Geologic time scale

upright=1.35|alt=Geologic time scale proportionally represented as a log-spiral. The image also shows some notable events in Earth's history and the general evolution of life.|thumb|The geologic time scale, proportionally represented as a [[Logarithmic spiral|log-spiral with some major events in Earth's history. A megaannum (Ma) represents one million (106) years.]]

The geologic time scale or geological time scale describes how geologic time is divided into standardised intervals. It uses the rock record together with the principles of chronostratigraphy to place rock sequences into their relative age positions, and geochronology techniques, such as radiometric dating, to precisely date the boundaries between them. 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 geological 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 is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC) that are used to define divisions of geological time. The chronostratigraphic divisions are in turn used to define geochronologic units. It arranges the rock record in chronological order by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. It combines the disciplines of chronostratigraphy, which studies the relationships between rock sequences to determine their relative ages, and geochronology, the science of dating rocks and other geological materials.

Chronostratigraphy

Chronostratigraphy is the branch of stratigraphy that organises all the rocks of the Earth's crust into groups, known as chronostratigraphic units, based on their relative ages.

The law of superposition that states that in undeformed stratigraphic sequences the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface. but this principle is still a useful concept.
The principle of lateral continuity that states layers of sediments extend laterally in all directions until either thinning out or being cut off by a different rock layer, i.e. they are laterally continuous.

The age of a geochronologic unit can be refined and changed by improved dating techniques. However, the equivalent chronostratigraphic unit boundary remains unchanged. 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.

The Proterozoic (apart from the Ediacaran), Archean and Hadean are subdivided by absolute ages (Global Standard Stratigraphic Ages) rather than geological features. There are four formally defined eons: the Hadean, Archean, Proterozoic and Phanerozoic.

An is the smallest hierarchical geochronologic unit. It is equivalent to a chronostratigraphic stage. by Arthur Holmes (1890 – 1965), who drew inspiration from James Hutton (1726–1797), a Scottish Geologist who presented the idea of uniformitarianism or the theory that changes to the Earth's crust resulted from continuous and uniform processes. The broader concept of the relation between rocks and time can be traced back to (at least) the philosophers of Ancient Greece from 1200 BC to 600 AD. Xenophanes of Colophon (c. 570–487 BCE) observed rock beds with fossils of seashells 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. This view was shared by a few of Xenophanes's scholars and those that followed, including Aristotle (384–322 BC) 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 recognized by Chinese naturalist Shen Kuo (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. with the 13th-century Dominican bishop Albertus Magnus (c. 1200–1280), who drew from Aristotle's natural philosophy, extending this into a theory of a petrifying fluid. 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. After studying rock layers and the fossils they contained, Smith concluded that each layer of rock contained distinct material that could be used to identify and correlate rock layers across different regions of the world. Smith developed the concept of faunal succession or the idea that fossils can serve as a marker for the age of the strata they are found in and published his ideas in his 1816 book, "Strata identified by organized fossils."

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.

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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. Hutton's theory would later become known as uniformitarianism, popularised by John Playfair (1748–1819) and later Charles Lyell (1797–1875) in his Principles of Geology. 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

thumb|One example of an obsolete geological time scale (France, mid-1940s).

During the 19th century, the debate regarding Earth's age was renewed, with geologists estimating ages based on denudation rates and sedimentary thicknesses or ocean chemistry, and physicists determining ages for the cooling of the Earth or the Sun using basic thermodynamics or orbital physics. The discovery of isotopes in 1913 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.

Modern international geological time scale

The establishment of the IUGS in 1961 and acceptance of the Commission on Stratigraphy (applied in 1965) 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". 1989, 2004, 2008, 2012, 2016, and 2020. 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. The use of subseries/subepochs has been ratified by the ICS.

{| class="wikitable collapsible sticky-header" style="clear:both;margin:0; font-size:95%"

!Eonothem/ Eon

!Erathem/ Era

!System/ Period

!Series/ Epoch

!Stage/ Age

!Major events

!Start, million years ago

| rowspan="102" style="background:" |Phanerozoic

| rowspan="24" style="background:" |Cenozoic

| rowspan="7" style="background:" |Quaternary

| rowspan="3" style="background:" |Holocene

| style="background:#fcf0f2" |Meghalayan

|4.2 ka cool period, dry climate leads to decline of agriculture-related civilisations in Egypt, Mesopotamia and India. Medieval Warm Period (about 900 - 1350 CE) and Little Ice Age (about 1400 to 1900 CE). Rapidly warming climate as CO2 added to atmosphere from burning fossil fuels.

| style="background:#fcf0e8" | *

| style="background:#fcf0de" |Greenlandian

|Younger Dryas and Last Glacial Period end. Rise of agriculture. Homo heidelbergensis evolves in Africa and spreads to Europe. Homo neanderthalensis appear in western Eurasia. Homo sapiens evolve in Africa. Homo erectus and Homo heidelbergensis die out. Laramide orogeny ends. Cooling climate with brief warm period. End Eocene Australia and South America move away from Antarctica opening Drake and Tasmanian passages. Antarctic Circumpolar current forms. Rapid drop in global temperatures. Ice sheets on Antarctica. Greenhouse temperatures continue from Paleocene-Eocene Thermal Maximum (PETM) as climate affected by North Atlantic LIP eruptions, but global cooling begins from about 50 Ma with changing paleogeography and oceanography conditions. Bering Straits land bridge present during low sea level periods. Nevadan orogeny, North America. Rise in global sea levels. Change from aragonite to calcite seas. Sonoma (western Laurussia), and Hunter-Bowen (Australia) orogenies continue. Archosaurs divide into pseudosuchia (crocodiles), and ornithodirans (dinosaurs and pterosaurs). Mammaliaformes evolve from cynodonts. First teleosts (modern ray-finned fish). Ichthyosaurs, and sauropterygians plesiosaurs, nothosaurs, placodonts) appear. Sonoma orogeny, western Laurussia. Kazakhstania and Tarim collide with Siberia. Orogenic collapse of Variscan orogeny and early extension along the lines of the future Atlantic, Indian and Southern Oceans. Opening of Neo-Tethys Ocean as Cimmerian terranes rift from northeast Gondwana. Global average temperatures rise from c. 12° to over 30° at Permo-Triassic boundary. Humid, coal swamps form in foreland basins of the Central Pangean Mountains and around North and South China cratons. As the Late Paleozoic icehouse (LPIA) continues, waxing and waning of ice sheets causes rapid changes in global sea level, flooding these regions and depositing cyclothem sequences. Atmospheric oxygen levels rise to over 25% before decreasing again. Appearance of aragonite reef builders, including algae and sponges. Lepidodendron and Sigillaria lycopod trees dominate coal swamps, with smaller sphenopsids (horsetails) and seed ferns between. Gymnosperms, including conifers and cycads grow on drier ground. leads to change in woodland vegetation (Carboniferous rainforest collapse).

| style="background:" |

| style="background:" |Kasimovian

| style="background:" |

| style="background:" |Moscovian

| style="background:" |

| style="background:" |Bashkirian

| style="background:" | *

| rowspan="3" style="background:" |Mississippian

| style="background:" |Serpukhovian

| rowspan="3" |Continents form a near circle around the opening Paleo-Tethys Ocean. Gondwana forms the southern to southwestern margin; Laurussia the west; Siberia, Amuria and Kazakhstania the north; North and South China the northeast; and, Annamia the eastern margin. Closure of Ural Ocean between Kazakhstania and Laurussia during the Ural orogeny. Development of Altai accretionary complexes along north and eastern margin of the Paleo-Tethys. Main phase of LPIA begins. Drop in global sea levels, extensive glaciation across Gondwana. Rheic Ocean closes as ATA collides with Laurussia beginning the Variscan orogeny. Other orogenies: Antler, Ellesmerian, and Acadian (Laurussia); Achalian (Argentina); Tabberabberan/Lachlan (Australia); Ross (Antarctica); Kazakh (Kazakhstania). Vascular plants increase in size, develop large root systems and spread to upland areas. First forests, seed plants, and modern soil orders appear (alfisols and ultisols).

| style="background:" | *

| style="background:" |Frasnian

| style="background:" | *

| rowspan="2" style="background:" |Middle

| style="background:" |Givetian

| style="background:" | *

| style="background:" |Eifelian

| style="background:" | *

| rowspan="3" style="background:" |Lower/Early

| style="background:" |Emsian

| style="background:" | *

| style="background:" |Pragian

| style="background:" | *

| style="background:" |Lochkovian

| style="background:" | *

| rowspan="8" style="background:" |Silurian

| colspan="2" style="background:" |Pridoli

| rowspan="8" |Laurentia and Avalonia-Baltica collide as Iapetus Ocean closes, Caledonian-Scandian orogeny, and formation of Laurussia. Other orogenies: Salinic (Appalachians); Famatinian (South America) tapers off; Lachlan (Australia). Series of microcontinents and North China separate opening Paleo-Tethys and closing Paleoasian Ocean. Widespread evaporite deposition and hothouse climate by late Silurian. Great Ordovician Biodiversification Event, major increase in new genera e.g. brachiopods, trilobites, corals, echinoderms, bryozoans, gastropods, bivalves, nautiloids, graptolites, and conodonts. Very high sea levels expand shallow continental seas, increase range of ecological niches. Modern marine ecosystems established. Early aragonite seas replaced by mixed aragonite-calcite seas with many animals developing CaCO3 skeletons. Rapid diversification of animals (Cambrian Explosion), most modern animal phyla appear, e.g. arthropods; molluscs; annelids; echinoderms; bryozoa; priapulids; brachiopods; hemichordates; and, chordates. Radiations of small shelly fossils. Giant anomalocarids (arthropods) dominant predators. Increase in bioturbation and grazing led to decline in stromatolites. Varying oxygen levels in oceans led to series of extinction events followed by radiations, including: earliest Cambrian loss of the Ediacaran acritarchs; end-Botomian extinction, linked to the Kalkarindji large igneous province eruptions (c. 514 Ma) with loss of archaeocyathids (early Cambrian reef builders) and hyoliths; and, end-Cambrian reduction in trilobite diversity. Rapid rise in eukaryote diversity and numbers, including early animals. First biomineralising animals. Diversification of eukaryotes as oxygen levels increase. All major modern day clades, including Archaeplastida (e.g. red and green algae), Opisthokonta (e.g. fungi) and Amoebozoa appear. Evidence for life on land.

| style="background:" |

| style="background:" |Ectasian

| colspan="3" |Extensive dyke swarms found across all cratons mark completion of breakup of Columbia (Nuna) supercontinent. Oceans have oxygen-rich surface layers and euxinic (no oxygen, high levels of H2S) deep waters, leading to widespread formation of giant massive sulphide deposits (SEDEX) on the seafloor.

| style="background:" |

| style="background:" |Orosirian

| colspan="3" |2.0–1.8 Ga Columbia supercontinent assembles during collisional events including Trans-Hudson orogeny (North America), Limpopo Belt (South Africa), Capricorn orogeny (Australia) and Trans-North China orogeny. Drop in atmospheric oxygen as increased volcanism releases carbon dioxide. Major diversification of cyanobacteria with multicellularity, increasing cell size and specialisation. Evidence for oldest crust from detrital zircon c. 4.37 Ga. 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. The definition of the Anthropocene as a geologic time period rather than a geologic event remains controversial and difficult.

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. The formal proposal was completed and submitted to the Subcommission on Quaternary Stratigraphy in late 2023 for a section in Crawford Lake, Ontario, with heightened Plutonium levels corresponding to 1952 CE. This proposal was rejected as a formal geologic epoch in early 2024, to be left instead as an "invaluable descriptor of human impact on the Earth system"

Proposals for revisions to pre-Cryogenian timeline

Shields et al. 2021

The ICS Subcommission for Cryogenian Stratigraphy has 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. 2012, Their recommend revisions

Jack Hillsian or Zirconian Era/Erathem (4404–4030 Ma) – both names allude to the Jack Hills Greenstone Belt which provided the oldest mineral grains on Earth, zircons. first fossil appearance of eukaryotes. The Moon is unique in the Solar System in that it is the only other body from which humans have rock samples with a known geological context.

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).

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).

ImageSize = width:800 height:50

PlotArea = left:15 right:15 bottom:20 top:5

AlignBars = early

Period = from:-4500 till:0

TimeAxis = orientation:horizontal

ScaleMajor = unit:year increment:500 start:-4500

ScaleMinor = unit:year increment:100 start:-4500

Colors =

id:sidericol value:rgb(1,0.4,0.3)

id:theiicol value:rgb(1,0.2,0.5)

id:phyllocol value:rgb(0.7,0.4,1)

PlotData=

align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5)

text:Siderikan from:-3500 till:0 color:sidericol

text:Theiikian from:-4000 till:-3500 color:theiicol

text:Phyllocian from:start till:-4000 color:phyllocol

</timeline>

Notes

References

External links

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
NASA: Geologic Time (archived 18 April 2005)
GSA: Geologic Time Scale (archived 20 January 2019)
British Geological Survey: Geological Timechart
GeoWhen Database (archived 23 June 2004)
National Museum of Natural History – Geologic Time (archived 11 November 2005)
SeeGrid: Geological Time Systems. . Information model for the geologic time scale.
Exploring Time from Planck Time to the lifespan of the universe
Lane, Alfred C, and Marble, John Putman 1937. Report of the Committee on the measurement of geologic time
Lessons for Children on Geologic Time (archived 14 July 2011)
Deep Time – A History of the Earth : Interactive Infographic
Geology Buzz: Geologic Time Scale. .

Geologic time scale

Chronostratigraphy

The advent of geochronometry

Modern international geological time scale

Proposals for revisions to pre-Cryogenian timeline

Shields et al. 2021

Martian geologic time scale

See also

Notes

References

Further reading

External links