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. Bibcode:2004AuJES..51...31W. doi:10.1046/j.1400-0952.2003.01042.x.

External links

Amelia Creek crater

Amelia Creek crater is an impact structure (or astrobleme), the eroded remnant of a former impact crater, located in the Davenport Range, Northern Territory, Australia. It lies within a low range of Paleoproterozoic sedimentary and volcanic rocks, which are extensively folded and faulted, thus making an eroded impact crater difficult to recognize. It was only discovered by the identification of shatter cones near its centre. The central shatter cone locality is surrounded by a 20 by 12 km (12.4 by 7.5 mi) area of anomalous deformation, the asymmetry being possibly related to very oblique impact, but may be at least partly due to the pre-existing structural complexity of the rocks. This deformed zone gives the best estimate for the original size of the crater. Impact took place after folding of the Paleoproterozoic rocks but before deposition of Neoproterozoic and Cambrian rocks which overlie them, thus constraining the impact event to the interval between about 1660 and 600 Ma.

Archean

The Archean Eon ( , also spelled Archaean or Archæan) is one of the four geologic eons of Earth history, occurring 4,000 to 2,500 million years ago (4 to 2.5 billion years ago). During the Archean, the Earth's crust had cooled enough to allow the formation of continents and life started to form.

Cambrian explosion

The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record. It lasted for about 20–25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification of other organisms.Before the Cambrian explosion, most organisms were simple, composed of individual cells occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. Almost all present animal phyla appeared during this period.

Canadian Shield

The Canadian Shield, also called the Laurentian Plateau, or Bouclier canadien (French), is a large area of exposed Precambrian igneous and high-grade metamorphic rocks (geological shield) that forms the ancient geological core of the North American continent (the North American Craton or Laurentia). Composed of igneous rock resulting from its long volcanic history, the area is covered by a thin layer of soil. With a deep, common, joined bedrock region in eastern and central Canada, it stretches north from the Great Lakes to the Arctic Ocean, covering over half of Canada; it also extends south into the northern reaches of the United States. Human population is sparse, and industrial development is minimal, while mining is prevalent.

Danakil Alps

The Danakil Alps are a highland region in Ethiopia and Eritrea with peaks over 1000 metres in height and a width varying between 40 and 70 kilometres. The alps lie to the east of the Danakil Depression and separate it from the southern Red Sea. A rift escarpment facing the Red Sea forms the eastern boundary of the range.

Geologically these highlands are described as a horst and are sometimes referred to as the Danakil Horst or Danakil Block. They were formed by geological faulting which has occurred since the Miocene epoch. There is Precambrian basement rock underlying the region and in coastal Eritrea Precambrian and Mesozoic rocks are exposed. The basement rock of the alps has become overlaid with flood basalt since the Oligocene epoch. About 20 million years ago the Afar rift zone opened up. This resulted in the alps breaking away from the Ethiopian plateau to which they had previously been attached and drifting to the east/northeast.The Danakil Alps contains many volcanic edifices, such as those forming the Nabro Volcanic Range. The largest of the Nabro Volcanic Range edifices are the Mallahle, Nabbro, and Dubbi. The volcanic range extends northwestward to the Red Sea, ending with the Kod Ali volcano offshore.The Danakil Alps have been cut off from the sea since the late Pleistocene.

Eoarchean

The Eoarchean ( ; also spelled Eoarchaean) is the first era of the Archean Eon of the geologic record for which the Earth has a solid crust. It spans 400 million years from the end of the Hadean Eon 4 billion years ago (4000 Mya) to the start of the Paleoarchean Era 3600 Mya. The beginnings of life on Earth have been dated to this era and evidence of cyanobacteria date to 3500 Mya, just outside this era. At that time, the atmosphere was without oxygen and the pressure values ranged from 10 to 100 bar (around 10 to 100 atmospheres).

Guiana Shield

The Guiana Shield is one of the three cratons of the South American Plate. It is a 1.7 billion-year-old Precambrian geological formation in northeast South America that forms a portion of the northern coast. The higher elevations on the shield are called the Guiana Highlands, which is where the table-like mountains called tepuis are found. The Guiana Highlands are also the source of some of the world's most spectacular waterfalls such as Angel Falls, Kaieteur Falls and Kuquenan Falls.

The Guiana Shield underlies Guyana (previously British Guiana), Suriname (previously Dutch Guiana) and French Guiana (or Guyane), much of southern Venezuela, as well as parts of Colombia, and Brazil. The rocks of the Guiana Shield consist of metasediments and metavolcanics (greenstones) overlain by sub-horizontal layers of sandstones, quartzites, shales and conglomerates intruded by sills of younger mafic intrusives such as gabbros.

Hadean

The Hadean ( ) is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. As of 2016, the ICS describes its status as "informal". Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the "Priscoan period", from priscus, the Latin word for "ancient". Other, older texts simply refer to the eon as the Pre-Archean.

Ironstone

Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron compound from which iron can be smelted commercially. This term is customarily restricted to hard coarsely banded, nonbanded, and noncherty sedimentary rocks of post-Precambrian age. The Precambrian deposits, which have a different origin, are generally known as banded iron formations. The iron minerals comprising ironstones can consist either of oxides, i.e. limonite, hematite, and magnetite; carbonates, i.e. siderite; silicates, i.e. chamosite; or some combination of these minerals.

Laurentia

Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of the North American continent. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although originally it also included the cratonic areas of Greenland and also the northwestern part of Scotland, known as the Hebridean Terrane. During other times in its past, Laurentia has been part of larger continents and supercontinents and itself consists of many smaller terranes assembled on a network of Early Proterozoic orogenic belts. Small microcontinents and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian craton seen today.The craton is named after the Laurentian Shield, through the Laurentian Mountains, which received their name from the Saint Lawrence River, named after Lawrence of Rome.

Mesoarchean

The Mesoarchean (, also spelled Mesoarchaean) is a geologic era within the Archean Eon, spanning 3,200 to 2,800 million years ago. The era is defined chronometrically and is not referenced to a specific level in a rock section on Earth. Fossils from Australia show that stromatolites have lived on Earth since the Mesoarchean. The Pongola glaciation occurred around 2,900 million years ago. The first supercontinent Vaalbara broke up during this era about 2,800 million years ago.

The earliest reefs date from this era, and were probably formed by stromatolites. The surface temperature during the Mesoarchean was likely not much higher than modern-day temperatures. Atmospheric carbon dioxide concentration was only a few times higher than its pre-industrial value, and the Sun's luminosity was only 70% of its current value, cancelling out the influence of a greater degree of greenhouse effect that may be operating.

Neoarchean

Not to be confused with archaea, in spite of biological discussion commonly alluding to the only life forms of that era, e.g. microbes

The Neoarchean (; also spelled Neoarchaean) is a geologic era within the Archaean Eon.

The Neoarchean spans the period from 2,800 to 2,500 million years ago—the period being defined chronometrically and not referenced to a specific level in a rock section on Earth.

North Stack

North Stack (Welsh: Ynys Arw, meaning "rugged island") is a small island situated just off Holy Island on the north-west coast of Anglesey.North Stack can also refer to the headland opposite the island. This is the site of a redundant fog warning station, comprising a number of buildings, including the Trinity House Magazine, built in 1861, where shells for the warning cannon were stored. These buildings now house a bird watching observatory, giving a view of South Stack lighthouse across Gogarth Bay, and the studio of artist Philippa Jacobs. The Precambrian quartzite cliffs of this headland, including North Stack Walls and Wen Zawn, provide one of the largest ranges of rock climbs in Britain, such as "A Dream of White Horses", "The Cad" and "The Bells! The Bells!" The climbing areas at North Stack have been documented since the 1970s in a series of local guidebooks. Leading climbers such as Paul Pritchard have put up many extreme new routes.

The stack was the site of a crash of a US Eighth Air Force B-24 Bomber on 22 December 1944 that killed the eight crew on board.

Paleoarchean

The Paleoarchean (), also spelled Palaeoarchaean (formerly known as early Archean), is a geologic era within the Archaean Eon. It spans the period of time 3,600 to 3,200 million years ago—the era is defined chronometrically and is not referenced to a specific level of a rock section on Earth. The name derives from Greek "Palaios" ancient. The oldest ascertained life form of fossilized bacteria in microbial mats, 3,480 million years old, found in Western Australia, is from this era. The first supercontinent Vaalbara formed during this period.

During this era, a large asteroid, about 37 to 58 kilometres (23–36 mi) wide, collided with the Earth in the area of South Africa about 3.26 billion years ago, creating the features known as the Barberton greenstone belt.

Proterozoic

The Proterozoic ( ) is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two forms of ultimately Greek origin: protero- meaning "former, earlier", and -zoic, a suffix related to zoe "life". The Proterozoic Eon extended from 2500 mya to 541 mya (million years ago), and is the most recent part of the Precambrian "supereon". The Proterozoic is the longest eon of the Earth's geologic time scale and it is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; several glaciations, which produced the hypothesized Snowball Earth during the Cryogenian Period in the late Neoproterozoic Era; and the Ediacaran Period (635 to 541 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms and provides us with the first obvious fossil evidence of life on earth.

Shield (geology)

A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years. They have been little affected by tectonic events following the end of the Precambrian, and are relatively flat regions where mountain building, faulting, and other tectonic processes are greatly diminished compared with the activity that occurs at the margins of the shields and the boundaries between tectonic plates.

The term shield, used to describe this type of geographic region, appears in the 1901 English translation of Eduard Suess's Face of Earth by H. B. C. Sollas, and comes from the shape "not unlike a flat shield" of the Canadian Shield which has an outline that "suggests the shape of the shields carried by soldiers in the days of hand-to-hand combat."Shields occur on all continents.

Shoemaker crater

Shoemaker (formerly known as Teague Ring) is an impact structure, the deeply eroded remnant of a former impact crater, situated in arid central Western Australia, about 100 km (62 mi) north-northeast of Wiluna. It is named in honour of planetary geologist Eugene Shoemaker.

Strangways crater

Strangways is a large impact structure, the eroded remnant of a former impact crater, situated in the Northern Territory, Australia. It was named after the nearby Strangways River. The location is remote and difficult to access.

Yarrabubba crater

Yarrabubba crater refers to an impact structure (or astrobleme), the eroded remnant of a former impact crater, situated in the northern Yilgarn Craton near Yarrabubba Station between the towns of Sandstone and Meekatharra, central Western Australia.

Cenozoic era
(present–66.0 Mya)
Mesozoic era
(66.0–251.902 Mya)
Paleozoic era
(251.902–541.0 Mya)
Proterozoic eon
(541.0 Mya–2.5 Gya)
Archean eon (2.5–4 Gya)
Hadean eon (4–4.6 Gya)

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