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Geologic Timescale

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Geologic Timescale


The geologic time scale (GTS) is a system of chronological measurement that relates stratigraphy to time, and is used by geologists, paleontologists, and other earth scientists to describe the timing and relationships between events that have occurred throughout Earth's history. The table of geologic time spans presented here agrees with the dates and nomenclature set forth by the International Commission on Stratigraphy standard color codes of the International Commission on Stratigraphy.

Evidence from radiometric dating indicates that the Earth is about 4.54 billion years old. The geology or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by the absolute age.

Terminology

e  h
Units in geochronology and stratigraphy[2]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
Eonothem
Eon
4 total, half a billion years or more
Erathem
Era
10 total, several hundred million years
System
Period
Series
Epoch
tens of millions of years
Stage
Age
millions of years
Chronozone
Chron
subdivision of an age, not used by the ICS timescale

The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time in earth's history.

Geologists qualify these units as Early, Mid, and Late when referring to time, and Lower, Middle, and Upper when referring to the corresponding rocks. For example, the Lower Jurassic Series in chronostratigraphy corresponds to the Early Jurassic Epoch in geochronology.[3] The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic."

Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on succession of trilobites. In East Asia and Siberia, the same unit is split into Alexian, Atdabanian, and Botomian stages. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.[4]

History and nomenclature of the time scale



In Ancient Greece, Aristotle saw that fossil seashells from rocks were similar to those found on the beach and inferred that the fossils were once part of living animals. He reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.[5]

The 11th-century Persian geologist Avicenna (Ibn Sina) and the 13th century Dominican bishop Albertus Magnus (Albert of Saxony) extended Aristotle's explanation into a theory of a petrifying fluid.[6] Avicenna also first proposed one of the principles underlying geologic time scales, the law of superposition of strata, while discussing the origins of mountains in The Book of Healing in 1027.[7][8] The Chinese naturalist Shen Kuo (1031–1095) also recognized the concept of 'deep time'.[9]

The principles underlying geologic (geological) time scales were later laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th century geologists realized that:

  1. Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition;
  2. Strata laid down at the same time in different areas could have entirely different appearances;
  3. The strata of any given area represented only part of the Earth's long history.

The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Paleocene - Pliocene) and "Quaternary" (now Pleistocene and Holocene) remained in use as names of geological periods well into the 20th century.

The Neptunist theories popular at this time (expounded by Werner) proposed that all rocks had precipitated out of a single enormous flood. A major shift in thinking came when James Hutton presented his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe before the Royal Society of Edinburgh in March and April 1785. It has been said that "as things appear from the perspective of the 20th century, James Hutton in those readings became the founder of modern geology"[10] Hutton proposed that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the "Neptunist" flood-oriented theory.

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today.

The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian", (the classical name for Wales) and the "Ordovician", and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales.[11] The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures", the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by Scottish geologist Roderick Murchison. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) —red beds, capped by chalk, followed by black shales— that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous" (from Latin creta meaning 'chalk') as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin[12] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates).

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. In 1841 John Phillips published the first global geological time scale based on the types of fossils found in each era. Phillips' scale helped standardize the use of terms like Paleozoic ("old life") which he extended to cover a larger period than it had in previous usage, and Mesozoic ("middle life") which he invented.[13]

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate.

The first geologic time scale that included absolute dates was published in 1913 by the British geologist Arthur Holmes.[14] He greatly furthered the newly created discipline of geochronology and published the world renowned book The Age of the Earth in which he estimated the Earth's age to be at least 1.6 billion years.[15]

In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) started an effort to define global references known as GSSP (Global Boundary Stratotype Sections and Point)s for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al.[16] A UML model for how the timescale is structured, relating it to the GSSP, is also available.[17]

Condensed graphical timelines

The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this compresses the most recent eon. Therefore the second scale shows the most recent eon with an expanded scale. Finally, the second scale again compresses the most recent era, so the latest era is expanded in the third scale. Since the Quaternary is a very short period with short epochs, it is expanded in the fourth scale. The second, third, and fourth timelines are therefore each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on the third timeline on the right, another reason for expanding the fourth scale. The Pleistocene (P) epoch. Q stands for the Quaternary period.

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Millions of Years

Table of geologic time

The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. (See lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon.) This table is arranged with the most recent geologic periods at the top, and the most ancient at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time.

The content of the table is based on the current official geologic time scale of the International Commission on Stratigraphy,[18] with the epoch names altered to the early/late format from lower/upper as recommended by the ICS when dealing with chronostratigraphy.[3]


Proposed Precambrian Timeline

The Geologic Time Scale 2012 book from which the ICS approved the new time scale also included a proposal to radically revise the Precambrian Timescale. Three periods are named after supercontinents.

  • Hadean Eon - 4567-4030 MYA
    • Chaotian Era - 4567-4404 MYA
    • Jack Hillsian or Zirconian Era - 4404-4030 MYA
  • Archean Eon - 4030-2420 MYA
    • Paleoarchean Era - 4030-3490 MYA
      • Acastan Period - 4030-3810 MYA
      • Isuan Period - 3810-3490 MYA
    • Mesoarchean Era - 3490-2780 MYA
      • Vaalbaran Period - 3490-3020 MYA
      • Pongolan Period - 3020-2780 MYA
    • Neoarchean Era - 2780-2420 MYA
      • Methanian Period - 2780-2630 MYA
      • Siderian Period - 2630-2420 MYA
  • Proterozoic Eon - 2420-541 MYA

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 bar:eon
 from: -2420 till: -542  text:Proterozoic color:proterozoic   
 from: -4030 till: -2420 text:Archean     color:archean   
 from: start till: -4030 text:Hadean      color:hadean


 bar:era
 from: -850 till: -542  text:Neoproterozoic color:neoproterozoic   
 from: -1780 till: -850 text:Mesoproterozoic color:mesoproterozoic  
 from: -2420 till: -1780 text:Paleoproterozoic color:paleoproterozoic
 from: -2780 till: -2420 text:Neoarchean color:neoarchean       
 from: -3490 till: -2780 text:Mesoarchean color:mesoarchean      
 from: -4030 till: -3490 text:Paleoarchean color:paleoarchean     
 from: -4400 till: -4030 text:Zirconian color:zirconian
 from: start till: -4400 text:Chaotian color:chaotian
 bar:period
 fontsize:6
 from: -635  till: -542  text:Ediacaran color:ediacaran
 from: -850  till: -635  text:Cryogenian color:cryogenian
 from: -1780 till: -850  text:Rodinian color:rodinian
 from: -2060 till: -1780 text:Columbian color:columbian
 from: -2250 till: -2060 text:Eukaryian color:eukaryian
 from: -2420 till: -2250 text:Oxygenian color:oxygenian
 from: -2630 till: -2420 text:Siderian color:siderian
 from: -2780 till: -2630 text:Methanian color:methanian
 from: -3020 till: -2780 text:Pongolan color:pongolan
 from: -3490 till: -3020 text:Vaalbaran color:vaalbaran
 from: -3810 till: -3490 text:Isuan color:isuan
 from: -4030 till: -3810 text:Acastan color:acastan
 from: start till: -4030 color:white

Heres a comparison, shown below is the current official one.

ImageSize = width:800 height:120 PlotArea = left:65 right:15 bottom:20 top:5 AlignBars = justify

Colors =

 id:precambrian value:rgb(0.968,0.262,0.439)
 id:proterozoic value:rgb(0.968,0.207,0.388)
 id:neoproterozoic value:rgb(0.996,0.701,0.258)
 id:ediacaran value:rgb(0.996,0.85,0.415)
 id:cryogenian value:rgb(0.996,0.8,0.36)
 id:tonian value:rgb(0.996,0.75,0.305)
 id:mesoproterozoic value:rgb(0.996,0.705,0.384)
 id:stenian value:rgb(0.996,0.85,0.604)
 id:ectasian value:rgb(0.996,0.8,0.541)
 id:calymmian value:rgb(0.996,0.75,0.478)
 id:paleoproterozoic value:rgb(0.968,0.263,0.44)
 id:statherian value:rgb(0.968,0.459,0.655)
 id:orosirian value:rgb(0.968,0.408,0.596)
 id:rhyacian value:rgb(0.968,0.357,0.537)
 id:siderian value:rgb(0.968,0.306,0.478)
 id:archean value:rgb(0.996,0.157,0.498)
 id:neoarchean value:rgb(0.976,0.608,0.757)
 id:mesoarchean value:rgb(0.968,0.408,0.662)
 id:paleoarchean value:rgb(0.96,0.266,0.624)
 id:eoarchean value:rgb(0.902,0.114,0.549)
 id:hadean value:rgb(0.717,0,0.494)
 id:black value:black
 id:white value:white

Period = from:-4567 till:-542 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-4500 ScaleMinor = unit:year increment:100 start:-4500

Define $markred = text:"*" textcolor:red shift:(0,3) fontsize:10

PlotData=

 align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5)
 bar:supereon
 from: start till: -542 text:Precambrian color:precambrian


 bar:eon

 from: -2500 till: -542  text:Proterozoic color:proterozoic   
 from: -4000 till: -2500 text:Archean     color:archean   
 from: start till: -4000 text:Hadean      color:hadean


 bar:era
 from: -1000 till: -542  text:Neoproterozoic color:neoproterozoic   
 from: -1600 till: -1000 text:Mesoproterozoic color:mesoproterozoic  
 from: -2500 till: -1600 text:Paleoproterozoic color:paleoproterozoic
 from: -2800 till: -2500 text:Neoarchean color:neoarchean       
 from: -3200 till: -2800 text:Mesoarchean color:mesoarchean      
 from: -3600 till: -3200 text:Paleoarchean color:paleoarchean     
 from: -4000 till: -3600 text:Eoarchean color:eoarchean
 from: start till: -4000 color:white
 bar:period
 fontsize:6
 from: -635  till: -542  text:Ediacaran color:ediacaran
 from: -850  till: -635  text:Cryogenian color:cryogenian
 from: -1000 till: -850  text:Tonian color:tonian
 from: -1200 till: -1000 text:Stenian color:stenian
 from: -1400 till: -1200 text:Ectasian color:ectasian
 from: -1600 till: -1400 text:Calymmian color:calymmian
 from: -1800 till: -1600 text:Statherian color:statherian
 from: -2050 till: -1800 text:Orosirian color:orosirian
 from: -2300 till: -2050 text:Rhyacian color:rhyacian
 from: -2500 till: -2300 text:Siderian color:siderian
 from: start till: -2500 color:white

See also

References and footnotes

Further reading

External links

  • NASA: Geologic Time
  • GSA: Geologic Time Scale
  • British Geological Survey: Geological Timechart
  • GeoWhen Database
  • International Commission on Stratigraphy Time Scale
  • Chronos.org
  • National Museum of Natural History - Geologic Time
  • SeeGrid: Geological Time Systems Information model for the geologic time scale
  • Exploring Time from Planck Time to the lifespan of the universe
  • Episodes, Gradstein, Felix M. et al. (2004) A new Geologic Time Scale, with special reference to Precambrian and Neogene, Episodes, Vol. 27, no. 2 June 2004 (pdf)
  • Lane, Alfred C, and Marble, John Putman 1937. Report of the Committee on the measurement of geologic time
  • Lessons for Children on Geologic Time
  • Deep Time - A History of the Earth : Interactive Infographic