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. The geologic time scale is used primarily by those in the Earth sciences (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[1] is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC)[2] that are used to define divisions of geological time. The chronostratigraphic divisions are in turn used to define geochronologic units.[2]
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±0.05billion years.[3] 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,[4] and geochronology, the science of dating rocks and other geological materials.[5]
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.[4] A chronostratigraphic unit includes all rock sequences globally that were deposited during a particular time interval.[6]
Chronostratigraphy uses several key principles to determine the relative relationships of rocks and thus their chronostratigraphic position in the rock record.[7][8]
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.[9][10][11][8]:150
The principle of original horizontality that states layers of sediments will originally be deposited horizontally under the action of gravity.[9][11][8] However, it is now known that not all sedimentary layers are deposited purely horizontally,[8][12] 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.[9] Layers do not extend indefinitely; their limits are controlled by the amount and type of sediment in a sedimentary basin, and the geometry of that basin.[13]
The law of included fragments that states small fragments of one type of rock that are embedded in a second type of rock must have formed first, and were included when the second rock was forming.[11][8]
The relationships of unconformities which are geologic features representing a gap in the geologic record. Unconformities are formed during periods of erosion or non-deposition, indicating non-continuous sediment deposition.[8] Observing the type and relationships of unconformities in strata allows geologist to understand the relative timing of the strata.[13]
The principle of faunal succession (where applicable) that states rock strata contain distinctive sets of fossils that succeed each other vertically in a specific and reliable order.[14][8] This allows for a correlation of strata even when the horizon between them is not continuous.[13]
Geochronology
Geochronology is the study of geological time. It uses quantitative measurements (geochronometry), such as radiometric dating, to provide precise ages, and relative methods of dating (e.g. paleomagnetism and stable isotope ratios) to establish a timeframe for events in Earth's history.[5][7] A geochronologic unit is an interval of time during which a chronostratigraphic unit formed.[6] For example, all the rocks of the Silurian System (a chronostratigraphic unit) were deposited during the Silurian Period (a geochronologic unit).[15]
The age of a geochronologic unit can be refined and changed by improved dating techniques. However, the equivalent chronostratigraphic unit boundary remains unchanged.[2][15] For example, in early 2022, the base of the Cambrian Period (a geochronologic unit) 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 been changed; rather, the absolute age has merely been refined.[2]
Global Boundary Stratotype Section and Point (GSSP)
Historically, regional geologic time scales were used[16] 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.[7] A Global Boundary Stratotype Section and Point (GSSP) defines the lower boundary of a stage as being at a precise point in a specific rock succession in a particular geographic location. These reference points are known informally as "golden" spikes.[15] All the beds above the spike belong to one time interval and all those below it to another. This allows beds of a similar age around the world to be correlated with the strata that contain the golden spike. For example, the iridium anomaly produced by the Chicxulub asteroid impact marks the lower boundary of the Paleogene System and thus the boundary between the Cretaceous and Paleogene. Whilst the GSSP is defined at Oued Djerfane in Tunisia, strata containing the iridium anomaly are found worldwide.[17]
The Proterozoic (apart from the Ediacaran), Archean and Hadean are subdivided by absolute ages (Global Standard Stratigraphic Ages) rather than geological features.[7] Proposals have been made to better reconcile these divisions with the rock record.[18][16]
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 Chronostratigraphic Chart are represented by the unit Ma (megaannum, for 'million years'). For example, 201.4±0.2 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 units commonly used by geologists are Ga (gigaannum, billion years), and ka (kiloannum, thousand years), with the latter often represented in calibrated units (before present).[5]
The geologic time scale is divided into chronostratigraphic units and their corresponding geochronologic units:
A period is equivalent to a chronostratigraphic system.[4][19] There are 22 defined periods, with the current being the Quaternary period.[2] As an exception, two subperiods are used for the Carboniferous Period.[4]
An epoch is the second smallest geochronologic unit. It is equivalent to a chronostratigraphic series.[4][19] 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.[2] The use of subepochs as formal units in international chronostratigraphy was ratified in 2022.[20]
An age is the smallest hierarchical geochronologic unit. It is equivalent to a chronostratigraphic stage.[4][19] There are 96 formal and five informal ages.[2] The current age is the Meghalayan.[21]
A chron is a non-hierarchical formal geochronology unit of unspecified rank and is equivalent to a chronostratigraphic chronozone. These correlate with magnetostratigraphic, lithostratigraphic, or biostratigraphic units as they are based on previously defined stratigraphic units or geologic features.[4]
Formal, hierarchical units of the geologic time scale (largest to smallest)
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 Early and Late 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 System (chronostratigraphic unit).[4]
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 suffix (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 International Commission on Stratigraphy 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.[4]
Informally, the time before the Cambrian is often referred to as the Precambrian or pre-Cambrian (Supereon).[18][note 2]
The most modern geological time scale was not formulated until 1911[22] 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.[23] 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. 570–487BCE) 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.[24] 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 naturalistShen Kuo[25] (1031–1095) and Islamicscientist-philosophers, notably the Brothers of Purity, who wrote on the processes of stratification over the passage of time in their treatises.[24] Their work likely inspired that of the 11th-century PersianpolymathAvicenna (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.[24] Avicenna also recognized fossils as "petrifications of the bodies of plants and animals",[26] with the 13th-century DominicanbishopAlbertus Magnus (c. 1200–1280), who drew from Aristotle's natural philosophy, extending this into a theory of a petrifying fluid.[27] 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.[24] 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':[28][24]
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 neighbouring 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.
Sketch of the Succession of Strata and their Relative Altitudes (William Smith)
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.[24] Although many theories surrounding philosophy and concepts of rocks were developed in earlier years, "the first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made in the late 18th century."[27] Later, in the 19th century, academics further developed theories on stratification. William Smith, often referred to as the "Father of Geology"[29] developed theories through observations rather than drawing from the scholars that came before him. Smith's work was primarily based on his detailed study of rock layers and fossils during his time and he created "the first map to depict so many rock formations over such a large area".[29] 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.[30] 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."[30]
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.[24] In De solido intra solidum naturaliter contento dissertationis prodromus Steno states:[9][31]
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.[24] 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:[32]
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 during the era of Biblical models by Thomas Burnet who applied a two-fold terminology to mountains by identifying "montes primarii" for rock formed at the time of the 'Deluge', and younger "monticulos secundarios" formed later from the debris of the "primarii".[33][24]Anton Moro (1687–1784) also used primary and secondary divisions for rock units but his mechanism was volcanic.[34][24] In this early version of the Plutonism theory, the interior of Earth was seen as hot, and this drove the creation of primary igneous and metamorphic rocks and secondary rocks formed contorted and fossiliferous sediments. These primary and secondary divisions were expanded on by Giovanni Targioni Tozzetti (1712–1783) and Giovanni Arduino (1713–1795) to include tertiary and quaternary divisions.[24] 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.[35][10][36] Hutton's theory would later become known as uniformitarianism, popularised by John Playfair[37] (1748–1819) and later Charles Lyell (1797–1875) in his Principles of Geology.[11][38][39] 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.[40]
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.[41]
The advent of geochronometry
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.[3] These estimations varied from 15,000 million years to 0.075 million years depending on method and author, but the estimations of 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.[42]
Graph maps depth in sediment layers to the geological time interval during which each layer formed. Deeper layers correspond to older times.
The establishment of the IUGS in 1961[48] and acceptance of the Commission on Stratigraphy (applied in 1965)[49] 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,[50] 1989,[51] 2004,[52] 2008,[53] 2012,[54] 2016,[55] and 2020.[56] 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[55] and 2020[56]) 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.[41]
The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of 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.
(Horizontal scale is millions of years for the above timelines; thousands of years for the timeline below)
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 ~538.8 Ma (~11.8% of Earth's history), whilst the previous three eons[note 2] collectively span ~4,028.2 Ma (~88.2% 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).[18][57] The use of subseries/subepochs has been ratified by the ICS.[20]
While some regional terms are still in use,[16] the table of geologic time conforms to the nomenclature, ages, and colour codes set forth by the International Commission on Stratigraphy in the official International Chronostratigraphic Chart.[1][58]
Start of Pleistocene Ice Age: 40,000 year cycles of glacials/interglacials with ice cap growth and retreat, and sea level falls and rises.[63] Rise of Pleistocene megafauna. Homo habilis and Homo erectus evolve in Africa.[66]
Connection between Mediterranean Sea and Atlantic is blocked, resulting in Messinian salinity crisis with evaporites accumulating across Mediterranean as its waters dry up. Collision of Banda Arc with Australia and Timor begins.[68] Global climate cools and permanent ice cap forms in Arctic. Sea levels drop as ice sheets grow.[63] Spread of C4 grasses result in extinction of many herbivores.[6]Sea snakes evolve. Gorilla-human-chimpanzee lineages split, then chimpanzees and humans diverge.[66] Earliest hominid Sahelanthropus.[6]
Australia begins to collide with Southeast Asia, blocking equatorial circulation between western Pacific and Indian Oceans.[68][63] Antarctic ice cap shrinks as global temperatures warm (Middle Miocene climatic optimum).[63] Last creodonts (early predatory mammals) become extinct. Megalodon (giant shark) evolves.[66]
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.[68] The terranes collide with southeastern Laurussia during the Variscan orogeny. Antler orogeny continues, and opening of the Slide Mountain Ocean along western margin of Laurussia.[85] 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.[86] Main phase of LPIA begins. Drop in global sea levels, extensive glaciation across Gondwana.[83] Increasing atmospheric oxygen levels.[82] Change from calcite to aragonite seas.[6]Evolutionary radiations after the Late Devonian extinctions include brachiopods, bivalves, echinoderms, ammonoids, gastropods, sharks and ray-finned bony fish. Placoderms and graptolites die out. Proetida only group of trilobites.[6][66] First freshwater molluscs and sharks.[6]Arthropleura (millipede) largest ever terrestrial arthropod. First flying insects Paleodictyopora. Fish-like (Pederpes) and semi-aquatic tetrapods (Eucritta) appear on land.[66] Seedless vascular plants and seed ferns diversify.[6]
Paleo-Tethys continues to open as the Armorican Terrane Assemblage (ATA) drifts north and Annamia-South China moves away from Gondwana.[68][87]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).[68] Period of high sea-levels, greenhouse conditions but decreasing atmospheric CO2 levels and slowly cooling climate with glaciations towards end.[88]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).[88] Growth of massive reef systems. Major radiation of jawed fish with appearance of ray-finned, lobe-finned, and cartilaginous fish. Appearance of tetrapods (evolved from lobe-finned fish). Early amphibians move on to land. First ammonoids.[6] Emplacement of the Viley and Pripyat–Dniepr–Donets large igneous provinces coincide with global marine anoxic events and the Kellwasser (c. 372 Ma) and Hangenberg (c. 359 Ma) mass extinctions.[88] Kellwasser extinction: c. 20% of families and c. 50% of genera of marine invertebrates lost. Tabulate coral and stromatoporoid reef ecosystems wiped out. Loss of placoderms and many groups of jawless fish. Hangenberg extinction: loss of c. 16% of marine families and c. 21% of marine genera, including ammonoids, ostracods and sharks.[88][89]
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).[68][90] Series of microcontinents and North China separate opening Paleo-Tethys and closing Paleoasian Ocean.[90] Rheic Ocean widens between Gondwana and Laurussia. Siberia drifts north of equator.[68] Temperatures increase as Hirnantian glaciation ends. Sea levels rise. Deposition of black shales, North Africa and Arabia, major hydrocarbonsource rocks.[68] Fluctuating climate with glacial advances results in changing ocean conditions causes extinction events, followed by ecological recoveries.[91] Widespread evaporite deposition and hothouse climate by late Silurian.[6][63] After end-Ordovician mass extinction, major radiation of graptolites, bivalves, gastropods, nautiloids, brachiopods, and crinoids. Increase in trilobites, but never fully recover. Corals and stromatoporiods diversify to produce large reefs. Proliferation of eurypterid arthropods. Earliest jawed fish (acanthodians). Appearance of ostracoderms. Appearance of vascular plants. First land animals including myriapods. First freshwater fish.[6]
Most continents lay in equatorial regions. Gondwana stretched to south pole. Panthalassic Ocean covered northern hemisphere. Avalonia rifted from Gondwana closing Iapetus Ocean in front, opening Rheic Ocean behind. South China close to Gondwana; North China between Siberia and Gondwana. Orogenies: Famatinian (South America); Benambran (Australia); Taconic (Laurentia). Baltica and Siberia drift north.[68] Early greenhouse climate, cooling to icehouse conditions during Hirnantian Ice Age. Increase in atmospheric O2.[92]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.[93] Modern marine ecosystems established.[92] Earliest jawless fish. Tabulate corals and stromatoporoids dominant reef builders. Nautiloids main predators.[6] Appearance of eurypterids and asteroids. Spread of early land plants.[92]Late Ordovician mass extinction, loss of ~85% of marine invertebrate species. Two pulses: first with onset of glaciation affects tropical fauna; second at end of ice age, warming climate impacts cool water species.[6] Drastic reduction in trilobite, brachiopod, graptolite, echinoderm, conodont, coral, and chitinozoan genera.[93]
Gondwana stretched from the south pole to equator, separated from Laurentia and Baltica by the Iapetus Ocean. Siberia lay close to the equator, north of Baltica; North and South China close to equatorial Gondwana. Orogenies: Cadomian (N.Africa/southern Europe); Kuunga (central Gondwana); Famatinian orogeny (South America); Delamerian (Australia).[68] Greenhouse climate. High atmospheric CO2 levels. Atmospheric oxygen levels rose with increase in photosynthesising organisms.[94] Early aragonite seas replaced by mixed aragonite-calcite seas with many animals developing CaCO3 skeletons.[95] 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.[96] Giant anomalocarids (arthropods) dominant predators. Increase in bioturbation and grazing led to decline in stromatolites.[6] 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.[94][97][6] Many fossil lagerstätten, including Burgess Shale and Chengjiang Formation, formed by rapid burial in anoxic conditions.[94]
Rodinia continues to breakup. 720 Ma eruptions of Franklin and Irkutsk LIPs mark rifting of Siberia from Laurentia. Iapetus Ocean begins to open as Amazonia and Baltica drift from Laurentia (from c. 650 Ma).[98]Sturtian (720–658 Ma) and Marinoan (655–635 Ma) Snowball Earth glaciations.[16]
900 Ma Rodinia at its maximum extent. Intracontinental rifting begins c. 850 Ma, associated magmatism becoming widespread from 825 Ma, including the Malani Igneous Suite eruptions, India (c. 775 Ma). Beginning of breakup of Rodinia from c. 750 Ma.[98]
Extensive dyke swarms found across all cratons mark completion of breakup of Columbia (Nuna) supercontinent.[102] 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.[16]
Columbia continues to fragment with widespread rift-related magmatism.[103] Stromatolites reach their maximum extent and diversity as cyanobacteria diversify and flourish.[99] Primitive seaweeds appear.[16]
Columbian supercontinent continues to grow along its margins by subduction-related magmatism and terrane accretion. Extension and rift zones begin to develop from c. 1.6 Ga. Eukaryotic red algae appear.[99]Vredefort impact event (2.19 Ga).[104]
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.[103] Drop in atmospheric oxygen as increased volcanism releases carbon dioxide.[16]Grypania represents a possible early eukaryote.[99]Sudbury Impact (1.85 Ga).[104]
Widespread mantle melting and crustal growth followed by formation of supercratons Superia (North America, northwest Europe, South Africa and northwest Australia) and Sclavia (Canada, Zimbabwe, southern India, southwestern Australia, Brazil and North China).[16][105] Major diversification of cyanobacteria with multicellularity, increasing cell size and specialisation.[99] Proliferation of oxygen-producing life leads to stepwise increase in atmospheric oxygen and deposition of banded iron formation.[99][16]
Possible onset of plate tectonics c. 3 Ga.[68] Cratons with low relief and extensive shallow marine environments. Weathering increased supply of nutrients to seas. Localised free oxygen associated with carbonate platform stromatolites. Evidence for oxygen-producing photosynthesisers (and possible eukaryotes) c. 3.2 Ga, and terrestrial life c. 3 Ga.[99] Oldest evidence of glaciation c. 2.9 Ga.[16]
First suggested in 2000,[106] 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.[107] The definition of the Anthropocene as a geologic time period rather than a geologic event remains controversial and difficult.[108][109][110][111]
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.[112] 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.[113] 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"[114]
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.[18] This work assessed the geologic history of the currently defined eons and eras of the Precambrian,[note 2] and the proposals in the "Geological Time Scale" books 2004,[115]2012,[16] and 2020.[116] Their recommend revisions[18] of the pre-Cryogenian geologic time scale were as below (changes from the current scale [v2023/09] are italicised). This suggestion was unanimously rejected by the Subcommission on Precryogenian Stratigraphy, based on scientific weaknesses.
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 κράτος (krátos) '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 Greek σκουριά (skouriá) 'rust'.
Kleisian or Syndian (1000–800 Ma) – respectively from Greek κλείσιμο (kleísimo) 'closure' and σύνδεση (sýndesi) '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 9]
ICC pre-Cambrian timeline (v2026-06, current as of August2026), 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 and the Subcommission on Precambrian Stratigraphy.[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.[16]As of April2022 these proposed changes have not been accepted by the ICS. The proposed changes (changes from the current scale [v2023/09]) are italicised:
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.[54][117]
Vaalbaran Period/System (3490–3020 Ma) – based on the names of the Kaapvaal (Southern Africa) and Pilbara (Western Australia) cratons, to reflect the growth of stable continental nuclei or proto-cratonic kernels.[54]
Pongolan Period/System (3020–2780 Ma) – named after the Pongola Supergroup, in reference to the well preserved evidence of terrestrial microbial communities in those rocks.[54]
Oxygenian Period/System (2420–2250 Ma) – named for displaying the first evidence for a global oxidising atmosphere.[54]
Jatulian or Eukaryian Period/System (2250–2060 Ma) – names are respectively for the Lomagundi–Jatuli δ13C isotopic excursion event spanning its duration, and for the (proposed)[119][120] first fossil appearance of eukaryotes.[54]
Some other planets and satellites in the Solar System have sufficiently rigid structures to have preserved records of their own histories, for example, Venus, Mars and the Earth's Moon. Dominantly fluid planets, such as the giant planets, do not comparably preserve their history. Apart from the Late Heavy Bombardment, events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets. Construction of a time scale that links the planets is, therefore, of only limited relevance to the Earth's time scale, except in a Solar System context. The existence, timing, and terrestrial effects of the Late Heavy Bombardment are still a matter of debate.[121]
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.[122] 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.
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).[123][124]
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).[125]
Plutonian time scale
The geologic history of Pluto has been tentatively divided into four periods for the regions closely observed by New Horizons, each named after an associated significant feature: the Burneyan, representing the oldest, most cratered terrain; the Hayabusan, representing middle-aged, eroded terrain; the Tartaran, representing relatively young terrain associated with the sublimation and deposition of volatiles; and Sputnikian, representing the very young ice deposits of Sputnik Planitia.[126][127]
↑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.
123Precambrian or pre-Cambrian is an informal geological term for time before the Cambrian period
↑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.[59][60]
12Geochronometric 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.
↑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.
↑Smith, William (1816). Strata identified by organized fossils: Containing prints on colored paper of the most characteristic specimens in each stratum. doi:10.5962/bhl.title.106808. OCLC654668607.
123Nichols, Gary (2010). Sedimentology and stratigraphy (2. ed., [Nachdr.]ed.). Chichester: Wiley-Blackwell. ISBN978-1-4051-3592-4.
12Smith, William (1816). Strata identified by organized fossils: Containing prints on colored paper of the most characteristic specimens in each stratum. doi:10.5962/bhl.title.106808.
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