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This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga [1]. Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey.

Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by 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–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.

Each era on the scale is separated from the next by a major event or change.

Contents

Graphical timelines

The second and third timelines are each subsections of their preceding timeline as indicated by asterisks.

Millions of Years

The Holocene (the latest epoch) is too short to be shown clearly on this timeline.

Terminology

e  h
Units in geochronology and stratigraphy[2]
Segments of rock (strata) in chronostratigraphy Periods of time in geochronology Notes
Eonothem
Eon
4 total, half a billion years or more
Erathem
Era
12 total, several hundred million years
System
Period
Series
Epoch
tens of millions of years
Stage
Age
millions of years
Chronozone
Chron
smaller than an age/stage, not used by the ICS timescale

The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time.

Geologists qualify these units as Upper, Middle, and Lower. Examples are "Upper Jurassic" and "Middle Cambrian". The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." The names of subdivisions can vary when discussing either position in the geologic record or corresponding time period, by use of either upper/lower (for series) or late/early (for epochs). For example, the Lower Jurassic Series in geochronology corresponds to the Early Jurassic Epoch in chronostratigraphy.[3]

Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period 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 Tommotian, 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.[4]

History of the time scale and names

Animation showing Earth's palaeogeographic reconstruction beginning from early Cambrian period.
Diagram of geological time scale, where the past is toward the bottom of the spiral

In classical antiquity, Aristotle saw that fossil seashells from rocks were similar to those found on the beach and deduced that the fossils were once part of living animals. He reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.[5]

The 11th-century Persian geologist Avicenna (Ibn Sina) examined various fossils and deduced that they originated from the petrifaction of plants and animals.[6] He 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 in 1027, he outlined the principle as follows:[7][8]

It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on. Over each layer there spread a substance of different material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimentary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains.

His contemporary, Abu Rayhan Biruni (973-1048), discovered the existence of shells and fossils in regions that once were seas and later became dry land, such as the Indian subcontinent. Based on this evidence, he realized that the Earth is constantly evolving and proposed that the Earth had an age, but that its origin was too distant to measure.[9] Later in the 11th century, the Chinese naturalist, Shen Kuo (1031–1095), also recognized the concept of 'deep time'.[10]

The principles underlying geologic (geological) time scales were later laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are 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 to real rocks proved complex. Over the course of the 18th century geologists realized that:

  1. Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition;
  2. Strata laid down at the same time in different areas could have entirely different appearances;
  3. The strata of any given area represented only part of the Earth's long history.
A comparative geological timescale

The first serious attempts to formulate a geological 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 Abraham Werner, among others) divided the rocks of the 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 Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 20th century.

The Neptunist theories popular at this time (expounded by Werner) 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 before the Royal Society of Edinburgh in March and April 1785. It has been said that "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology"[11] Hutton proposed that the interior of the 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 was dubbed "Plutonist" in contrast to the flood-oriented theory.

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brogniart 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 geological periods still used today.

The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for Wales) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales.[12] The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply 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 Brogniart 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).

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs.

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 various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the 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 (pioneered by such geologists as Arthur Holmes) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate.

The first geologic time scale was eventually published in 1913 by the British geologist Arthur Holmes.[14] He greatly furthered the newly created discipline of geochronology and published the world renowned book The Age of the Earth in 1913 in which he estimated the Earth's age to be at least 1.6 billion years.[15]

In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) started an effort to define global references (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al.[16]. A UML model for how the timescale is structured, relating it to the GSSP, is also available[17].

Table of geologic time

The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. (See lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon.) 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,[18] with the epoch names altered to the early/late format from lower/upper as recommended by the ICS when dealing with chronostratigraphy.[3]

See also

References and footnotes

  1. ^ International Commission on Stratigraphy 2008:http://www.stratigraphy.org/column.php?id=Chart/Time%20Scale, retrieved 9 March 2009.
  2. ^ International Commission on Stratigraphy. "International Stratigraphic Chart". http://www.stratigraphy.org/upload/ISChart2009.pdf. Retrieved 2009-09-25. 
  3. ^ a b International Commission on Stratigraphy. "Chronostratigraphic Units." International Stratigraphic Guide. Accessed 14-DEC-2009. [1]
  4. ^ Statutes of the International Commission on Stratigraphy, retrieved 26 November 2009
  5. ^ Correlating Earth's History, Paul R. Janke
  6. ^ Rudwick, M. J. S. (1985). The Meaning of Fossils: Episodes in the History of Palaeontology. University of Chicago Press. p. 24. ISBN 0226731030. 
  7. ^ Munim M. Al-Rawi and Salim Al-Hassani (November 2002). "The Contribution of Ibn Sina (Avicenna) to the development of Earth sciences". FSTC. http://www.muslimheritage.com/uploads/ibnsina.pdf. Retrieved 2008-07-01. 
  8. ^ Stephen Toulmin and June Goodfield (1965), The Ancestry of Science: The Discovery of Time, p. 64, University of Chicago Press (cf. The Contribution of Ibn Sina to the development of Earth sciences)
  9. ^ Scheppler, Bill (2006), Al-Biruni: Master Astronomer and Muslim Scholar of the Eleventh Century, The Rosen Publishing Group, p. 86, ISBN 1404205128 
  10. ^ Sivin, Nathan (1995). Science in Ancient China: Researches and Reflections. Brookfield, Vermont: Ashgate Publishing Variorum series. III, 23–24. 
  11. ^ John McPhee, Basin and Range, New York:Farrar, Straus and Giroux, 1981, pp.95-100.
  12. ^ John McPhee, Basin and Range, pp.113-114.
  13. ^ (in Russian) Great Soviet Encyclopedia (3rd ed.). Moscow: Sovetskaya Enciklopediya. 1974. vol. 16, p. 50. 
  14. ^ Geologic Time Scale
  15. ^ How the discovery of geologic time changed our view of the world, Bristol University
  16. ^ Felix M. Gradstein, James G. Ogg, Alan G. Smith (Editors); A Geologic Time Scale 2004, Cambridge University Press, 2005, (ISBN 0-521-78673-8)
  17. ^ Cox & Richard, A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards, Geosphere, volume 1, pp 119-137, Geological Society of America, 2005
  18. ^ International Commission on Stratigraphy. "International Stratigraphic Chart". http://www.stratigraphy.org/upload/ISChart2009.pdf. Retrieved 2009-09-25. 
  19. ^ Paleontologists often refer to faunal stages rather than geologic (geological) periods. The stage nomenclature is quite complex. See "The Paleobiology Database". http://flatpebble.nceas.ucsb.edu/cgi-bin/bridge.pl?action=startScale. Retrieved 2006-03-19.  for an excellent time ordered list of faunal stages.
  20. ^ a b Dates are slightly uncertain with differences of a few percent between various sources being common. This is largely due to uncertainties in radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where they would be most useful. The dates and errors quoted above are according to the International Commission on Stratigraphy 2004 time scale. Dates labeled with a * indicate boundaries where a Global Boundary Stratotype Section and Point has been internationally agreed upon: see List of Global Boundary Stratotype Sections and Points for a complete list.
  21. ^ a b Historically, the Cenozoic has been divided up into the Quaternary and Tertiary sub-eras, as well as the Neogene and Paleogene periods. The 2009 version of the ICS time chart recognizes a slightly extended Quaternary as well as the Paleogene and a truncated Neogene, the Tertiary having been demoted to informal status.
  22. ^ a b c d e f For more information on this, see the following articles: Earth's atmosphere, carbon dioxide, Carbon dioxide in the Earth's atmosphere, Image:Phanerozoic_Carbon_Dioxide.png, Image:65 Myr Climate Change.png, Image:Five Myr Climate Change.png, and Template:DF temperature
  23. ^ The start time for the Holocene epoch is here given as 11,430 years ago ± 130 years (that is, between 9610 BC-9560 BC and 9350 BC-9300 BC). For further discussion of the dating of this epoch, see Holocene.
  24. ^ a b In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods.
  25. ^ The Precambrian is also known as Cryptozoic.
  26. ^ a b c d e f g h i j k l m n The Proterozoic, Archean and Hadean are often collectively referred to as the Precambrian Time or sometimes, also the Cryptozoic.
  27. ^ a b c d e f g h i j k l Defined by absolute age (Global Standard Stratigraphic Age).
  28. ^ The age of the oldest measurable craton, or continental crust, is dated to 3600-3800 Ma
  29. ^ Though commonly used, the Hadean is not a formal eon and no lower bound for the Archean and Eoarchean have been agreed upon. The Hadean has also sometimes been called the Priscoan or the Azoic. Sometimes, the Hadean can be found to be subdivided according to the lunar geologic time scale. These eras include the Cryptic and Basin Groups (which are subdivisions of the pre-Nectarian era), Nectarian, and Early Imbrian units.
  30. ^ a b c d These unit names were taken from the Lunar geologic timescale and refer to geologic events that did not occur on Earth. Their use for Earth geology is unofficial.
  31. ^ Bowring, Samuel A. (1999). "Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada". Contributions to Mineralogy and Petrology 134: 3. doi:10.1007/s004100050465.  The oldest rock on earth is the Acasta Gneiss, and it dates to 4.03 Ga, located in the Northwest Territories of Canada.
  32. ^ http://www.geology.wisc.edu/%7Evalley/zircons/Wilde2001Nature.pdf

External links

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