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.

The topic of geological ages encompasses three basic fields of research

• Stratigraphy: Encompasses the study of the relationships of strata in time and space.
• Biostratigraphy: Addresses fossilised organisms for ordering and correlating strata in relative time.
• Geochronology: The study of absolute geological time itself, particularly by the use of radiometric dating.

Different schemata (geological time scales) 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

The underlying premises or basic ideas about geological ages and time scales in modern earth science are from people who worked during the period spanning the late 17th to the19th centuries. In 1669, Nicolaus Steno described the principle of original horizontality composed of four basic geologic principles that today are the fundamental concepts of stratigraphy:

• Superposition of strata: Sedimentary materials are deposited from a fluid (particles in solution) onto a solid surface, ergo the strata above did not exist when lower stratum was formed;
• Initial horizontality of strata: Sedimentary materials are laid down laterally in a continuous horizontal manner (stratum),
• Stratal continuity: Superposition (stacking) occurs according to age—younger rocks are deposited on top of older rock;
• Cross cutting relationships: Deviation from horizontal layers (stratum) is altered after the material is deposited—earthquakes, volcanoes, etc.

So, like layers of paint, the different layers are laid down with the oldest on the very bottom and the youngest on top, then the next and so on. The key to this is understanding that at the time, the processes forming the earth were a complete mystery and explanations could be very convoluted and completely wrong. Steno’s work was a major breakthrough and he has been called the father of modern geology.

In 1795, James Hutton proposed the principle of uniformitarianism, later developed by Charles Lyell (Principles of Geology, published 1830). This principle defines natural geologic processes as uniform in frequency and magnitude throughout time. This is a critical concept in other fields as well, the principle that natural phenomena in process in the far distant past are proceeding in the present time. The magnitude of the processes were considered to be unchanged and all change was the result of the ‘sum of small effects’. Conceptually this came to mean that the earth is very old. How old was originally posited to be in the tens of millions of years but later work by Ernest Rutherford changed this dramatically.

The next phase of development in this field was from a canal builder in 19th century England, William Smith, In 1815 Smith produced a geologic map of England demonstrating the principle of biotic succession: The different types of animals that had lived and died in that region over immense periods of time had changed and left their fossils attesting to their existence. Simply put the principle of biotic succession says that fossils are found in rocks in a very definite order. Smith’s work has been incorporated to use fossils to define increments or stages within a relative time scale. Many of the ages are in fact named for the presence of life at those times.

Determining geological ages and configuring current schema today 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. (Lead is the product of the radioactive decay of uranium and thorium.) 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. Phanerozoic is constructed of two Greek roots. Phaneros (Greek: φανερος) means “visible”, and zooikos (Greek: ζωικός) means “of or pertaining to animal”, denoting the period when abundant life forms became evident. Stratigraphically it demarcates geologic time represented by rocks containing abundant fossil evidence.

• Precambrian

The name is derived from Cambria, the Roman word for Wales, the area where the rocks of the Cambrian Eon was first studied. Pre- indicates the ages before the rocks dated in Wales.Not actually termed an ‘Eon' by the ISC. Beginning approxmately 4,500 mya and ending approximately 543 mya, the Precambrian encompasses about 88% of the earth's entire existance. The Precambrian encompasses three eons (in descending order to oldest):

• Proterozoic Eon (2500 to 543 mya)
• Archaean Eon (3800 to 2500 mya)
• Hadean Eon (4500 to 3800 mya – Not Recognised by the International Commission on Stratigraphy)

Era

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

• Cenozoic Era (65 mya to today)

From the Greek word kainos (καινος), meaning fresh or new and zoikos (ζωικός), of or pertaining to animals, combined here to denote new life. This marked the end of a major extinction of dinosaurs, foraminifers, calcareous nannofossils etc. and the emergence of an abundance of mammalian life.

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

The Proterozoic Eon has three eras:

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

The Archean Eon has four eras:

• 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 Eon

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

The 'Hadean Eon is divided into varying era schemes but none are recognised by the ISC. The Hadean 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 is not a geologic eon 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.

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 with a provisional demarcation of the Quaternary at the end of the era

Period

Cenozoic Era

In descending order to oldest:

• Quaternary (end of Neogene to present day - see note below)
• Neogene (23.03 mya to Quaternary Period)
• Paleogene (65.5 to 23.03 mya)

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’.

Mesozoic Era

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

Paleozoic Era

• 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)

Epoch

Quaternary Period

• Holocene Epoch (11,800 years ago to present time)
• Pleistocene Epoch (1.8 mya to 11,800 yrs)

Neogene Period

• Pliocene Epoch (5.3 to 1.8 mya)
• Miocene Epoch (23.03 to 5.3 mya)

Paleogene

• Oligocene Epoch (33.9 to 23.03 mya)
• Eocene Epoch (55.8 to 33.9 mya)
• Paleocene Epoch (65 to 55.8 mya)

Cretaceous

• Upper Epoch (99.6 to 65.5 mya)
• Lower Epoch (145.5 to 99.6 mya)

Jurassic

• Upper Epoch (161.2 to 145.5 mya)
• Middle (175.6 to 161.2 mya)
• Lower 199.6 to 175.6 mya)

Triassic

• Upper Epoch (228.0 199.6 mya)
• Middle Epoch (245.0 to 228.0 mya)
• Lower Epoch (251.0 to 245.0 mya

Permian

• Lopingian Epoch (260.4 to 251.0 mya)
• Guadalupian Epoch (270.8 to 260.4 mya)
• Cisuralian Epoch (299.0 to 270.8 mya)

Carboniferous

• Pennsylvanian (318.1 to 299.0 mya)
• Mississippian (359.2 to 318.1 mya)

Devonian

• Upper Epoch (385.3 to 359.2 mya)
• Middle Epoch (397.5 to 385.3 mya)
• Lower Epoch (416.0 to 397.5 mya)

Silurian

• Pridoli Epoch (418.7 to 416.0 mya)
• Ludlow Epoch (422.9 to 418.7 mya)
• Wenlock Epoch (428.2 to 422.9 mya)
• Llandovery Epoch (443.7 to 428.2 mya)

Ordovician

• Upper Epoch (460.9 to 443.7 mya)
• Middle Epoch (471.8 to 460.9 mya)
• Lower Epoch (488.3 to 471.8 mya)

Cambrian

• Furongian Epoch (501.0 to 488.3 mya)
• Series 3 Epoch (510.0 to 501.0 mya)
• Series 2 Epoch (521.0 to 510.0 mya)
• Series 1 Epoch (542.0 to 521.0 mya)

The ISC does not divide any part of the Precambrian into Epochs

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