Geologic ages of earth history

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The geological ages refer to periods of marked change in the processes and events in the entire history of the earth. These changes have been delineated by physical evidence found in the earth’s lithosphere. Ages are also noted in reference to the types of organisms found in the fossil record down to the present day. Changes are marked by (but not exclusive of) such processes as volcanic activity, flooding, and seismic activity.

Different schemata for the geologic ages do exist but there is a great deal of consensus as well as generalised omissions. Therefore many sources will note ages that others do not and visa versa.

History

Determining geological ages and configuring current schema is a direct outgrowth of research into natural radioactive decay that began with French physicist Henry Becquerel's discovery of the radioactive decay of uranium in 1896. Employing radioactivity to determine geologic time was first suggested in 1905 by New Zealand physicist Ernest Rutherford. Rutherford had determined that radioactive decay in any given material proceeds at a constant rate. Those elements are present in geologic samples and their radioactivity can be measured in extreme lengths of time.[1] In 1907, radiochemist Bertram Borden Boltwood (Massachusets) published his schema for geologic ages that incorporated radioactive decay measures.[2] Boltwood’s premise for his dating method was that the decay of uranium suggested that geologic age could be determined by ratios of uranium, thorium and lead contained in the rocks. This produced values of extreme lengths of time, some exceeding two billion years. Boltwood’s ages have since been revised but his calculations indicating from hundreds to thousands of millions of years are still considered correct.[3][4]

Nomenclature

A common nomenclature for geological ages are in descending order (largest span of time to narrowest):

  • Eon
  • Era
  • Period
  • Subperiod
  • Epoch
  • Age

These demarcations are delineated by length of time in millons of years

Schemata

Eons

  • Phanerozoic Eon
Beginning approxmately 543 million years ago (mya) to present
  • Precambrian Eon
Beginning approxmately 4,500 mya and ending approximately 543 mya

Era

The Phanerozoic Eon encompasses three eras (in descending order to oldest)

  • Cenozoic Era (65 mya to today)
  • Mesozoic Era (248 to 65 mya)
  • Paleozoic Era (543 to 248 mya)

The Precambrian Eon encompasses three eras (in descending order to oldest)

  • Proterozoic Era (2500 to 543 mya)
  • Archaean (3800 to 2500 mya)
  • Hadean (4500 to 3800 mya)

Period

The Cenozoic Era encompasses two periods (in descending order to oldest)

  • Quaternary (1.8 mya to today)
  • Tertiary (65 to 1.8 mya)

The Cenozoic was originally demarcated into Primary, Secondary, Tertiary and Quaternary Eras. The eras have been merged into Cenozoic and the Quaternary and Tertiary have been retained as “periods” not eras.Cite error: Invalid <ref> tag; invalid names, e.g. too many

The Mesozoic Era has three periods

  • Cretaceous (144 to 65 mya)
  • Jurassic (206 to 144 mya)
  • Triassic (248 to 206 mya)

The Paleozoic Era has six periods

  • Permian (290 to 248 mya)
  • Carboniferous (354 to 290 mya)
  • Devonian (417 to 354 mya)
  • Silurian (443 to 417 mya)
  • Ordovician (490 to 443 mya)
  • Cambrian (543 to 490 mya)

The Proterozoic Era has three periods

  • Neoproterozoic (900 to 543 mya)
  • Mesoproterozoic (1600 to 900 mya)
  • Paleoproterozoic (2500 to 1600 mya)

The Archean Era and the Hadean Era are not divided into periods

Sources

  • US Geological Survey [1] Major Divisions of Geologic Time. Retrieved 15, April, 2007
  • USGS [2] Geologic Age. Retrieved 15, April, 2007
  • OTS Heavy Oil Science Center [3] Geological Ages of the Earth. Retrieved 15, April, 2007
  • University of California Museum of Paleontology [4] Web Geological Time Machine. Retrieved 15, April, 2007
  • Dinosauria On-line [5] Geologic Ages of Earth History. Retrieved 15, April, 2007
  • Raven, P. H., R. F. Evert, and S. E. Eichorn. (1999). Biology of Plants. New York: W. H. Freeman and Co. Worth Publishers.
  • Harland, W. Brian, Richard Armstrong, Allan Cox, Craig Lorraine, Alan Smith and David Smith. (1990). A Geologic Time Scale 1989. New York: Cambridge University Press. Cited in Dinosauria. Retrieved 15, April, 2007
  • Gradstein, F.M., F.P. Agterberg, J.G. Ogg, J. Hardenbol, P. van Veen, J. Thierry and Z. Huang. (1995). A Triassic, Jurassic and Cretaceous time scale, pp. 95-126. IN W.A. Bergrgren, D.V. Kent, M.-P. Aubry & J. Hardenbol (eds.), Geochronology, Time Scales, and Global Stratigraphic Correlation. SEPM Special Publication No. 54. Cited in Dinosauria. Retrieved 15, April, 2007

References

  1. ↑ E. Rutherford’s work was reported in numerous publications: Radioactivity 1904, Cambridge University Press 2nd Edition 1905, (pp580); Radioactive Transformations (1906) Charles Scribner's Sons, NY (pp287) From his Silliman Memorial Lectures at Yale University, March 1905. For a comprehensive list of his related publications refer to Rutherford's Publications Rutherford.org.nz
  2. ↑ Boltwood, Bertram (1907) The Ultimate Disintegration Products of the Radio-active Elements. Part II. The disintegration products of uranium. American Journal of Science series 4, volume 23, pages 77-88
  3. ↑ Geologic Age USGS Learning Web. Retrieved 16 April, 2007
  4. ↑ A Brief History of the Division of Earth Sciences National Academy of Sciences. Retrieved 16 April, 2007
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