Eon

A geological period is one of the 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.

StructureEdit

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 geological period is known as a system; 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 also: Another proposed set of periods between 4600 and 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 issuesEdit

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 the uniform organization of the stratigraphic record.

Notable changes

• Changes in recent years have included the abandonment of the former Tertiary Period in favour of the Paleogene and succeeding Neogene periods.
• The abandonment of the Quaternary period was also considered but it has been retained for continuity reasons.
• Even earlier in the history of the science, the Tertiary was considered to be an 'era' and its subdivisions (Paleocene, Eocene, Oligocene, Miocene and Pliocene) were themselves referred to as 'periods' but they now enjoy the status of 'epochs' within the more recently delineated Paleogene and Neogene periods.

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.