View source for Geologic time scale

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{{redirectstohere|'''geological time scale'''|'''geological timescale'''|'''geologic timescale'''}}

The '''geologic time scale''' is used by [[geologist]]s and other [[scientists]] to describe the timing and relationships between events that have occurred during the [[history of Earth]]. The table of [[geologic periods]] presented here agrees with the dates and [[Scientific classification|nomenclature]] proposed by the [[International Commission on Stratigraphy]], and uses the standard color codes of the [[United States Geological Survey]].
{{TOCnestright}}

Evidence from [[radiometric dating]] indicates that the [[age of the Earth|Earth]] is about 4.570 billion years old. The geological 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 time scale are usually delimited by major [[geology|geological]] or [[paleontology|paleontological]] events, such as [[mass extinction]]s. For example, the boundary between the [[Cretaceous]] period and the [[Paleogene]] period is defined by the [[extinction event]], known as the [[Cretaceous–Tertiary extinction event]], that marked the demise of the [[dinosaur]]s and of many marine [[species]]. Older periods which predate the reliable fossil record are defined by absolute age.

==Graphical timelines==
The second and third timelines are each subsections of their preceding timeline as indicated by asterisks.

{{Timeline Geological Timescale}}

The [[Holocene]] (the latest [[epoch]]) is too small to be shown clearly on this timeline.

==Terminology== <!-- Various redirects target this section title -->

The largest defined unit of time is the supereon composed of [[Eon (geology)|Eons]]. Eons are divided into [[Era (geology)|Eras]], which are in turn divided into [[Period (geology)|Periods]], [[Epoch (geology)|Epochs]] and [[Stage (geology)|Stages]]. At the same time paleontologists define a system of [[faunal stage]]s, of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geological nomenclature, though in general there are far more recognized faunal stages than defined geological time units.

[[Geologist]]s tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper [[Jurassic]]", and "Middle [[Cambrian]]". ''Upper'', ''Middle'', and ''Lower'' are terms applied to the rocks themselves, as in "Upper Jurassic [[sandstone]]," while ''Late'', ''Middle'', and ''Early'' are applied to time, as in "Early Jurassic [[deposition]]" or "[[fossil]]s of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in [[North America]] the Lower [[Cambrian]] is referred to as the [[Waucoban]] series that is then subdivided into zones based on [[trilobita|trilobites]]. The same timespan is split into [[Tommotian]], [[Atdabanian]] and [[Botomian]] stages in [[East Asia]] and [[Siberia]]. 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.

==History of the time scale==
{{dablink|See the main articles: [[history of geology]] and [[history of paleontology]].}}


[[Image:Earth Clock ENG.svg‎|thumb|right|Earth history mapped to 24 hours]]
The principles underlying geologic (geological) time scales were 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|principle 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: 
# Sequences of strata were often eroded, distorted, tilted, or even inverted after [[Deposition (sediment)|deposition]]; 
# Strata laid down at the same time in different areas could have entirely different appearances; 
# The strata of any given area represented only part of the Earth's long history.

[[Image:Geological Time Scale.png‎|thumb|right|A comparative geological timescale]]

The first serious attempts to formulate a geological time scale that could be applied anywhere on [[Earth]] took place 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-Holocene) remained in use as names of geological periods well into the 20th century.

In opposition to the then-popular [[Neptunist]] theories expounded by Werner (that all rocks had precipitated out of a single enormous flood), a major shift in thinking came with the reading by [[James Hutton]] of 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, events which "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology"<ref>John McPhee, ''Basin and Range'', New York:Farrar, Straus and Giroux, 1981, pp.95-100.</ref> What Hutton proposed was 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 flood-oriented theory.

The identification of strata by the fossils they contained, pioneered by [[William Smith (geologist)|William Smith]], [[Georges Cuvier]], [[Jean Baptiste Julien d'Omalius d'Halloy|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 [[United Kingdom|British]] geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for [[Wales]]) and the "Ordovician," and "Silurian", named after ancient [[Welsh people|Welsh]] tribes, were periods defined using stratigraphic sequences from Wales.<ref>John McPhee, ''Basin and Range'', pp.113-114.</ref> The "Devonian" was named for the [[England|English]] [[Counties of England|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 a [[Scotland|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) &mdash;[[red bed]]s, capped by [[chalk]], followed by black [[shale]]s&mdash; that are found throughout [[Germany]] and [[Northwest Europe]], called the 'Trias'. The "Jurassic" was named by a [[France|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 a [[Belgium|Belgian]] geologist [[Jean Baptiste Julien d'Omalius d'Halloy|Jean d'Omalius d'Halloy]] in 1822, using strata in the [[Paris Basin (geology)|Paris basin]]<ref>{{cite book|title=[[Great Soviet Encyclopedia]]|publisher=Sovetskaya Enciklopediya|edition=3rd ed.|pages=vol. 16, p. 50|year=1974|location=Moscow|language=Russian}}</ref> and named for the extensive beds of chalk ([[calcium carbonate]] deposited by the shells of marine [[invertebrate]]s). 

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs.

When [[William Smith (geologist)|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 [[creationist]]s 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 [[paleontologist]]s constructed the geologic table based on the relative positions of different strata and [[fossil]]s, and estimated the time scales based on studying rates of various kinds of [[weathering]], [[erosion]], [[sedimentation]], and [[lithification]]. Until the discovery of [[Radioactive decay|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.

In 1977, the ''Global Commission on Stratigraphy'' (now the [[International Commission on Stratigraphy]]) started an effort to define global references ([[Global Boundary Stratotype Section and Point|Global Boundary Stratotype Sections and Points]]) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al.<ref>Felix M. Gradstein, James G. Ogg, Alan G. Smith (Editors); ''A Geologic Time Scale 2004'', Cambridge University Press, 2005,  (ISBN 0-521-78673-8)</ref>. A UML model for how the timescale is structured, relating it to the GSSP, is also available<ref>Cox & Richard, ''A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards'', [http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2FGES00022.1 Geosphere, volume 1, pp 119-137], Geological Society of America, 2005</ref>.

==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.)  The height of each table entry does not correspond to the duration of each subdivision of time.

{{Geologic time scale}}
==See also==
<!-- This does not work for many browsers. MOST readers still use IE6 and IE7, neither of which have no clue on this mozilla extension. See/use Column templates (as I did below)

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* [[Age of the Earth]]
* [[Anthropocene]]
* [[Cosmological timeline]]
* [[Deep time]]
* [[Geological history of Earth]]
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* [[History of Earth]]
* [[List of fossil sites]] ''(with link directory)''
* [[Logarithmic timeline]]
* [[Lunar geologic timescale]]
* [[Martian geologic timescale]]
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* [[Natural history]]
* [[Timeline of evolution]]
* [[Timeline of human evolution]]
* [[Time line of the geologic history of the United States]]
* [[Timetable of the Precambrian]]
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==External links==
*[http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html NASA: Geologic Time]
*[http://www.geosociety.org/science/timescale/timescl.htm GSA: Geologic Time Scale]
*[http://www.bgs.ac.uk/Education/britstrat/home.html British Geological Survey: Geological Timechart]
* [http://www.stratigraphy.org/geowhen/ GeoWhen Database ]
* [http://www.stratigraphy.org/gssp.htm International Commission on Stratigraphy Time Scale ]
* [http://www.chronos.org CHRONOS ]
* [http://www.nmnh.si.edu/paleo/geotime/index.htm National Museum of Natural History - Geologic Time ]
* [https://www.seegrid.csiro.au/twiki/bin/view/CGIModel/GeologicTime SeeGrid: Geological Time Systems] Information model for the geologic time scale
* [http://exploringtime.org/?page=segments Exploring Time] from Planck Time to the lifespan of the universe


==References and footnotes==
{{reflist|2}}

{{Time Topics}}
{{Time measurement and standards}}
{{Chronology}}

[[Category:Earth sciences]]
[[Category:Geochronology]]
[[Category:Geologic time scale| ]]
[[Category:Geology timelines]]
[[Category:Historical geology|Geologic time scale]]
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