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Geologic time scale

The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronology (a scientific branch of geology that aims to determine the age of rocks). It is used primarily by Earth scientists (including geologists, paleontologists, geophysicists, geochemists, and paleoclimatologists) to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties, and fossils. The definition of standardised international units of geologic time is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective[1] is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC)[2] that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units.[2]

While some regional terms are still in use,[3] the table of geologic time conforms to the nomenclature, ages, and colour codes set forth by the ICS.[1][4]

– Newer rock beds will lie on top of older rock beds unless the succession has been overturned.

Superposition

– All rock layers were originally deposited horizontally.[note 1]

Horizontality

– Originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer.

Lateral continuity

Biologic succession (where applicable) – This states that each stratum in a succession contains a distinctive set of fossils. This allows for a correlation of the stratum even when the horizon between them is not continuous.

– A rock feature that cuts across another feature must be younger than the rock it cuts across.

Cross-cutting relationships

– Small fragments of one type of rock but embedded in a second type of rock must have formed first, and were included when the second rock was forming.

Inclusion

Relationships of – Geologic features representing periods of erosion or non-deposition, indicating non-continuous sediment deposition.

unconformities

The geologic time scale is a way of representing deep time based on events that have occurred throughout Earth's history, a time span of about 4.54 ± 0.05 Ga (4.54 billion years).[5] It chronologically organises strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. For example, the Cretaceous–Paleogene extinction event, marks the lower boundary of the Paleogene System/Period and thus the boundary between the Cretaceous and Paleogene systems/periods. For divisions prior to the Cryogenian, arbitrary numeric boundary definitions (Global Standard Stratigraphic Ages, GSSAs) are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record.[6][3]


Historically, regional geologic time scales were used[3] due to the litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardising globally significant and identifiable stratigraphic horizons that can be used to define the lower boundaries of chronostratigraphic units. Defining chronostratigraphic units in such a manner allows for the use of global, standardised nomenclature. The ICC represents this ongoing effort.


The relative relationships of rocks for determining their chronostratigraphic positions use the overriding principles of:[7][8][9][10]

An eon is the largest geochronologic time unit and is equivalent to a chronostratigraphic .[16] There are four formally defined eons: the Hadean, Archean, Proterozoic and Phanerozoic.[2]

eonothem

An era is the second largest geochronologic time unit and is equivalent to a chronostratigraphic .[11][16] There are ten defined eras: the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic and Cenozoic, with none from the Hadean eon.[2]

erathem

A period is equivalent to a chronostratigraphic .[11][16] There are 22 defined periods, with the current being the Quaternary period.[2] As an exception two subperiods are used for the Carboniferous Period.[11]

system

An epoch is the second smallest geochronologic unit. It is equivalent to a chronostratigraphic .[11][16] There are 37 defined epochs and one informal one. There are also 11 subepochs which are all within the Neogene and Quaternary.[2] The use of subepochs as formal units in international chronostratigraphy was ratified in 2022.[17]

series

An age is the smallest hierarchical geochronologic unit and is equivalent to a chronostratigraphic .[11][16] There are 96 formal and five informal ages.[2]

stage

A chron is a non-hierarchical formal geochronology unit of unspecified rank and is equivalent to a chronostratigraphic .[11] These correlate with magnetostratigraphic, lithostratigraphic, or biostratigraphic units as they are based on previously defined stratigraphic units or geologic features.

chronozone

Sequences of strata often become eroded, distorted, tilted, or even inverted after deposition

Strata laid down at the same time in different areas could have entirely different appearances

The strata of any given area represented only part of Earth's long history

[note 5]

Aubry, Marie-Pierre; Van Couvering, John A.; Christie-Blick, Nicholas; Landing, Ed; Pratt, Brian R.; Owen, Donald E.; Ferrusquia-Villafranca, Ismael (2009). "Terminology of geological time: Establishment of a community standard". Stratigraphy. 6 (2): 100–105. :10.7916/D8DR35JQ.

doi

Gradstein, F. M.; Ogg, J. G. (2004). (PDF). Lethaia. 37 (2): 175–181. Bibcode:2004Letha..37..175G. doi:10.1080/00241160410006483. Archived from the original (PDF) on 17 April 2018. Retrieved 30 November 2018.

"A Geologic Time scale 2004 – Why, How and Where Next!"

Gradstein, Felix M.; Ogg, James G.; Smith, Alan G. (2004). . Cambridge, UK: Cambridge University Press. ISBN 978-0-521-78142-8. Retrieved 18 November 2011.

A Geologic Time Scale 2004

Gradstein, Felix M.; Ogg, James G.; Smith, Alan G.; Bleeker, Wouter; Laurens, Lucas, J. (June 2004). . Episodes. 27 (2): 83–100. doi:10.18814/epiiugs/2004/v27i2/002.{{cite journal}}: CS1 maint: multiple names: authors list (link)

"A new Geologic Time Scale, with special reference to Precambrian and Neogene"

Ialenti, Vincent (28 September 2014). . NPR. NPR Cosmos & Culture.

"Embracing 'Deep Time' Thinking"

Ialenti, Vincent (21 September 2014). . NPR. NPR Cosmos & Culture.

"Pondering 'Deep Time' Could Inspire New Ways To View Climate Change"

; Walter, Malcolm R.; Narbonne, Guy M.; Christie-Blick, Nicholas (30 July 2004). "A New Period for the Geologic Time Scale" (PDF). Science. 305 (5684): 621–622. doi:10.1126/science.1098803. PMID 15286353. S2CID 32763298. Archived (PDF) from the original on 15 December 2011. Retrieved 18 November 2011.

Knoll, Andrew H.

Levin, Harold L. (2010). . The Earth Through Time. Hoboken, New Jersey: John Wiley & Sons. ISBN 978-0-470-38774-0. Retrieved 18 November 2011.

"Time and Geology"

Montenari, Michael (2016). (1st ed.). Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-811549-7.

Stratigraphy and Timescales

Montenari, Michael (2017). (1st ed.). Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-813077-3.

Advances in Sequence Stratigraphy

Montenari, Michael (2018). (1st ed.). Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-815098-6.

Cyclostratigraphy and Astrochronology

Montenari, Michael (2019). (1st ed.). Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-817552-1.

Case Studies in Isotope Stratigraphy

Montenari, Michael (2020). (1st ed.). Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-820991-2.

Carbon Isotope Stratigraphy

Montenari, Michael (2021). (1st ed.). Amsterdam: Academic Press (Elsevier). ISBN 978-0-12-824624-5.

Calcareous Nannofossil Biostratigraphy

Nichols, Gary (2013). (2nd ed.). Hoboken: Wiley-Blackwell. ISBN 978-1-4051-3592-4

Sedimentology and Stratigraphy

Williams, Aiden (2019). (1st ed.). Forest Hills, NY: Callisto Reference. ISBN 978-1-64116-075-9

Sedimentology and Stratigraphy

The current version of the International Chronostratigraphic Chart can be found at

stratigraphy.org/chart

Interactive version of the International Chronostratigraphic Chart is found at

stratigraphy.org/timescale

A list of current Global Boundary Stratotype and Section Points is found at

stratigraphy.org/gssps

(archived 18 April 2005)

NASA: Geologic Time

(archived 20 January 2019)

GSA: Geologic Time Scale

British Geological Survey: Geological Timechart

(archived 23 June 2004)

GeoWhen Database

(archived 11 November 2005)

National Museum of Natural History – Geologic Time

. Archived 23 July 2008 at the Wayback Machine. Information model for the geologic time scale.

SeeGrid: Geological Time Systems

from Planck Time to the lifespan of the universe

Exploring Time

Gradstein, Felix M. et al. (2004) A new Geologic Time Scale, with special reference to Precambrian and Neogene, Episodes, Vol. 27, no. 2 June 2004 (pdf)

Episodes

Lane, Alfred C, and Marble, John Putman 1937.

Report of the Committee on the measurement of geologic time

(archived 14 July 2011)

Lessons for Children on Geologic Time

Deep Time – A History of the Earth : Interactive Infographic

. Archived 12 August 2021 at the Wayback Machine.

Geology Buzz: Geologic Time Scale