Geologic ages of earth history

The geologic history of the earth is preserved and documented in the rocks. The oldest earth's rock known is more than 3 billions of years old; since this remote time, the rocks provide a record of the events on our planet, which becomes increasingly complete approachin present time. Geologists describe and study the geologic time in two distinct ways.

On the one hand, they try to measure the geologic time as a physical quantity: this is the aim of a field of Earth sciences called geochronometry. The geologic time is most commonly measured with methods based on the radioactive decay of long-lived unstable isotopes present in the earth's crust, and is expressed in Ma (i.e., millions of years ago).

On other hand, the geologic time can be described by establishing a hyerarchical scheme of relative time subdivisions. This was the only representation of geologic time before radioactive decay was discovered, but it is also widely used in the current geologic practice and research. The boundaries of major relative time subdivisions, and expecially those of the erathems, are marked by mass extinctions, in turn associated with physical events as extreme volcanic activity, bolide impacts or climate changes recognized globally. However, the definition of time-rock units must follow a formal procedure, implying that some boundaries are placed at completely arbitrary positions. Indeed, this is the case for most of the minor divisions (ages) of the geologic time.

The identification, description and quantification of geologic time involves, in addition to geochronometry, the fields of stratigraphy (the sub-discipline of geology studying the relationships of strata in time and space), biostratigraphy (the use of fossils for ordering and correlating strata in relative time), and geochronology (the study of geological time itself).

When relative time subdivisions are coupled with absolute (measured) ages, they constitute a Geologic time scale, which is the main topic of this article.

Different geological time scales exist and the issues involved in arriving at a meaningful date can be very complex. Many regionalised time scales carry variations and omissions, but a reasonable consensus was reached on the 2004 geologic time scale published by International Commission on Stratigraphy (ICS). Subdivisions and absolute ages of this article are based mostly on this publication.

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.

Definitions

Determining the age of the earth from physical evidence involves a number of fields that are strongly interrelated. Often the terms are intermingled while the goals and methods may be similar or different.

• Stratigraphy: The study of the order in which units of rock are deposited.
• Stratum (pl. strata): A unit of sedimentary rock, bounded on the upper and lower limits by different layers of rock (bedding planes and that are reasonablely persistence on a lateral plane. In other words, a definable, physically obvious, layer of rock.
• Formation: A unit of rocks which is consistently recognizable over wide areas. Formations are employed to make geological maps.
• Period: The time-span over which a geological system was deposited. Time-units correspond to a recognised and defined body of rock defined by methods used in stratigraphic palaeontology.
• System: the rocks laid down during a Period (Carboniferous, Jurassic, etc.) For example, the Cretaceous System was created during the Cretaceous Period.

For largely pragmatic reasons, earth history needs to be subdivided into smaller portions. However, any period has different aspects:

• Lithostratigraphic Units: Rocks are deposited in successive layers which have chemically and mineralogically distinct characteristics. The subdivision of the rock successions leads to the distinction of Groups, Formations, and Members.
• Biostratigraphic Units: Distinctive characteristics of fossils of animals and plants which exist for limited periods of time in recognisable biozones permit the delineation rocks of certain ages in which they are deposited.
• Chronostratigprahic Units: Geochonographic measures of absolute time leads to chronostratigraphy and enables the subdivision of systems into Series and Stages.

These subdivisions are done locally as well as globally. All hierarchically ordered units better serve research if they are fixed by sound boundaries that can be easily correlated worldwide.

Nomenclature

The ICS nomenclature for stratigraphic units is both Chronostratigraphic and Geochronological. They are positioned here in descending order (largest span of time to narrowest):

Chronostratigraphic units	Geochronologic units
Eonothem Erathem System Series Stage Substage	Eon Era Period Epoch Age Subage

There are two meanings for "age": one is the fundamental chronostratigraphic unit (Age (geology)), the other is the result an absolute datation, e.g., a radiometric datation, and is a number (e.g., the Induan age begun ca. 252 Ma, where "252 Ma" is a radiometric age). So "Induan" is an age, but to construe the "Triassic" as an age is an error: "Triassic" is a period.

These demarcations are delineated below in length of time in millons of years.

Schemata

Global definitions of chronostratigraphic boundaries are the purview of the International Union of Geological Sciences (IUGS). While there are regional stratigraphic schemata that will vary considerably, there are international standards with which these various schema do agree and are referred to. The recommendations for Global Standard Stratotype-section and Points (GSSP) which demarcate the boundaries of the strata and ultimately define the age of the earth are processed through a sequence of stages. Stratigraphic boundaries are proposed to the sub-commission on a particular geologic stage who then submit their recommendations to the International Commission on Stratigraphy (ICS) for official acceptance. The ICS then refer their recommendations to the Executive Committee of IUGS for ratification. In this way, geologists the world over refer to a standard reference for the worldwide correlation of the boundary.

This distinguishes between lithostratigraphic definitions which can vary a great deal from region to region and chronostratigraphic definitions which are meant to set internationally defined standards. Whereas the lithostratigraphic studies denote physical characteristics of the strata (mineral and biotic sequences for example), the chronostratigraphic studies use various methods combining analysis of fossil records and radiometric studies to provide relative and absolute dating.

Eons

Phanerozoic

Beginning approximately 543 Ma (million years ago), the Phanerozoic eonothem is represented by rocks containing abundant fossil evidence. The word has Greek roots, deriving from Phaneros (Greek: φανερος) means “visible”, and zooikos (Greek: ζωικός) means “of or pertaining to animal”. The Phanerozoic spans, indeed, the time when abundant life forms became evident.

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)

Erathem/Era	System/Period	Series/Epoch	Stage/Age
Cenozoic	Neogene	Holocene
		Pleistocene	Upper/Late
			Middle/Mid
			Lower/Early
		Pliocene	Gelasian
			Piacenzian
			Zanclean
		Miocene	Messinian
			Tortonian
			Serravallian
			Langhian
			Burdigalian
			Aquitanian
	Paleogene	Oligocene	Chattian
			Rupelian
		Eocene	Priabonian
			Bartonian
			Lutetian
			Ypresian
		Paleocene	Thanetian
			Selandian
			Danian
Mesozoic	Cretaceous	Upper/Late	Maastrichtian
			Campanian
			Santonian
			Coniacian
			Turonian
			Cenomanian
		Lower/Early	Albian
			Aptian
			Barremian
			Hauterivian
			Valanginian
			Berriasian
	Jurassic	Upper/Late	Tithonian
			Kimmeridgian
			Oxfordian
		Middle/Mid	Callovian
			Bathonian
			Bajocian
			Aalenian
		Lower/Early	Toarcian
			Pliensbachian
			Sinemurian
			Hettangian
	Triassic	Upper/Late	Rhaetian
			Norian
			Carnian
		Middle/Mid	Ladinian
			Anisian
		Lower/Early	Olenekian
			Induan
Paleozoic	Permian	Lopingian	Changhsingian
			Wuchiapingian
		Guadalupian	Capitanian
			Wordian
			Roadian
		Cisuralian	Kungurian
			Artinskian
			Sakmarian
			Asselian
	Carboniferous	Pennsylvanian	Gzhelian
			Kasimovian
			Moscovian
			Bashkirian
		Mississippian	Serpukhovian
			Visean
			Tournaisian
	Devonian	Upper/Late	Famennian
			Frasnian
		Middle/Mid	Givetian
			Eifelian
		Lower/Early	Emsian
			Pragian
			Lochkovian
	Silurian	Pridoli	Pridoli
		Ludlow	Ludfordian
			Gorstian
		Wenlock	Homerian
			Sheinwoodian
		Llandovery	Telychian
			Aeronian
			Rhuddanian
	Ordovician	Upper/Late	Himantian
			Katian
			Sandbian
		Middle/Mid	Darriwilian
			Unnamed
		Lower/Early	Floian
			Tremadocian
	Cambrian	Furongian	Unnamed
			Unnamed
			Paibian
		Unnamed	Unnamed
			Drumian
			Unnamed
		Unnamed	Unnamed
			Unnamed
		Unnamed	Unnamed
			Unnamed

Precambrian

The name is derived from Cambria, the Roman word for Wales, the area where the rocks of the Cambrian Period was first studied. Pre- indicates the ages before the rocks dated in Wales.Not actually termed an ‘Eon' by the ICS. 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), subdivided into eras:

• Proterozoic Eon (2500 to 543 mya)
  • Neoproterozoic (900 to 543 mya)
  • Mesoproterozoic (1600 to 900 mya)
  • Paleoproterozoic (2500 to 1600 mya)
• Archaean Eon (3800 to 2500 mya)
  • Neoarchean (2800 mya to 2500 mya)
  • Mesoarchean (3200 to 2800 mya)
  • Paleoarchean (3600 to 3200 mya)
  • Eoarchean (no lower limit defined to 3600 mya)
• Hadean Eon (4500 to 3800 mya – Not Recognised by the International Commission on Stratigraphy)

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 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.

Eonothem/Eon	Erathem/Era	System/Period
Proterozoic	Neoproterozoic	Ediacaran
		Cryogenian
		Tonian
	Mesoproterozoic	Stenian
		Ectasian
		Calymmian
	Paleoproterozoic	Statherian
		Orosirian
		Rhyacian
		Siderian
Archean	Neoarchean
	Mesoarchean
	Paleoarchean
	Eoarchean

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 (48 to 443 mya)
• Cambrian (542 to 488 mya)

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
• Subcommission of ICS International Commission on Stratigraphy (ICS), International Union of Geological Sciences (IUGS).