Geological period

A geologic period is a time unit subdivision of geologic time defined as a span of years into which the larger era time units are divided into smaller timeframes, as Era's divide the Eon. In the Earth Sciences rocks and especially the sequences of rocks called stratum (plural: strata) arrayed in an ordered "rock column" occurring during a timespan are the focus of study so the time units are paired with corresponding Rock strata units whose characteristics define such points elsewhere that occurred concurrently as the local rock layers were laid down as sediments. For the Geological Period the paired rock strata term, a geologic stage is used to denote the corresponding rock layers of both the geologic record and the fossil record; thus the rocks of the Devonian System were laid down during the Devonian Period, and such equivalent units exist at each level of refinement of geological chronology and biogeological or stratigraphic classification.

Each unit, of strata no matter how interrupted the record recorded in the local rock column is mapped into the overall geologic record and classified carefully into chronological units of geologic time based on world wide efforts of the International Commission on Stratigraphy working to correlate the world's local stratigraphic record into one uniform planet wide benchmarked system, in an steady effort ongoing since 1974. While paleontologists often refer to faunal stages rather than geologic periods, they are often used in popular presentations of paleontology or plate reconstructions.

As illustrated in the article on the geologic time scale, most periods are subdivided into smaller units called epochs. In 2004 the International Union of Geological Sciences (I.U.G.S.) recognized the Ediacaran period of the Neoproterozoic era, the first such newly-designated period in 130 years.

Eon	Era	Period	Start, Million Years Ago
Phanerozoic	Cenozoic	Quaternary (Pleistocene/Holocene)	2.588
		Neogene (Miocene/Pliocene)	23.0
		Paleogene (Paleocene/Eocene/Oligocene)	65.5
	Mesozoic	Cretaceous	145.5
		Jurassic	200
		Triassic	251
	Paleozoic	Permian	300
		Carboniferous (Mississippian/Pennsylvanian)	359
		Devonian	416
		Silurian	444
		Ordovician	488
		Cambrian	542
Proterozoic	Neoproterozoic	Ediacaran	630

Units in geochronology and stratigraphy

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

Multidiscipline synchronization

The earth sciences are as a rule multi-disciplinary.

see also

• System (stratigraphy)