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 geologist
s have split the Earth's history
are larger subdivisions than periods while periods themselves may be divided into epochs
The rocks formed during a period belong to a stratigraphic
unit called a system
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
) 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.
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
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.
* Changes in recent years have included the abandonment of the former Tertiary
Period in favour of the Paleogene
and succeeding Neogene
* 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
) were themselves referred to as 'periods' but they now enjoy the status of 'epochs' within the more recently delineated Paleogene
*Geologic time scale
Category:Geologic time scales
Category:Units of geologic time by rank