Geologic timescale - Definition


Geologic timescale - Definition

The table and timeline of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy. When used in formal writing, specific geological and historical periods and events are capitalized. Some examples: the Silurian Period, the Ice Age, the Neolithic, the Archean; however, when not specific, some historical events remain uncapitalized, as in ice ages.

The Earth is thought by geologists to be 4.6 billion years old. The geologic or "deep" time of Earth's past has been organized into various periods according to events which took place in each period.

Contents

1 Table of geologic periods

2 Graphical timeline

3 References

4 See also

Table of geologic periods

Years Ago^3,5	Epoch	Period/Age⁴		Era	Eon	Major Events
Present day	Holocene	Neogene		Cenozoic	Phanerozoic	End of ice age and rise of modern civilization
11430	Pleistocene					Extinction of many large mammals. Evolution of fully modern humans
1.81 million	Pliocene
5.33 million	Miocene
23.0 million	Oligocene	Paleogene
37.2 million	Eocene					Appearance of first "modern" mammals
55.8 million	Paleocene
65.5 million^*		Cretaceous		Mesozoic		Dinosaurs reach peak, become extinct. Primitive placental mammals
146 million		Jurassic				Marsupial mammals, first birds, first flowering plants
200 million		Triassic				First dinosaurs, Egg-laying mammals, breakup of Pangea into Gondwana and Laurasia
251 million^*		Permian		Paleozoic		Permian extinction event- 95% of life on Earth becomes extinct
299 million		Carboniferous¹	Pennsylvanian			Abundant insects, first reptiles, coal forests
318 million		Carboniferous¹	Mississippian			Large primitive trees, first land vertebrates
359 million		Devonian				First amphibians, clubmosses and horsetails appear, progymnosperms (first seed bearing plants) appear
416 million^*		Silurian				First vascular land plants, first jawed fish
443 million^*		Ordovician				Invertebrates dominant; first land plants
488 million^*		Cambrian				Major diversification of life in the Cambrian explosion
542 million^*		Ediacaran		Neoproterozoic	Proterozoic²	First multi-celled animals
630 million^*,7		Cryogenian				Possible snowball Earth period, Rodinia begins to break up
850 million		Tonian				First acritarch radiation
1.0 billion		Stennian		Mesoproterozoic		Formation of Rodinia
1.2�10⁹		Ectasian
1.4�10⁹		Calymmian
1.6�10⁹		Statherian		Paleoproterozoic		First complex single-celled life
1.8�10⁹		Orosirian				Transition to oxygen atmosphere
2.05�10⁹		Rhyacian
2.3�10⁹		Siderian
2.5�10⁹				Neoarchean	Archaean²
2.8�10⁹				Mesoarchean
3.2�10⁹				Paleoarchean
3.6�10⁹				Eoarchean		Simple single-celled life
3.8�10⁹					Hadean^2,6	4.1�10⁹ - Oldest known rock; 4.4�10⁹ - Oldest known mineral; 4.57�10⁹ - Formation of Earth

In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods.
The Proterozoic, Archean and Hadean are often collectively referred to as Precambrian Time, and sometimes also as the Cryptozoic.
Dates are slightly uncertain with differences of a few percent between various sources being common. This is largely due to uncertainties in radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where we would most like to have them. Dates with an * are radiometrically determined based on internationally agreed to GSSPs. The dates quoted above are according to the International Commission on Stratigraphy 2004 time scale. All dates given are for the end of the interval in question.
Paleontologists often refer to faunal stages rather than geologic Periods. The Stage Nomenclature is quite complex. See Harland (http://flatpebble.nceas.ucsb.edu/cgi-bin/bridge.pl?action=startScale) for an excellent time ordered list of faunal stages. Also see the article on GSSPs.
The time shown in the "Years Ago" column is that of the end of the interval named beside it.
Hadean was sometimes called Priscoan.
The GSSP for the base of the Ediacaran, ratified in 2004, is a climatic/geochemical marker rather than biostratigraphic. It represents the end of a widespread glaciation event.

Graphical timeline

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 from:-3800   till: -2500  text:Archaean
 from: start  till: -3800  text:Hadean

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 from:  -64.3 till:    0   text:C~z shift:(0,1.5)
 from: -251.1 till:  -64.3 text:Meso~zoic shift:(0,1.5)
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 from: -900   till:  -545  text:Neoprote-~rozoic shift:(0,1.8)
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 from:-3800   till: -3600  text:Eo-~arch-~ean shift:(0,2)
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 from: -208   till: -146
 from: -251.1 till: -208
 from: -286   till: -251.1
 from: -360   till: -286
 from: -408.5 till: -360
 from: -443.5 till: -408.5
 from: -490   till: -443.5
 from: -545   till: -490

 from: -620   till:  -545  text:Ed. 
 from: -850   till:  -620  text:Cryo-~genian
 from: -1000  till:  -850  text:Ton-~ian
 from: -1200  till:  -1000 text:Sten-~nian
 from: -1400  till:  -1200 text:Ect-~asian
 from: -1600  till:  -1400 text:Calym-~mian
 from: -1800  till:  -1600 text:Stath-~erian
 from: -2050  till:  -1800 text:Oro-~sirian
 from: -2300  till:  -2050 text:Rhy-~acian
 from: -2500  till:  -2300 text:Sid-~erian
 from: start  till:  -2500

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 from: -443.5 till: -408.5 text:Silurian
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References

GeoWhen Database (http://www.stratigraphy.org/geowhen/)
International Commission on Stratigraphy Time Scale (http://www.stratigraphy.org/gssp.htm)