Period (geology)

A geologic period is one of several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place.

These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history.

Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages.

The rocks formed during a period belong to a stratigraphic unit called a system.

Structure

The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy at particular locations around the world.

In 2004 the Ediacaran Period of the latest Precambrian was defined in similar fashion (and was the first such newly designated period in 130 years) but earlier periods are simply defined by age.

A consequence of this approach to the Phanerozoic periods is that the ages of their beginnings and ends can change from time to time as the absolute age of the chosen rock sequences, which define them, is more precisely determined.

The set of rocks (sedimentary, igneous or metamorphic) that formed during a geologic period is known as a system, so for example the 'Jurassic System' of rocks was formed during the 'Jurassic Period' (between 201 and 145 million years ago).

The following table includes all currently recognized periods. The table omits the time before 2500 million years ago, which is not divided into periods.

Eon Era Period Extent, Million
Years Ago		 Duration, Millions
of Years
Phanerozoic Cenozoic Quaternary (Pleistocene/Holocene) 2.588–0 2.588+
Neogene (Miocene/Pliocene) 23.03–2.588 20.4
Paleogene (Paleocene/Eocene/Oligocene) 66.0–23.03 42.9
Mesozoic Cretaceous 145.5–66.0 79.5
Jurassic 201.3–145.0 56.3
Triassic 252.17–201.3 50.9
Paleozoic Permian 298.9–252.17 46.7
Carboniferous (Mississippian/Pennsylvanian) 358.9–298.9 60
Devonian 419.2–358.9 60.3
Silurian 443.4–419.2 24.2
Ordovician 485.4–443.4 42
Cambrian 541.0–485.4 55.6
Proterozoic Neoproterozoic Ediacaran 635.0–541.0 94
Cryogenian 850–635 215
Tonian 1000–850 150
Mesoproterozoic Stenian 1200–1000 200
Ectasian 1400–1200 200
Calymmian 1600–1400 200
Paleoproterozoic Statherian 1800–1600 200
Orosirian 2050–1800 250
Rhyacian 2300–2050 250
Siderian 2500–2300 200

See Geologic time scale#Proposed Precambrian timeline for another set of periods 4600–541 MYA.

Units in geochronology and stratigraphy[1]
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 defined, several hundred million years
System Period 22 defined, tens to ~one hundred million years
Series Epoch 34 defined, tens of millions of years
Stage Age 99 defined, millions of years
Chronozone Chron subdivision of an age, not used by the ICS timescale

Correlation issues

In a steady effort ongoing since 1974, the International Commission on Stratigraphy has been working to correlate the world's local stratigraphic record into one uniform planet-wide benchmarked system.

American geologists have long considered the Mississippian and Pennsylvanian to be periods in their own right though the ICS now recognises them both as 'subperiods' of the Carboniferous Period recognised by European geologists. Cases like this in China, Russia and even New Zealand with other geological eras has slowed down the uniform organization of the stratigraphic record.

Notable changes

See also

References

  1. ↑ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy.
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