Geologic time scale

The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" and "Quaternary" remained in use as names of geological periods well into the 20th century.

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies of the strata and fossils of Europe produced, between 1820 and 1850, the sequence of geological periods still used today.

British geologists dominated the process, and the names of the periods reflect that dominance. The "Cambrian," "Ordovician," and "Silurian" periods were named after ancient British tribes (and defined using stratigraphic sequences from Wales). The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian," though defined using strata in Russia, was delineated and named by a British geologist: Roderick Murchison.

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs.

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on their Christian heritage, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate.

In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al. (ISBN 0521786738), and is used as the basis of this page.

Table of geologic time

Eon	Era	PeriodPaleontologists often refer to faunal stages rather than geologic (geological) periods. The stage nomenclature is quite complex. See for an excellent time ordered list of faunal stages.		Series/ Epoch	Major Events	Start, Million Years AgoDates are slightly uncertain with differences of a few percent between various sources being common. This is largely due to uncertainties in radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where they would be most useful. The dates and errors quoted above are according to the International Commission on Stratigraphy 2004 time scale. Dates labeled with a * indicate boundaries where a Global Boundary Stratotype Section and Point has been internationally agreed upon: see List of Global Boundary Stratotype Sections and Points for a complete list.
Phane- rozoic	Cenozoic	NeogeneHistorically, the Cenozoic has been divided up into the Quaternary and Tertiary sub-eras, as well as the Neogene and Paleogene periods. However, the International Commission on Stratigraphy has recently decided to stop endorsing the terms Quaternary and Tertiary as part of the formal nomenclature.		Holocene	End of recent glaciation and rise of modern civilization.	0.011430 ± 0.00013The start time for the Holocene epoch is here given as 11,430 years ago ± 130 years (that is, between 9610 B.C. and 9350 B.C.). For further discussion of the dating of this epoch, see Holocene.
				Pleistocene	Flourishing and then extinction of many large mammals (Pleistocene megafauna). Evolution of anatomically modern humans.	1.806 ± 0.005 *
				Pliocene	Intensification of present ice age; cool and dry climate. Australopithecines, many of the existing genera of mammals, and recent mollusks appear.	5.332 ± 0.005 *
				Miocene	Moderate climate; Orogeny in northern hemisphere. Modern mammal and bird families became recognizable. Horses and mastodons diverse. Grasses become ubiquitous. First apes appear.	23.03 ± 0.05 *
		Paleogene		Oligocene	Warm climate; Rapid evolution and diversification of fauna, especially mammals. Major evolution and dispersal of modern types of flowering plants	33.9±0.1 *
				Eocene	Archaic mammals (e.g. Creodonts, Condylarths, Uintatheres, etc) flourish and continue to develop during the epoch. Appearance of several "modern" mammal families. Primitive whales diversify. First grasses. Reglaciation of Antarctica; current ice age begins.	55.8±0.2 *
				Paleocene	Climate tropical. Modern plants appear; Mammals diversify into a number of primitive lineages following the extinction of the dinosaurs. First large mammals (up to bear or small hippo size).	65.5±0.3 *
	Mesozoic	Cretaceous		Upper/Late	Flowering plants proliferate, along with new types of insects. More modern teleost fish begin to appear. Ammonites, belemnites, rudist bivalves, echinoids and sponges all common. Many new types of dinosaurs (e.g. Tyrannosaurs, Titanosaurs, duck bills, and horned dinosaurs) evolve on land, as do modern crocodilians; and mosasaurs and modern sharks appear in the sea. Primitive birds gradually replace pterosaurs. Monotremes, marsupials and placental mammals appear. Break up of Gondwana.	99.6±0.9 *
				Lower/Early		145.5 ± 4.0
		Jurassic		Upper/Late	Gymnosperms (especially conifers, Bennettitales and cycads) and ferns common. Many types of dinosaurs, such as sauropods, carnosaurs, and stegosaurs. Mammals common but small. First birds and lizards. Ichthyosaurs and plesiosaurs diverse. Bivalves, Ammonites and belemnites abundant. Sea urchins very common, along with crinoids, starfish, sponges, and terebratulid and rhynchonellid brachiopods. Breakup of Pangea into Gondwana and Laurasia.	161.2 ± 4.0
				Middle		175.6 ± 2.0 *
				Lower/Early		199.6 ± 0.6
		Triassic		Upper/Late	Archosaurs dominant on land as dinosaurs, in the oceans as Ichthyosaurs and nothosaurs, and in the air as pterosaurs. cynodonts become smaller and more mammal-like, while first mammals and crocodilia appear. Dicrodium flora common on land. Many large aquatic temnospondyl amphibians. Ceratitic ammonoids extremely common. Modern corals and teleost fish appear, as do many modern insect clades.	228.0 ± 2.0
				Middle		245.0 ± 1.5
				Lower/Early		251.0 ± 0.4 *
	Paleozoic	Permian		Lopingian	Landmasses unite into supercontinent Pangea, creating the Appalachians. End of Permo-Carboniferous glaciation. Synapsid reptiles (pelycosaurs and therapsids) become plentiful, while parareptiles and temnospondyl amphibians remain common. In the mid-Permian, coal-age flora are replaced by cone-bearing gymnosperms (the first true seed plants) and by the first true mosses. Beetles and flies evolve. Marine life flourishes in warm shallow reefs; productid and spiriferid brachiopods, bivalves, forams, and ammonoids all abundant. Permian-Triassic extinction event occurs 251 mya: 95 percent of life on Earth becomes extinct, including all trilobites, graptolites, and blastoids.	260.4 ± 0.7 *
				Guadalupian		270.6 ± 0.7 *
				Cisuralian		299.0 ± 0.8 *
		Carbon- iferousIn North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods./ Pennsyl- vanian		Upper/Late	Winged insects radiate suddenly; some (esp. Protodonata and Palaeodictyoptera) are quite large. Amphibians common and diverse. First reptiles and coal forests (scale trees, ferns, club trees, giant horsetails, Cordaites, etc.). Highest-ever oxygen levels. Goniatites, brachiopods, bryozoa, bivalves, and corals plentiful in the seas. Testate forams proliferate.	306.5 ± 1.0
				Middle		311.7 ± 1.1
				Lower/Early		318.1 ± 1.3 *
		Carbon- iferous/ Missis- sippian		Upper/Late	Large primitive trees, first land vertebrates, and amphibious sea-scorpions live amid coal-forming coastal swamps. Lobe-finned rhizodonts are big fresh-water predators. In the oceans, early sharks are common and quite diverse; echinoderms (esp. crinoids and blastoids) abundant. Corals, bryozoa, goniatites and brachiopods (Productida, Spiriferida, etc.) very common. But trilobites and nautiloids decline. Glaciation in East Gondwana.	326.4 ± 1.6
				Middle		345.3 ± 2.1
				Lower/Early		359.2 ± 2.5 *
		Devonian		Upper/Late	First clubmosses, horsetails and ferns appear, as do the first seed-bearing plants (progymnosperms), first trees (the tree-fern Archaeopteris), and first (wingless) insects. Strophomenid and atrypid brachiopods, rugose and tabulate corals, and crinoids are all abundant in the oceans. Goniatite ammonoids are plentiful, while squid-like coleoids arise. Trilobites and armoured agnaths decline, while jawed fishes (placoderms, lobe-finned and ray-finned fish, and early sharks) rule the seas. First amphibians still aquatic. "Old Red Continent" of Euramerica.	385.3 ± 2.6 *
				Middle		397.5 ± 2.7 *
				Lower/Early		416.0 ± 2.8 *
		Silurian		Pridoli	First vascular plants (the whisk ferns and their relatives), first millipedes and arthropleurids on land. First jawed fishes, as well as many armoured jawless fish, populate the seas. Sea-scorpions reach large size. Tabulate and rugose corals, brachiopods (Pentamerida, Rhynchonellida, etc.), and crinoids all abundant. Trilobites and mollusks diverse; graptolites not as varied.	418.7 ± 2.7 *
				Upper/Late (Ludlow)		422.9 ± 2.5 *
				Wenlock		428.2 ± 2.3 *
				Lower/Early (Llandovery)		443.7 ± 1.5 *
		Ordovician		Upper/Late	Invertebrates diversify into many new types (e.g., long straight-shelled cephalopods). Early corals, articulate brachiopods (Orthida, Strophomenida, etc.), bivalves, nautiloids, trilobites, ostracods, bryozoa, many types of echinoderms (crinoids, cystoids, starfish, etc.), branched graptolites, and other taxa all common. Conodonts (early planktonic vertebrates) appear. First green plants and fungi on land. Ice age at end of period.	460.9 ± 1.6 *
				Middle		471.8 ± 1.6
				Lower/Early		488.3 ± 1.7 *
		Cambrian		Upper/Late (Furongian)	Major diversification of life in the Cambrian Explosion. Many fossils; most modern animal phyla appear. First chordates appear, along with a number of extinct, problematic phyla. Reef-building Archaeocyatha abundant; then vanish. Trilobites, priapulid worms, sponges, inarticulate brachiopods (unhinged lampshells), and many other animals numerous. Anomalocarids are giant predators, while many Edicarian fauna die out. Prokaryotes, protists (e.g., forams), fungi and algae continue to present day. Gondwana emerges.	501.0 ± 2.0 *
				Middle		513.0 ± 2.0
				Lower/Early		542.0 ± 0.3 *
Proter- ozoic The Proterozoic, Archean and Hadean are often collectively referred to as Precambrian Time, and sometimes also as the Cryptozoic.	Neo- proterozoic	Ediacaran		Good fossils of multi-celled animals. Ediacaran fauna (or Vendobionta) flourish worldwide in seas. Trace fossils of worm-like Trichophycus, etc. First sponges and trilobitomorphs. Enigmatic forms include oval-shaped Dickinsonia, frond-shaped Charniodiscus, and many soft-jellied creatures.		630 +5/-30 *
		Cryogenian		Possible "snowball Earth" period. Fossils still rare. Rodinia landmass begins to break up.		850 Defined by absolute age (Global Standard Stratigraphic Age).
		Tonian		Rodinia supercontinent persists. Trace fossils of simple multi-celled eukaryotes. First radiation of dinoflagellate-like acritarchs.		1000
	Meso- proterozoic	Stenian		Narrow highly metamorphic belts due to orogeny as supercontinent Rodinia is formed.		1200
		Ectasian		Platform covers continue to expand. Green algae colonies in the seas.		1400
		Calymmian		Platform covers expand.		1600
	Paleo- proterozoic	Statherian		First complex single-celled life: protists with nuclei. Columbia is the primoidal supercontinent.		1800
		Orosirian		The atmosphere became oxygenic. Vredefort and Sudbury Basin asteroid impacts. Much orogeny.		2050
		Rhyacian		Bushveld Formation occurs. Huronian glaciation.		2300
		Siderian		Oxygen Catastrophe: banded iron formations result.		2500
Archean	Neoarchean	Stabilization of most modern cratons; possible mantle overturn event.				2800
	Mesoarchean	First stromatolites, probably colonial cyanobacteria). Oldest macrofossils.				3200
	Paleoarchean	First known oxygen-producing bacteria. Oldest definitive microfossils.				3600
	Eoarchean	Simple single-celled life (probably bacteria and perhaps archaea). Oldest probable microfossils.				3800
Hadean Though commonly used, the Hadean is not a formal eon and no lower bound for the Eoarchean has been agreed upon. The Hadean has also sometimes been called the Priscoan or the Azoic.	Lower ImbrianThese era names were taken from the Lunar geologic timescale. Their use for Earth geology is unofficial.					c.3850
	Nectarian					c.3920
	Basin Groups	Oldest known rock (4100 mya).				c.4150
	Cryptic	Formation of earth (4570 mya). Oldest known mineral (4400 mya).				c.4570

References and footnotes