Question: How many geologic eons are there?

Geologic time scale




The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. The table of geologic time spans, presented here, agree with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).


Terminology
The primary defined divisions of time are eons, in sequence the Hadean, the Archean, the Proterozoic and the Phanerozoic.
The first three of these can be referred to collectively as the Precambrian supereon. 
Eons are divided into eras, which are in turn divided into periods, epochs and ages.
The following four timelines show the geologic time 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. Therefore, 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, and the most recent period is expanded in the fourth timeline.




Millions of Years
Corresponding to eons, eras, periods, epochs and ages, the terms "eonothem", "erathem", "system", "series", "stage" are used to refer to the layers of rock that belong to these stretches of geologic time in Earth's history.
Geologists qualify these units as "early", "mid", and "late" when referring to time, and "lower", "middle", and "upper" when referring to the corresponding rocks. For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology.[2] The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic."
Rationale
Evidence from radiometric dating indicates that Earth is about 4.54 billion years old.[3][4] The geology or deep time of Earth's past has been organized into various units according to events which took place. Different spans of time on the GTS are usually marked by corresponding changes in the composition of strata which indicate major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life. Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.
Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same time-span was historically given different names in different locales. For example, in North America, the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on succession of trilobites. In East Asia and Siberia, the same unit is split into Alexian, Atdabanian, and Botomian stages. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.[5]
Some other planets and moons 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 gas giants, do not preserve their history in a comparable manner. 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 debated.[lower-alpha 1]
History and nomenclature of the time scale
Life timelineThis box: viewtalk-4500—–-4000—–-3500—–-3000—–-2500—–-2000—–-1500—–-1000—–-500—–0—waterSingle-celled
lifephotosynthesisEukaryotesMulticellular
lifeArthropods    MolluscsPlantsDinosaursMammalsFlowersBirdsPrimates←Earth (−4540)←Earliest water←Earliest life←Earliest oxygen←Atmospheric oxygen←Oxygen crisis←Sexual reproduction←Earliest plants←Ediacara biota←Cambrian explosion←Tetrapoda←Earliest apesP
h
a
n
e
r
o
z
o
i
c






P
r
o
t
e
r
o
z
o
i
c


A
r
c
h
e
a
nH
a
d
e
a
nPongolaHuronianCryogenianAndeanKarooQuaternaryIceAges(million years ago)
(


Early history
In Ancient Greece, Aristotle (384-322 BCE) observed that fossils of seashells in rocks resembled those found on beaches – he inferred that the fossils in rocks were formed by organisms, and he reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci (1452–1519) concurred with Aristotle's interpretation that fossils represented the remains of ancient life.[6]
The 11th-century Persian geologist Avicenna (Ibn Sina, died 1037) and the 13th-century Dominican bishop Albertus Magnus (died 1280) extended Aristotle's explanation into a theory of a petrifying fluid.[7] Avicenna also first proposed one of the principles underlying geologic time scales, the law of superposition of strata, while discussing the origins of mountains in The Book of Healing (1027).[8][9] The Chinese naturalist Shen Kuo (1031–1095) also recognized the concept of "deep time".[10]
Establishment of primary principles
In the late 17th century Nicholas Steno (1638–1686) pronounced the principles underlying geologic (geological) time scales. Steno argued that rock layers (or strata) were laid down in succession, and that each represents a "slice" of time. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them proved challenging. Steno's ideas also lead to other important concepts geologists use today, such as relative dating. Over the course of the 18th century geologists realized 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
The Neptunist theories popular at this time (expounded by Abraham Werner (1749–1817) in the late 18th century) proposed that all rocks had precipitated out of a single enormous flood. A major shift in thinking came when James Hutton presented his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe[11]
before the Royal Society of Edinburgh in March and April 1785. John McPhee asserts that "as things appear from the perspective of the 20th century, James Hutton in those readings became the founder of modern geology".[12]:95–100
Hutton proposed that the interior of Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory, known as "Plutonism", stood in contrast to the "Neptunist" flood-oriented theory.
Formulation of geologic time scale
The first serious attempts to formulate a geologic time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Werner, among others) divided the rocks of Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Paleogene and Neogene) remained in use as the name of a geological period well into the 20th century and  "Quaternary" remains in formal use as the name of the current period.
The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brongniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geologic periods still used today.
Naming of geologic periods, eras and epochs
Early work on developing the geologic time scale was dominated by British geologists, and the names of the geologic periods reflect that dominance. The "Cambrian", (the classical name for Wales) and the "Ordovician", and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales.[12]:113–114 The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was an adaptation of "the Coal Measures", the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by Scottish geologist Roderick Murchison. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad)—red beds, capped by chalk, followed by black shales—that are found throughout Germany and Northwest Europe, called the ‘Trias’. The "Jurassic" was named by a French geologist Alexandre Brongniart for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous" (from Latin creta meaning ‘chalk’) as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin[13] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates) found in Western Europe.
British geologists were also responsible for the grouping of periods into eras and the subdivision of the Tertiary and Quaternary periods into epochs. In 1841 John Phillips published the first global geologic time scale based on the types of fossils found in each era. Phillips' scale helped standardize the use of terms like Paleozoic ("old life") which he extended to cover a larger period than it had in previous usage, and Mesozoic ("middle life") which he invented.[14]
Dating of time scales

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since estimates of rates of change were uncertain. While creationists had been proposing dates of around six or seven thousand years for the age of Earth based on the Bible, early geologists were suggesting millions of years for geologic periods, and some were even suggesting a virtually infinite age for Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century, the ages of various rock strata and the age of Earth were the subject of considerable debate.
The first geologic time scale that included absolute dates was published in 1913 by the British geologist Arthur Holmes.[15] He greatly furthered the newly created discipline of geochronology and published the world-renowned book The Age of the Earth in which he estimated Earth's age to be at least 1.6 billion years.[16]
In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) began to define global references known as GSSP (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's work is described in the 2012 geologic time scale of Gradstein et al.[17] A UML model for how the timescale is structured, relating it to the GSSP, is also available.[18]
The Anthropocene
Popular culture and a growing number of scientists use the term "Anthropocene"  informally to label the current epoch in which we are living. The term was coined by Paul Crutzen and Eugene Stoermer in 2000 to describe the current time in which humans have had an enormous impact on the environment.  It has evolved to describe an "epoch" starting some time in the past and on the whole defined by anthropogenic carbon emissions and production and consumption of plastic goods that are left in the ground.[19]
Critics of this term say that the term should not be used because it is difficult, if not nearly impossible, to define a specific time when humans started influencing the rock strata—defining the start of an epoch.[20] Others say that humans have not even started to leave their biggest impact on Earth, and therefore the Anthropocene has not even started yet.
The ICS has not officially approved the term as of September2015.[21]  The Anthropocene Working Group met in Oslo in April 2016 to consolidate evidence supporting the argument for the Anthropocene as a true geologic epoch.[21] Evidence was evaluated and the group voted to recommend "Anthropocene" as the new geological age in August 2016.[22]
Should the International Commission on Stratigraphy approve the recommendation, the proposal to adopt the term will have to be ratified by the International Union of Geological Sciences before its formal adoption as part of the geologic time scale.[23]
Table of geologic time
The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. This table is arranged with the most recent geologic periods at the top, and the most ancient at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time.
The content of the table is based on the current official geologic time scale of the International Commission on Stratigraphy,[1] with the epoch names altered to the early/late format from lower/upper as recommended by the ICS when dealing with chronostratigraphy.[2]
A service providing a Resource Description Framework/Web Ontology Language representation of the timescale is available through the Commission for the Management and Application of Geoscience Information GeoSciML project as a service[24] and at a SPARQL end-point.[25][26]

Proposed Precambrian timeline
The ICS's Geologic Time Scale 2012 book which includes the new approved time scale also displays a proposal to substantially revise the Precambrian time scale to reflect important events such as the formation of the Earth or the Great Oxidation Event, among others, while at the same time maintaining most of the previous chronostratigraphic nomenclature for the pertinent time span.[40] (See also Period (geology)#Structure.)
Hadean Eon – 4600–4031 MYA
Chaotian Era – 4600–4404 MYA – the name alluding both to the mythological Chaos and the chaotic phase of planet formation[40][38][41]
Jack Hillsian or Zirconian Era – 4404–4031 MYA – both names allude to the Jack Hills Greenstone Belt which provided the oldest mineral grains on Earth, zircons[40][38]
Archean Eon – 4031–2420 MYA
Paleoarchean Era – 4031–3490 MYA
Acastan Period – 4031–3810 MYA – named after the Acasta Gneiss[40][38]
Isuan Period – 3810–3490 MYA – named after the Isua Greenstone Belt[40]
Mesoarchean Era – 3490–2780 MYA
Vaalbaran Period – 3490–3020 MYA – based on the names of the Kapvaal (Southern Africa) and Pilbara (Western Australia) cratons[40]
Pongolan Period – 3020–2780 MYA – named after the Pongola Supergroup[40]
Neoarchean Era – 2780–2420 MYA
Methanian Period – 2780–2630 MYA – named for the inferred predominance of methanotrophic prokaryotes[40]
Siderian Period – 2630–2420 MYA – named for the voluminous banded iron formations formed within its duration[40]
Proterozoic Eon – 2420–541 MYA
Paleoproterozoic Era – 2420–1780 MYA
Oxygenian Period – 2420–2250 MYA – named for displaying the first evidence for a global oxidizing atmosphere[40]
Jatulian or Eukaryian Period – 2250–2060 MYA – names are respectively for the Lomagundi–Jatuli δ13C isotopic excursion event spanning its duration, and for the (proposed)[42][43] first fossil appearance of eukaryotes[40]
Columbian Period – 2060–1780 MYA – named after the supercontinent Columbia[40]
Mesoproterozoic Era – 1780–850 MYA
Rodinian Period – 1780–850 MYA – named after the supercontinent Rodinia, stable environment[40]
Neoproterozoic Era – 850–541 MYA
Cryogenian Period – 850–630 MYA – named for the occurrence of several glaciations[40]
Ediacaran Period – 630–541 MYA
Shown to scale:

Compare with the current official timeline, not shown to scale:

See also


Age of the Earth
Bubnoff unit
Cosmic calendar
Deep time
Evolutionary history of life
Geological history of Earth
Geology of Mars/areology
Geon
Graphical timeline of the universe
History of the Earth
History of geology
History of paleontology
List of fossil sites
Logarithmic timeline
Lunar geologic timescale
Martian geologic timescale
Natural history
New Zealand geologic time scale
Prehistoric life
Timeline of the Big Bang
Timeline of evolution
Timeline of the geologic history of the United States
Timeline of human evolution
Timeline of natural history
Timeline of paleontology


Notes



Further reading











 Information model for the geologic time scale
 from Planck Time to the lifespan of the universe
,  Gradstein, Felix M. et al. (2004) A new Geologic Time Scale, with special reference to Precambrian and Neogene, Episodes, Vol. 27, no. 2 June 2004 (pdf)
Lane,  Alfred C, and Marble, John Putman 1937.