Editing Geologic time scale (section)

== Divisions of geologic time ==
{{See also|Stratigraphy|Chronostratigraphy|Biostratigraphy|Magnetostratigraphy|Lithostratigraphy|Geochronology}}
The geologic time scale is divided into chronostratigraphic units and their corresponding geochronologic units.

* An '''{{visible anchor|eon}}''' is the largest geochronologic time unit and is equivalent to a chronostratigraphic [[eonothem]].<ref name="dictionary_of_geology_2020">{{Cite book |url=https://www.worldcat.org/oclc/1137380460 |title=A dictionary of geology and earth sciences |date=2020 |author=Michael Allaby |isbn=978-0-19-187490-1 |edition=Fifth |location=Oxford |oclc=1137380460}}</ref> There are four formally defined eons: the [[Hadean]], [[Archean]], [[Proterozoic]] and [[Phanerozoic]].<ref name="ICC_Cohen_2013" />
* An '''{{visible anchor|era}}''' is the second largest geochronologic time unit and is equivalent to a chronostratigraphic [[erathem]].<ref name="ICS_chronostrat" /><ref name="dictionary_of_geology_2020" /> There are ten defined eras: the [[Eoarchean]], [[Paleoarchean]], [[Mesoarchean]], [[Neoarchean]], [[Paleoproterozoic]], [[Mesoproterozoic]], [[Neoproterozoic]], [[Paleozoic]], [[Mesozoic]] and [[Cenozoic]], with none from the Hadean eon.<ref name="ICC_Cohen_2013" />
* A '''{{visible anchor|period}}''' is equivalent to a chronostratigraphic [[system (stratigraphy)|system]].<ref name="ICS_chronostrat" /><ref name="dictionary_of_geology_2020" /> There are 22 defined periods, with the current being the [[Quaternary]] period.<ref name="ICC_Cohen_2013" /> As an exception, two subperiods are used for the [[Carboniferous|Carboniferous Period]].<ref name="ICS_chronostrat" />
* An '''{{visible anchor|epoch}}''' is the second smallest geochronologic unit. It is equivalent to a chronostratigraphic [[series (stratigraphy)|series]].<ref name="ICS_chronostrat" /><ref name="dictionary_of_geology_2020" /> There are 37 defined epochs and one informal one. The current epoch is the [[Holocene]]. There are also 11 subepochs which are all within the [[Neogene]] and Quaternary.<ref name="ICC_Cohen_2013" /> The use of subepochs as formal units in international chronostratigraphy was ratified in 2022.<ref name="Aubry_2022_subseries">{{Cite journal |last1=Aubry |first1=Marie-Pierre |last2=Piller |first2=Werner E. |last3=Gibbard |first3=Philip L. |last4=Harper |first4=David A. T. |last5=Finney |first5=Stanley C. |date=2022-03-01 |title=Ratification of subseries/subepochs as formal rank/units in international chronostratigraphy |journal=Episodes |language=en |volume=45 |issue=1 |pages=97–99 |doi=10.18814/epiiugs/2021/021016 |s2cid=240772165 |issn=0705-3797|doi-access=free }}</ref>
* An '''{{visible anchor|age}}''' is the smallest hierarchical geochronologic unit. It is equivalent to a chronostratigraphic [[stage (stratigraphy)|stage]].<ref name="ICS_chronostrat" /><ref name="dictionary_of_geology_2020" /> There are 96 formal and five informal ages.<ref name="ICC_Cohen_2013" /> The current age is the [[Meghalayan]].
* A ''{{visible anchor|chron}}'' is a non-hierarchical formal geochronology unit of unspecified rank and is equivalent to a chronostratigraphic [[chronozone]].<ref name="ICS_chronostrat" /> These correlate with [[Magnetostratigraphy|magnetostratigraphic]], [[Lithostratigraphy|lithostratigraphic]], or [[Biostratigraphy|biostratigraphic]] units as they are based on previously defined stratigraphic units or geologic features.

{| class="wikitable mw-collapsible"
|+Formal, hierarchical units of the geologic time scale (largest to smallest)
!Chronostratigraphic unit (strata)
!Geochronologic unit (time)
!Time span{{Efn|Time spans of geologic time units vary broadly, and there is no numeric limitation on the time span they can represent. They are limited by the time span of the higher rank unit they belong to, and to the chronostratigraphic boundaries they are defined by.|group=note|name=timespan}}
|-
|Eonothem
|Eon
|Several hundred million years to two billion years
|-
|Erathem
|Era
|Tens to hundreds of millions of years
|-
|System
|Period
|Millions of years to tens of millions of years
|-
|Series
|Epoch
|Hundreds of thousands of years to tens of millions of years
|-
|Subseries
|Subepoch
|Thousands of years to millions of years
|-
|Stage
|Age
|Thousands of years to millions of years
|}

The subdivisions {{em|Early}} and {{em|Late}} are used as the geochronologic equivalents of the chronostratigraphic {{em|Lower}} and {{em|Upper}}, e.g., Early [[Triassic]] Period (geochronologic unit) is used in place of Lower Triassic System (chronostratigraphic unit).

Rocks representing a given chronostratigraphic unit are that chronostratigraphic unit, and the time they were laid down in is the geochronologic unit, e.g., the rocks that represent the [[Silurian]] System {{em|are}} the Silurian System and they were deposited {{em|during}} the Silurian Period. This definition means the numeric age of a geochronologic unit can be changed (and is more often subject to change) when refined by geochronometry while the equivalent chronostratigraphic unit (the revision of which is less frequent) remains unchanged. For example, in early 2022, the boundary between the [[Ediacaran]] and [[Cambrian]] [[period (geologic time)|periods]] (geochronologic units) was revised from 541 Ma to 538.8 Ma but the rock definition of the boundary (GSSP) at the base of the Cambrian, and thus the boundary between the Ediacaran and Cambrian [[System (stratigraphy)|systems]] (chronostratigraphic units) has not been changed; rather, the absolute age has merely been refined.

=== Terminology ===

{{em|[[Chronostratigraphy]]}} is the element of [[stratigraphy]] that deals with the relation between rock bodies and the relative measurement of geological time.<ref name="ICS_chronostrat">{{Cite web |title=Chapter 9. Chronostratigraphic Units |url=https://stratigraphy.org/guide/chron |access-date=2022-04-02 |website=stratigraphy.org |publisher=International Commission on Stratigraphy}}</ref> It is the process where distinct strata between defined stratigraphic horizons are assigned to represent a relative interval of geologic time.

A '''{{em|chronostratigraphic unit}}'''{{Anchor|Chronostratigraphic unit}} is a body of rock, layered or unlayered, that is defined between specified stratigraphic horizons which represent specified intervals of geologic time. They include all rocks representative of a specific interval of geologic time, and only this time span. Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units.<ref name="ICS_chronostrat"/><!-- this citation cites two previous sentences -->

A '''{{em|geochronologic unit}}'''{{Anchor|Geochronologic unit}} is a subdivision of geologic time. It is a numeric representation of an intangible property (time).<ref name="ICS_definitions">{{Cite web |title=Chapter 3. Definitions and Procedures |url=https://stratigraphy.org/guide/defs |access-date=2022-04-02 |website=stratigraphy.org |publisher=International Commission on Stratigraphy}}</ref> These units are arranged in a hierarchy: eon, era, period, epoch, subepoch, age, and subage.<ref name="ICS_chronostrat" /> 
{{em|Geochronology}} is the scientific branch of geology that aims to determine the age of rocks, fossils, and sediments either through absolute (e.g., [[radiometric dating]]) or relative means (e.g., [[Law of superposition|stratigraphic position]], [[paleomagnetism]], [[stable isotope ratio]]s). 
{{em|[[Geochronometry]]}} is the field of geochronology that numerically quantifies geologic time.<ref name="ICS_definitions"/> <!-- this citation cites two previous sentences -->

A {{em|[[Global Boundary Stratotype Section and Point]]}} (GSSP) is an internationally agreed-upon reference point on a [[stratigraphic section]] that defines the lower boundaries of stages on the geologic time scale.<ref name="ICS_GSSP">{{Cite web |title=Global Boundary Stratotype Section and Points |url=https://stratigraphy.org/gssps/ |access-date=2022-04-02 |website=stratigraphy.org |publisher=International Commission on Stratigraphy}}</ref> (Recently this has been used to define the base of a system)<ref name="Knoll_2006_Ediacaran">{{Cite journal |last1=Knoll |first1=Andrew |last2=Walter |first2=Malcolm |last3=Narbonne |first3=Guy |last4=Christie-Blick |first4=Nicholas |date=2006 |title=The Ediacaran Period: a new addition to the geologic time scale |url=http://doi.wiley.com/10.1080/00241160500409223 |journal=Lethaia |language=en |volume=39 |issue=1 |pages=13–30 |doi=10.1080/00241160500409223|bibcode=2006Letha..39...13K }}</ref>

A {{em|[[Global Standard Stratigraphic Age]]}} (GSSA)<ref name="Remane_1996_GSSP">{{Cite journal |last1=Remane |first1=Jürgen |last2=Bassett |first2=Michael G |last3=Cowie |first3=John W |last4=Gohrbandt |first4=Klaus H |last5=Lane |first5=H Richard |last6=Michelsen |first6=Olaf |last7=Naiwen |first7=Wang |last8=the cooperation of members of ICS |date=1996-09-01 |title=Revised guidelines for the establishment of global chronostratigraphic standards by the International Commission on Stratigraphy (ICS) |journal=Episodes |language=en |volume=19 |issue=3 |pages=77–81 |doi=10.18814/epiiugs/1996/v19i3/007 |issn=0705-3797|doi-access=free }}</ref> is a numeric-only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined.<ref name="ICS_chronostrat"/> They are used where GSSPs have not yet been established. Research is ongoing to define GSSPs for the base of all units that are currently defined by GSSAs.

The standard international units of the geologic time scale are published by the International Commission on Stratigraphy on the International Chronostratigraphic Chart; however, regional terms are still in use in some areas. The numeric values on the International Chronostratigrahpic Chart are represented by the unit [[Megaannum|Ma]] (megaannum, for 'million [[year]]s'). For example, {{Period start|Jurassic}} {{Period start error|Jurassic}} Ma, the lower boundary of the [[Jurassic]] Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other [[Si prefix|SI prefix]] units commonly used by geologists are [[Gigaannum|Ga]] (gigaannum, billion years), and [[Kiloannums|ka]] (kiloannum, thousand years), with the latter often represented in calibrated units ([[Before Present|before present]]).