A geologic period is one of several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. They form one element of a hierarchy of divisions into which geologists have split the earth's history. Eons and eras are larger sudivisions than periods while periods themselves may be divided into epochs and ages.
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 ages of the chosen rock sequences which define them is more precisely determined.
The suite of rocks (sedimentary, igneous or metamorphic) associated with a geologic period is known as a system, so for example the 'Jurassic System' of rocks was put in place during the 'Jurassic Period' (between around 200 and 145.5 million years ago). 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.
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. Even earlier in the history of the science, the Tertiary was considered to be an 'era' and its subdivisions (Pliocene, Miocene, Paleocene, Eocene, Oligocene, Miocene and Pliocene) were themselves referred to as 'periods' but they now enjoy the status of 'epochs' within the more recently erected Paleogene and Neogene periods.
|Phanerozoic||Cenozoic||Quaternary (Pleistocene/Holocene)||2.588 - 0|
|Neogene (Miocene/Pliocene)||23.03 - 2.588|
|Paleogene (Paleocene/Eocene/Oligocene)||66.0 - 23.03|
|Mesozoic||Cretaceous||145.5 - 66.0|
|Jurassic||201.3 - 145.0|
|Triassic||252.17 - 201.3|
|Paleozoic||Permian||298.9 - 252.17|
|Carboniferous (Mississippian/Pennsylvanian)||358.9 - 298.9|
|Devonian||419.2 - 358.9|
|Silurian||443.4 - 419.2|
|Ordovician||485.4 - 443.4|
|Cambrian||541.0 - 485.4|
|Proterozoic||Neoproterozoic||Ediacaran||635.0 - 541.0|
|Cryogenian||850 - 635|
|Tonian||1000 - 850|
|Mesoproterozoic||Stenian||1200 - 1000|
|Ectasian||1400 - 1200|
|Calymmian||1600 - 1400|
|Paleoproterozoic||Statherian||1800 - 1600|
|Orosirian||2050 - 1800|
|Rhyacian||2300 - 2050|
|Siderian||2500 - 2300|
|<span style="float:right;"><span class="plainlinksneverexpand">e  h</span></span><center><big>Units in geochronology and stratigraphy</big><ref>Cohen, K.M., Finney, S., Gibbard, P.L. (2013), International Chronostratigraphic Chart, International Commission on Stratigraphy.</ref></center>|
|Segments of rock (strata) in chronostratigraphy||Time spans in geochronology||Notes to<br/>geochronological units|
|<center>Eonothem</center>||<center>Eon</center>||4 total, half a billion years or more|
|<center>Erathem</center>||<center>Era</center>||10 total, several hundred million years|
|<center>Series</center>||<center>Epoch</center>||tens of millions of years|
|<center>Stage</center>||<center>Age</center>||millions of years|
|<center>Chronozone</center>||<center>Chron</center>||subdivision of an age, not used by the ICS timescale|