Geologic ages of earth history

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.

Nomenclature

A common nomenclature for geologica 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 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

On-line 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

References

• 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