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.

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. In 1907, radiochemist Bertram Borden Boltwood (Massachusets) published his schema for geologic ages that incorporated radioactive decay measures. 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.

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 – Not Recognised by the International Commission on Stratigraphy)

Period

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

• Neogene (23.03 mya to present era)
• Paleogene (65.5 to 23.03 mya)

The geological ages were originally demarcated into Primary, Secondary, Tertiary and Quaternary Eras. Giovanni Arduino (circa 1759) proposed that all of geologic time should be broken up into three great orders: the Primary, Secondary, and Tertiary. In 1829, Jules Desnoyers proposed the term "Quaternary" to describe of rocks younger than the youngest Tertiary deposits. The eras have since been renamed and Tertiary and Quaternary were merged into the Cenozoic as “periods” not eras. However, the boundary between the Tertiary (65 to 1.8 mya) and Quaternary (1.8 mya to today) were ambiguously defined and these terms have in turn been replaced. The International Commission on Stratigraphy now endorses the terms Paleogene and Neogene to subdivide the Cenozoic Era.

The current stratigraphic schema for the Cenozoic places the Quaternary at the end of the Neogene spanning a period of time beginning 2.6 million years ago or beginning 1.8 mya till the present time. However, the ICS has asterisked the Quaternary as ‘not yet decided’.

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 has four periods:

• Neoarchean (2800 mya to 2500 mya)
• Mesoarchean (3200 to 2800 mya)
• Paleoarchean (3600 to 3200 mya)
• Eoarchean (no lower limit defined to 3600 mya)

While the International Commission on Stratigraphy does not define a beginning of the Archean, it is commonly regarded as that time when the earth solidified. The oldest known earth rocks are about 3.8 billion years old and that is a figure often used to mark the beginning of the Archean Era.

The Hadean Era

• Hadean (4500 to 3800 mya - not recognised by ICS))

The Hadean Era is not divided into periods. The Hadean era is the period of time the earth was forming literally from a gaseous cloud as the solar system and the sun were forming. The only rocks on earth that date from that period are meteorites which are about 4.5 billion years old (the oldest lunar rocks are also about 4.5 billion years old). Technically, the Hadean it is not a geologic era since there are no rocks on the planet from the time of formation. The oldest known earth rocks are about 3.8 billion years old and would have formed after the earth turned from a molten to a solid state. The Hadean is not recognised by the International Commission on Stratigraphy.

Sources

• International Commission on Stratigraphy Retrieved 17 April, 2007
• Major Divisions of Geologic Time US Geological Survey. Retrieved 15, April, 2007
• Geologic Age USGS. Retrieved 15, April, 2007
• Geological Ages of the Earth OTS Heavy Oil Science Center. Retrieved 15, April, 2007
• Web Geological Time Machine University of California Museum of Paleontology . Retrieved 15, April, 2007
• Division of Earth Sciences National Science Academies Press. Retrieved 17 April, 2007
• Geologic Ages of Earth History Dinosauria On-line. 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