Geologic time scale

Source: Wikipedia, the free encyclopedia.

megaannus
(Ma) represents one million (106) years.

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]

Principles

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]

  • Superposition – Newer rock beds will lie on top of older rock beds unless the succession has been overturned.
  • Horizontality – All rock layers were originally deposited horizontally.[note 1]
  • Lateral continuity – Originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer.
  • 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.
  • Cross-cutting relationships – A rock feature that cuts across another feature must be younger than the rock it cuts across.
  • Inclusion – 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.
  • Relationships of unconformities – Geologic features representing periods of erosion or non-deposition, indicating non-continuous sediment deposition.

Terminology

The GTS is divided into chronostratigraphic units and their corresponding geochronologic units. These are represented on the ICC published by the ICS; however, regional terms are still in use in some areas.

Chronostratigraphy is the element of stratigraphy that deals with the relation between rock bodies and the relative measurement of geological time.[11] It is the process where distinct strata between defined stratigraphic horizons are assigned to represent a relative interval of geologic time.

A 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.[11] Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units.[11] 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., stratigraphic position, paleomagnetism, stable isotope ratios).[12]

A geochronologic unit is a subdivision of geologic time. It is a numeric representation of an intangible property (time).[12] Eon, era, period, epoch, subepoch, age, and subage are the hierarchical geochronologic units.[11] Geochronometry is the field of geochronology that numerically quantifies geologic time.[12]

A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundaries of stages on the geologic time scale.[13] (Recently this has been used to define the base of a system)[14]

A Global Standard Stratigraphic Age (GSSA)[15] is a numeric only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined.[11] 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 numeric (geochronometric) representation of a geochronologic unit can, and is more often subject to change when geochronology refines the geochronometry, while the equivalent chronostratigraphic unit remains the same, and their revision is less common. For example, in early 2022 the boundary between the

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 systems
(chronostratigraphic units) has not changed, merely the geochronometry has been refined.

The numeric values on the ICC are represented by the unit

ka (kiloannum, thousand years), with the latter often represented in calibrated units (before present
).

Divisions of geologic time

The Early and Late subdivisions are used as the geochronologic equivalents of the chronostratigraphic Lower and Upper, e.g., Early Triassic Period (geochronologic unit) is used in place of Lower Triassic Series (chronostratigraphic unit).

Rocks representing a given chronostratigraphic unit are that chronostratigraphic unit, and the time they were laid down in is the geochronologic unit, i.e., the rocks that represent the Silurian Series are the Silurian Series and they were deposited during the Silurian Period.

Formal, hierarchical units of the geologic time scale (largest to smallest)
Chronostratigraphic unit (strata) Geochronologic unit (time) Time span[note 2]
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

Naming of geologic time

The names of geologic time units are defined for chronostratigraphic units with the corresponding geochronologic unit sharing the same name with a change to the latter (e.g. Phanerozoic Eonothem becomes the Phanerozoic Eon). Names of erathems in the Phanerozoic were chosen to reflect major changes in the history of life on Earth: Paleozoic (old life), Mesozoic (middle life), and Cenozoic (new life). Names of systems are diverse in origin, with some indicating chronologic position (e.g., Paleogene), while others are named for lithology (e.g., Cretaceous), geography (e.g., Permian), or are tribal (e.g., Ordovician) in origin. Most currently recognised series and subseries are named for their position within a system/series (early/middle/late); however, the ICS advocates for all new series and subseries to be named for a geographic feature in the vicinity of its stratotype or type locality. The name of stages should also be derived from a geographic feature in the locality of its stratotype or type locality.[11]

Informally, the time before the Cambrian is often referred to as the Precambrian or pre-Cambrian (Supereon).[6][note 3]

Time span and etymology of ICS eonothem/eon names
Name Time span Duration (million years) Etymology of name
Phanerozoic 538.8 to 0 million years ago 538.8 From the Greek words φανερός (phanerós) meaning 'visible' or 'abundant', and ζωή (zoē) meaning 'life'.
Proterozoic 2,500 to 538.8 million years ago 1961.2 From the Greek words πρότερος (próteros) meaning 'former' or 'earlier', and ζωή (zoē) meaning 'life'.
Archean 4,031 to 2,500 million years ago 1531 From the Greek word
ἀρχή
(archē), meaning 'beginning, origin'.
Hadean 4,567.3 to 4,031 million years ago 536.3 From Hades, Greek: ᾍδης, translit. Háidēs, the god of the underworld (hell, the inferno) in Greek mythology.
Time span and etymology of ICS erathem/era names
Name Time span Duration (million years) Etymology of name
Cenozoic 66 to 0 million years ago 66 From the Greek words καινός (kainós) meaning 'new', and ζωή (zōḗ) meaning 'life'.
Mesozoic 251.9 to 66 million years ago 185.902 From the Greek words μέσο (méso) meaning 'middle', and ζωή (zōḗ) meaning 'life'.
Paleozoic 538.8 to 251.9 million years ago 286.898 From the Greek words παλιός (palaiós) meaning 'old', and ζωή (zōḗ) meaning 'life'.
Neoproterozoic 1,000 to 538.8 million years ago 461.2 From the Greek words νέος (néos) meaning 'new' or 'young', πρότερος (próteros) meaning 'former' or 'earlier', and ζωή (zōḗ) meaning 'life'.
Mesoproterozoic 1,600 to 1,000 million years ago 600 From the Greek words μέσο (méso) meaning 'middle', πρότερος (próteros) meaning 'former' or 'earlier', and ζωή (zōḗ) meaning 'life'.
Paleoproterozoic 2,500 to 1,600 million years ago 900 From the Greek words παλιός (palaiós) meaning 'old', πρότερος (próteros) meaning 'former' or 'earlier', and ζωή (zōḗ) meaning 'life'.
Neoarchean 2,800 to 2,500 million years ago 300 From the Greek words νέος (néos) meaning 'new' or 'young', and ἀρχαῖος (arkhaîos) meaning 'ancient'.
Mesoarchean 3,200 to 2,800 million years ago 400 From the Greek words μέσο (méso) meaning 'middle', and ἀρχαῖος (arkhaîos) meaning 'ancient'.
Paleoarchean 3,600 to 3,200 million years ago 400 From the Greek words παλιός (palaiós) meaning 'old', and ἀρχαῖος (arkhaîos) meaning 'ancient'.
Eoarchean 4,031 to 3,600 million years ago 431 From the Greek words Ηώς (Ēṓs) meaning 'dawn', and ἀρχαῖος (arkhaîos) meaning 'ancient'.
Time span and etymology of ICS system/period names
Name Time span Duration (million years) Etymology of name
Quaternary 2.6 to 0 million years ago 2.58 First introduced by Jules Desnoyers in 1829 for sediments in France's Seine Basin that appeared to be younger than Tertiary[note 4] rocks.[20]
Neogene 23 to 2.6 million years ago 20.45 Derived from the Greek words νέος (néos) meaning 'new', and γενεά (geneá) meaning 'genesis' or 'birth'.
Paleogene 66 to 23 million years ago 42.97 Derived from the Greek words παλιός (palaiós) meaning 'old', and γενεά (geneá) meaning 'genesis' or 'birth'.
Cretaceous ~145 to 66 million years ago ~79 Derived from Terrain Crétacé used in 1822 by
Jean d'Omalius d'Halloy in reference to extensive beds of chalk within the Paris Basin.[21] Ultimately derived from the Latin
crēta meaning chalk.
Jurassic 201.4 to 145 million years ago ~56.4 Named after the Jura Mountains. Originally used by Alexander von Humboldt as 'Jura Kalkstein' (Jura limestone) in 1799.[22] Alexandre Brongniart was the first to publish the term Jurassic in 1829.[23][24]
Triassic 251.9 to 201.4 million years ago 50.502 From the Trias of Friedrich August von Alberti in reference to a trio of formations widespread in southern Germany
Permian 298.9 to 251.9 million years ago 46.998 Named after the historical region of Perm, Russian Empire.[25]
Carboniferous 358.9 to 298.9 million years ago 60 Means 'coal-bearing', from the Latin carbō (coal) and ferō (to bear, carry).[26]
Devonian 419.2 to 358.9 million years ago 60.3 Named after Devon, England.[27]
Silurian 443.8 to 419.2 million years ago 24.6 Named after the Celtic tribe, the Silures.[28]
Ordovician 485.4 to 443.8 million years ago 41.6 Named after the Celtic tribe, Ordovices.[29][30]
Cambrian 538.8 to 485.4 million years ago 53.4 Named for Cambria, a latinised form of the Welsh name for Wales, Cymru.[31]
Ediacaran 635 to 538.8 million years ago ~96.2 Named for the Ediacara Hills. Ediacara is possibly a corruption of the Kuyani words 'Yata Takarra' meaning hard or stony ground.[32][33]
Cryogenian 720 to 635 million years ago ~85 From the Greek words κρύος (krýos) meaning 'cold', and, γένεσις (génesis) meaning 'birth'.[3]
Tonian 1,000 to 720 million years ago ~280 From the Greek word τόνος (tónos) meaning 'stretch'.[3]
Stenian 1,200 to 1,000 million years ago 200 From the Greek word στενός (stenós) meaning 'narrow'.[3]
Ectasian 1,400 to 1,200 million years ago 200 From the Greek word ἔκτᾰσῐς (éktasis) meaning 'extension'.[3]
Calymmian 1,600 to 1,400 million years ago 200 From the Greek word κάλυμμᾰ (kálumma) meaning 'cover'.[3]
Statherian 1,800 to 1,600 million years ago 200 From the Greek word σταθερός (statherós) meaning 'stable'.[3]
Orosirian 2,050 to 1,800 million years ago 250 From the Greek word ὀροσειρά (oroseirá) meaning 'mountain range'.[3]
Rhyacian 2,300 to 2,050 million years ago 250 From the Greek word ῥύαξ (rhýax) meaning 'stream of lava'.[3]
Siderian 2,500 to 2,300 million years ago 200 From the Greek word σίδηρος (sídēros) meaning 'iron'.[3]
Time span and etymology of ICS series/epoch names
Name Time span Duration (million years) Etymology of name
Holocene 0.012 to 0 million years ago 0.0117 From the Greek words ὅλος (hólos) meaning 'whole', and καινός (kainós) meaning 'new'
Pleistocene 2.58 to 0.012 million years ago 2.5683 Coined in the early 1830s from the Greek words πλεῖστος (pleîstos) meaning 'most', and καινός (kainós) meaning 'new'
Pliocene 5.33 to 2.58 million years ago 2.753 Coined in the early 1830s from the Greek words πλείων (pleíōn) meaning 'more', and καινός (kainós) meaning 'new'
Miocene 23.03 to 5.33 million years ago 17.697 Coined in the early 1830s from the Greek words μείων (meíōn) meaning 'less', and καινός (kainós) meaning 'new'
Oligocene 33.9 to 23.03 million years ago 10.87 Coined in the 1850s from the Greek words ὀλίγος (olígos) meaning 'few', and καινός (kainós) meaning 'new'
Eocene 56 to 33.9 million years ago 22.1 Coined in the early 1830s from the Greek words ἠώς (ēōs) meaning 'dawn', and καινός (kainós) meaning 'new', referring to the dawn of modern life during this epoch
Paleocene 66 to 56 million years ago 10 Coined by Wilhelm Philippe Schimper in 1874 as a portmanteau of paleo- + Eocene, but on the surface from the Greek words παλαιός (palaios) meaning 'old', and καινός (kainós) meaning 'new'
Upper Cretaceous
100.5 to 66 million years ago 34.5 See Cretaceous
Lower Cretaceous
145 to 100.5 million years ago 44.5
Upper Jurassic

161.5 to 145 million years ago 16.5 See Jurassic
Middle Jurassic 174.7 to 161.5 million years ago 13.2
Lower Jurassic

201.4 to 174.7 million years ago 26.7
Upper Triassic
237 to 201.4 million years ago 35.6 See Triassic
Middle Triassic
247.2 to 237 million years ago 10.2
Lower Triassic
251.9 to 247.2 million years ago 4.702
Lopingian 259.51 to 251.9 million years ago 7.608 Named for Loping, China, an anglicization of the Mandarin word 乐平 (lèpíng) meaning 'peaceful music'
Guadalupian 273.01 to 259.51 million years ago 13.5 Named for the Guadalupe Mountains of the American Southwest, ultimately from Arabic وَادِي ٱل (wādī al) meaning 'valley of the', and Latin lupus meaning 'wolf' via Spanish
Cisuralian 298.9 to 273.01 million years ago 25.89 From Latin cis- (before) + Russian Урал (Ural), referring to the western slopes of the Ural Mountains
Upper Pennsylvanian 307 to 298.9 million years ago 8.1 Named for the US state of Pennsylvania, from William Penn + Latin silvanus (forest) + -ia by analogy to Transylvania
Middle Pennsylvanian 315.2 to 307 million years ago 8.2
Lower Pennsylvanian 323.2 to 315.2 million years ago 8
Upper Mississippian 330.9 to 323.2 million years ago 7.7 Named for the Mississippi River, from the Ojibwe word ᒥᐦᓯᓰᐱ (misi-ziibi) meaning 'great river'
Middle Mississippian 346.7 to 330.9 million years ago 15.8
Lower Mississippian 358.9 to 346.7 million years ago 12.2
Upper Devonian
382.7 to 358.9 million years ago 23.8 See Devonian
Middle Devonian 393.3 to 382.7 million years ago 10.6
Lower Devonian
419.2 to 393.3 million years ago 25.9
Pridoli 423 to 419.2 million years ago 3.8 Named for the Homolka a Přídolí nature reserve near Prague, Czechia
Ludlow 427.4 to 423 million years ago 4.4 Named after Ludlow, England
Wenlock 433.4 to 427.4 million years ago 6 Named for the Wenlock Edge in Shropshire, England
Llandovery 443.8 to 433.4 million years ago 10.4 Named after Llandovery, Wales
Upper Ordovician
458.4 to 443.8 million years ago 14.6 See Ordovician
Middle Ordovician
470 to 458.4 million years ago 11.6
Lower Ordovician
485.4 to 470 million years ago 15.4
Furongian 497 to 485.4 million years ago 11.6 From the Mandarin word 芙蓉 (fúróng) meaning 'lotus', referring to the state symbol of Hunan
Miaolingian 509 to 497 million years ago 12 Named for the Miao Ling [zh] mountains of Guizhou, Mandarin for 'sprouting peaks'
Cambrian Series 2 (informal) 521 to 509 million years ago 12 See Cambrian
Terreneuvian 538.8 to 521 million years ago 17.8 Named for Terre-Neuve, a French calque of Newfoundland

History of the geologic time scale

Early history

While a modern geological time scale was not formulated until 1911

BCE) observed rock beds with fossils of shells located above the sea-level, viewed them as once living organisms, and used this to imply an unstable relationship in which the sea had at times transgressed over the land and at other times had regressed.[35] This view was shared by a few of Xenophanes' contemporaries and those that followed, including Aristotle (384–322 BCE) who (with additional observations) reasoned that the positions of land and sea had changed over long periods of time. The concept of deep time was also recognised by Chinese naturalist Shen Kuo[36] (1031–1095) and Islamic scientist-philosophers, notably the Brothers of Purity, who wrote on the processes of stratification over the passage of time in their treatises.[35] Their work likely inspired that of the 11th-century Persian polymath Avicenna (Ibn Sînâ, 980–1037) who wrote in The Book of Healing (1027) on the concept of stratification and superposition, pre-dating Nicolas Steno by more than six centuries.[35] Avicenna also recognised fossils as "petrifications of the bodies of plants and animals",[37] with the 13th-century Dominican bishop Albertus Magnus (c. 1200–1280) extending this into a theory of a petrifying fluid.[38][verification needed] These works appeared to have little influence on scholars in Medieval Europe who looked to the Bible to explain the origins of fossils and sea-level changes, often attributing these to the 'Deluge', including Ristoro d'Arezzo in 1282.[35] It was not until the Italian Renaissance when Leonardo da Vinci (1452–1519) would reinvigorate the relationships between stratification, relative sea-level change, and time, denouncing attribution of fossils to the 'Deluge':[39][35]

Of the stupidity and ignorance of those who imagine that these creatures were carried to such places distant from the sea by the Deluge...Why do we find so many fragments and whole shells between the different layers of stone unless they had been upon the shore and had been covered over by earth newly thrown up by the sea which then became petrified? And if the above-mentioned Deluge had carried them to these places from the sea, you would find the shells at the edge of one layer of rock only, not at the edge of many where may be counted the winters of the years during which the sea multiplied the layers of sand and mud brought down by the neighboring rivers and spread them over its shores. And if you wish to say that there must have been many deluges in order to produce these layers and the shells among them it would then become necessary for you to affirm that such a deluge took place every year.

These views of da Vinci remained unpublished, and thus lacked influence at the time; however, questions of fossils and their significance were pursued and, while views against Genesis were not readily accepted and dissent from religious doctrine was in some places unwise, scholars such as Girolamo Fracastoro shared da Vinci's views, and found the attribution of fossils to the 'Deluge' absurd.[35]

Establishment of primary principles

Niels Stensen, more commonly known as Nicolas Steno (1638–1686), is credited with establishing four of the guiding principles of stratigraphy.[35] In De solido intra solidum naturaliter contento dissertationis prodromus Steno states:[7][40]

  • When any given stratum was being formed, all the matter resting on it was fluid and, therefore, when the lowest stratum was being formed, none of the upper strata existed.
  • ...strata which are either perpendicular to the horizon or inclined to it were at one time parallel to the horizon.
  • When any given stratum was being formed, it was either encompassed at its edges by another solid substance or it covered the whole globe of the earth. Hence, it follows that wherever bared edges of strata are seen, either a continuation of the same strata must be looked for or another solid substance must be found that kept the material of the strata from being dispersed.
  • If a body or discontinuity cuts across a stratum, it must have formed after that stratum.

Respectively, these are the principles of superposition, original horizontality, lateral continuity, and cross-cutting relationships. From this Steno reasoned that strata were laid down in succession and inferred relative time (in Steno's belief, time from Creation). While Steno's principles were simple and attracted much attention, applying them proved challenging.[35] These basic principles, albeit with improved and more nuanced interpretations, still form the foundational principles of determining the correlation of strata relative to geologic time.

Over the course of the 18th-century geologists realised that:

  • 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

Formulation of a modern geologic time scale

The apparent, earliest formal division of the geologic record with respect to time was introduced by

Giovanni Arduino (1713–1795) to include tertiary and quaternary divisions.[35] These divisions were used to describe both the time during which the rocks were laid down, and the collection of rocks themselves (i.e., it was correct to say Tertiary rocks, and Tertiary Period). Only the Quaternary division is retained in the modern geologic time scale, while the Tertiary division was in use until the early 21st century. The Neptunism and Plutonism theories would compete into the early 19th century with a key driver for resolution of this debate being the work of James Hutton (1726–1797), in particular his Theory of the Earth, first presented before the Royal Society of Edinburgh in 1785.[44][8][45] Hutton's theory would later become known as uniformitarianism, popularised by John Playfair[46] (1748–1819) and later Charles Lyell (1797–1875) in his Principles of Geology.[9][47][48] Their theories strongly contested the 6,000 year age of the Earth as suggested determined by James Ussher
via Biblical chronology that was accepted at the time by western religion. Instead, using geological evidence, they contested Earth to be much older, cementing the concept of deep time.

During the early 19th century William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brongniart pioneered the systematic division of rocks by stratigraphy and fossil assemblages. These geologists began to use the local names given to rock units in a wider sense, correlating strata across national and continental boundaries based on their similarity to each other. Many of the names below erathem/era rank in use on the modern ICC/GTS were determined during the early to mid-19th century.

The advent of geochronometry

During the 19th century, the debate regarding Earth's age was renewed, with geologists estimating ages based on

Lord Kelvin and Clarence King
were held in high regard at the time due to their pre-eminence in physics and geology. All of these early geochronometric determinations would later prove to be incorrect.

The discovery of radioactive decay by Henri Becquerel, Marie Curie, and Pierre Curie laid the ground work for radiometric dating, but the knowledge and tools required for accurate determination of radiometric ages would not be in place until the mid-1950s.[5] Early attempts at determining ages of uranium minerals and rocks by Ernest Rutherford, Bertram Boltwood, Robert Strutt, and Arthur Holmes, would culminate in what are considered the first international geological time scales by Holmes in 1911 and 1913.[34][49][50] The discovery of isotopes in 1913[51] by Frederick Soddy, and the developments in mass spectrometry pioneered by Francis William Aston, Arthur Jeffrey Dempster, and Alfred O. C. Nier during the early to mid-20th century would finally allow for the accurate determination of radiometric ages, with Holmes publishing several revisions to his geological time-scale with his final version in 1960.[5][50][52][53]

Modern international geologic time scale

The establishment of the IUGS in 1961[54] and acceptance of the Commission on Stratigraphy (applied in 1965)[55] to become a member commission of IUGS led to the founding of the ICS. One of the primary objectives of the ICS is "the establishment, publication and revision of the ICS International Chronostratigraphic Chart which is the standard, reference global Geological Time Scale to include the ratified Commission decisions".[1]

Following on from Holmes, several A Geological Time Scale books were published in 1982,[56] 1989,[57] 2004,[58] 2008,[59] 2012,[60] 2016,[61] and 2020.[62] However, since 2013, the ICS has taken responsibility for producing and distributing the ICC citing the commercial nature, independent creation, and lack of oversight by the ICS on the prior published GTS versions (GTS books prior to 2013) although these versions were published in close association with the ICS.[2] Subsequent Geologic Time Scale books (2016[61] and 2020[62]) are commercial publications with no oversight from the ICS, and do not entirely conform to the chart produced by the ICS. The ICS produced GTS charts are versioned (year/month) beginning at v2013/01. At least one new version is published each year incorporating any changes ratified by the ICS since the prior version.

The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.

SiderianRhyacianOrosirianStatherianCalymmianEctasianStenianTonianCryogenianEdiacaranCOsDCPTJKPgnEoarcheanPaleoarcheanMesoarcheanNeoarcheanPaleoproterozoicMesoproterozoicNeoproterozoicPaleozoicMesozoicCenozoicHadeanArcheanProterozoicPhanerozoicPrecambrian
CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogeneQuaternaryPaleozoicMesozoicCenozoicPhanerozoic
PaleoceneEoceneOligoceneMiocenePliocenePleistoceneHolocenePaleogeneNeogeneQuaternaryCenozoic
GelasianCalabrian (stage)ChibanianLate PleistocenePleistoceneHoloceneQuaternary

Horizontal scale is Millions of years (above timelines) / Thousands of years (below timeline)

GreenlandianNorthgrippianMeghalayanHolocene

Major proposed revisions to the ICC

Proposed Anthropocene Series/Epoch

First suggested in 2000,[63] the Anthropocene is a proposed epoch/series for the most recent time in Earth's history. While still informal, it is a widely used term to denote the present geologic time interval, in which many conditions and processes on Earth are profoundly altered by human impact.[64] As of April 2022 the Anthropocene has not been ratified by the ICS; however, in May 2019 the Anthropocene Working Group voted in favour of submitting a formal proposal to the ICS for the establishment of the Anthropocene Series/Epoch.[65] Nevertheless, the definition of the Anthropocene as a geologic time period rather than a geologic event remains controversial and difficult.[66][67][68][69]

Proposals for revisions to pre-Cryogenian timeline

Shields et al. 2021

An international working group of the ICS on pre-Cryogenian chronostratigraphic subdivision have outlined a template to improve the pre-Cryogenian geologic time scale based on the rock record to bring it in line with the post-Tonian geologic time scale.[6] This work assessed the geologic history of the currently defined eons and eras of the pre-Cambrian,[note 3] and the proposals in the "Geological Time Scale" books 2004,[70] 2012,[3] and 2020.[71] Their recommend revisions[6] of the pre-Cryogenian geologic time scale were (changes from the current scale [v2023/09] are italicised):

  • Three divisions of the Archean instead of four by dropping Eoarchean, and revisions to their geochronometric definition, along with the repositioning of the Siderian into the latest Neoarchean, and a potential Kratian division in the Neoarchean.
    • Archean (4000–2450 Ma)
      • Paleoarchean (4000–3500 Ma)
      • Mesoarchean (3500–3000 Ma)
      • Neoarchean (3000–2450 Ma)
        • Kratian (no fixed time given, prior to the Siderian) – from Greek word κράτος (krátos), meaning strength.
        • Siderian (?–2450 Ma) – moved from Proterozoic to end of Archean, no start time given, base of Paleoproterozoic defines the end of the Siderian
  • Refinement of geochronometric divisions of the Proterozoic, Paleoproterozoic, repositioning of the Statherian into the Mesoproterozoic, new Skourian period/system in the Paleoproterozoic, new Kleisian or Syndian period/system in the Neoproterozoic.
    • Paleoproterozoic (2450–1800 Ma)
      • Skourian (2450–2300 Ma) – from the Greek word σκουριά (skouriá), meaning 'rust'.
      • Rhyacian (2300–2050 Ma)
      • Orosirian (2050–1800 Ma)
    • Mesoproterozoic (1800–1000 Ma)
      • Statherian (1800–1600 Ma)
      • Calymmian (1600–1400 Ma)
      • Ectasian (1400-1200 Ma)
      • Stenian (1200–1000 Ma)
    • Neoproterozoic (1000–538.8 Ma)[note 5]
      • Kleisian or Syndian (1000–800 Ma) – respectively from the Greek words κλείσιμο (kleísimo) meaning 'closure', and σύνδεση (sýndesi) meaning 'connection'.
      • Tonian (800–720 Ma)
      • Cryogenian (720–635 Ma)
      • Ediacaran (635–538.8 Ma)

Proposed pre-Cambrian timeline (Shield et al. 2021, ICS working group on pre-Cryogenian chronostratigraphy), shown to scale:[note 6]

Current ICC pre-Cambrian timeline (v2023/09), shown to scale:

Van Kranendonk et al. 2012 (GTS2012)

The book, Geologic Time Scale 2012, was the last commercial publication of an international chronostratigraphic chart that was closely associated with the ICS.[2] It included a proposal to substantially revise the pre-Cryogenian time scale to reflect important events such as the formation of the Solar System and the Great Oxidation Event, among others, while at the same time maintaining most of the previous chronostratigraphic nomenclature for the pertinent time span.[72] As of April 2022 these proposed changes have not been accepted by the ICS. The proposed changes (changes from the current scale [v2023/09]) are italicised:

Proposed pre-Cambrian timeline (GTS2012), shown to scale:

Current ICC pre-Cambrian timeline (v2023/09), shown to scale:

Table of geologic time

The following table summarises the major events and characteristics of the divisions making up the geologic time scale of Earth. This table is arranged with the most recent geologic periods at the top, and the oldest at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time. As such, this table is not to scale and does not accurately represent the relative time-spans of each geochronologic unit. While the Phanerozoic Eon looks longer than the rest, it merely spans ~539 million years (~12% of Earth's history), whilst the previous three eons[note 3] collectively span ~3,461 million years (~76% of Earth's history). This bias toward the most recent eon is in part due to the relative lack of information about events that occurred during the first three eons compared to the current eon (the Phanerozoic).[6][77] The use of subseries/subepochs has been ratified by the ICS.[17]

The content of the table is based on the official ICC produced and maintained by the ICS who also provide an online interactive version of this chart. The interactive version is based on a service delivering a machine-readable Resource Description Framework/Web Ontology Language representation of the time scale, which is available through the Commission for the Management and Application of Geoscience Information GeoSciML project as a service[78] and at a SPARQL end-point.[79][80]

Non-Earth based geologic time scales

Lunar (selenological) time scale

The geologic history of Earth's Moon has been divided into a time scale based on geomorphological markers, namely impact cratering, volcanism, and erosion. This process of dividing the Moon's history in this manner means that the time scale boundaries do not imply fundamental changes in geological processes, unlike Earth's geologic time scale. Five geologic systems/periods (Pre-Nectarian, Nectarian, Imbrian, Eratosthenian, Copernican), with the Imbrian divided into two series/epochs (Early and Late) were defined in the latest Lunar geologic time scale.[97] The Moon is unique in the Solar System in that it is the only other body from which we have rock samples with a known geological context.

Early ImbrianLate ImbrianPre-NectarianNectarianEratosthenianCopernican period
Millions of years before present


Martian geologic time scale

The geological history of Mars has been divided into two alternate time scales. The first time scale for Mars was developed by studying the impact crater densities on the Martian surface. Through this method four periods have been defined, the Pre-Noachian (~4,500–4,100 Ma), Noachian (~4,100–3,700 Ma), Hesperian (~3,700–3,000 Ma), and Amazonian (~3,000 Ma to present).[98][99]

NoachianNoachianHesperianAmazonian (Mars)
Martian time periods (millions of years ago)

A second time scale based on mineral alteration observed by the OMEGA spectrometer on board the Mars Express. Using this method, three periods were defined, the Phyllocian (~4,500–4,000 Ma), Theiikian (~4,000–3,500 Ma), and Siderikian (~3,500 Ma to present).[100]

See also

Notes

  1. ^ It is now known that not all sedimentary layers are deposited purely horizontally, but this principle is still a useful concept.
  2. ^ 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.
  3. ^ a b c Precambrian or pre-Cambrian is an informal geological term for time before the Cambrian period
  4. ^ a b The Tertiary is a now obsolete geologic system/period spanning from 66 Ma to 2.6 Ma. It has no exact equivalent in the modern ICC, but is approximately equivalent to the merged Palaeogene and Neogene systems/periods.[18][19]
  5. ^ a b Geochronometric date for the Ediacaran has been adjusted to reflect ICC v2023/09 as the formal definition for the base of the Cambrian has not changed.
  6. ^ Kratian time span is not given in the article. It lies within the Neoarchean, and prior to the Siderian. The position shown here is an arbitrary division.
  7. ^ The dates and uncertainties quoted are according to the International Commission on Stratigraphy International Chronostratigraphic chart (v2023/06). A * indicates boundaries where a Global Boundary Stratotype Section and Point has been internationally agreed.
  8. ^
    Carbon dioxide in the Earth's atmosphere, and climate change. Specific graphs of reconstructed CO2 levels over the past ~550, 65, and 5 million years can be seen at File:Phanerozoic Carbon Dioxide.png, File:65 Myr Climate Change.png, File:Five Myr Climate Change.png
    , respectively.
  9. ^ The Mississippian and Pennsylvanian are official sub-systems/sub-periods.
  10. ^ a b This is divided into Lower/Early, Middle, and Upper/Late series/epochs
  11. ^ a b c d e f g h i j k l m Defined by absolute age (Global Standard Stratigraphic Age).
  12. ^ The age of the oldest measurable craton, or continental crust, is dated to 3,600–3,800 Ma.
  13. ^ Not enough is known about extra-solar planets for worthwhile speculation.

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Further reading

External links