Based on radiometric dating techniques, the Earth is estimated to be about 4,570 million years (4570 "Ma") old. The geological time scale is a means of mapping the history of the earth. It combines estimates of the age of geological formations as provided by radiometric dating techniques with the direct evidence of sequences and events in the rock record as assembled by geologists. In this way the geologic or deep time of Earth's past can be organized into various units according to events that took place in each period. Different spans of time on the time scale are usually delimited by major geologic or paleontologic events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species.
The earth history mapped on the geologic time scale contrasts with that mapped by young-earth creationists, which see the earth as only thousands of years old.
In the geological time scale, the largest defined unit of time is the eon, which is further divided successively into eras, periods, epochs, and stages. Overlaid on this general pattern developed by geologists is a complementary mapping by paleontologists who have defined a system of faunal stages of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geologic nomenclature, though in general there are far more recognized faunal stages than defined geologic time units.
Geologists tend to talk in terms of Upper/Late, Lower/Early, and Middle parts of periods and other units—for example, "Upper Jurassic", "Middle Cambrian". Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Early Cambrian is referred to as the Waucoban series, which is then subdivided into zones based on trilobites. The same time span is split into Tommotian, Atdabanian, and Botomian stages in East Asia and Siberia. It is a key aspect of the work of the International Commission on stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world.
Nicholas Steno laid down the principles underlying geologic time scales in the late seventeenth century. Steno argued that rock layers (strata) are laid down in succession, and that each represents a “slice” of time. He also formulated the principle of superposition, which states that any given stratum is probably older than those above it and younger than those below it.
Steno's principles were simple, but applying them to real rocks proved complex. During the eighteenth century, geologists came to realize that: 1) Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition; 2) strata laid down at the same time in different areas could have entirely different appearances; and 3) the strata of any given area represented only part of the Earth's long history.
The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late eighteenth 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 twentieth century.
The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, and Alexandre Brogniart in the early nineteenth century, enabled geologists to divide Earth history more finely and 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 formed 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 for ancient British tribes (and defined using stratigraphic sequences from Wales). The "Devonian" was named for the British 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 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, there was no way to determine what time scale they represented. Young earth creationists proposed dates of only a few thousand years, while others suggested large (and even infinite) ages. For over one hundred years, the age of the Earth and of the rock strata was the subject of considerable debate until advances in the latter part of the twentieth century allowed radioactive dating to provide relatively firm dates to geologic horizons. In the intervening century and a half, geologists and paleontologists constructed time scales based solely on the relative positions of different strata and fossils.
In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references (Global Boundary Stratotype Section and Points) for geologic periods and faunal stages. Their most recent work is described in the 2004 geologic time scale of Gradstein, Ogg, and Smith (2005), and used as the foundation of the table on this page. The tables of geologic periods presented here are in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey.
An overview of the geologic time periods. The second and third timelines are each subsections of their preceding timeline as indicated by asterisks.
|Major Events||Start, Million
|Cenozoic||Neogene3||Holocene||End of recent glaciation and rise of modern civilization.||0.011430 ± 0.00013 4|
|Pleistocene||Flourishing and then extinction of many large mammals (Pleistocene megafauna); Creation of fully modern humans.||1.806 ± 0.005 *|
|Pliocene||Intensification of present ice age. Cool and dry climate; Australopithecines appear, many of the existing genera of mammals, and recent molluscs appear.||5.332 ± 0.005 *|
|Miocene||Moderate climate; Mountain building in northern hemisphere; Modern mammal and bird families became recognizable. Grasses become ubiquitous. First hominoids appear.||23.03 ± 0.05 *|
|Oligocene||Warm climate; Rapid evolution and diversification of fauna, especially mammals. Major evolution and dispersal of modern types of angiosperms.||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; start of current ice age.||55.8±0.2 *|
|Paleocene||Climate tropical. Modern plants; 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 appear, along with new types of insects. More modern teleost fish begin to appear. Ammonites, belemnites, rudists, 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, 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. Echinoids very common, also 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 and diverse on land, include many large forms; cynodonts become smaller and more mammal-like. First dinosaurs, mammals, pterosaurs, and crocodilia. Dicrodium flora common on land. Many large aquatic temnospondyl amphibians. Ichthyosaurs and nothosaurs common in the seas. Ceratite ammonoids extremely common. Modern corals and teleost fish appear.||228.0 ± 2.0|
|Middle||245.0 ± 1.5|
|Lower/Early||251.0 ± 0.4 *|
|Paleozoic||Permian||Lopingian||Landmass unites in the supercontinent of Pangea. Synapsid reptiles become common (Pelycosaurs and Therapsids), parareptiles and temnospondyl amphibians also remain common. Carboniferous flora replaced by gymnosperms in the middle of the period. Beetles and flies evolve. Marine life flourishes in the warm shallow reefs. Productid and spiriferid brachiopods, bivalves, foraminifera, and ammonoids all abundant. End of Permo-carboniferous ice age. At the end of the period, the Permian extinction event—95% of life on Earth becomes extinct.||260.4 ± 0.7 *|
|Guadalupian||270.6 ± 0.7 *|
|Cisuralian||299.0 ± 0.8 *|
|Upper/Late||Winged insects appear and are abundant, some growing to large size. Amphibians common and diverse. First reptiles, coal forests (Lepidodendron, Sigillaria, Calamites, Cordaites, etc), very high atmospheric oxygen content. In the seas, Goniatites, brachiopods, bryozoa, bivalves, corals, etc. all common.||306.5 ± 1.0|
|Middle||311.7 ± 1.1|
|Lower/Early||318.1 ± 1.3 *|
|Upper/Late||Large primitive trees, first land vertebrates, brackish water and amphibious eurypterids; rhizodonts dominant fresh-water predators. In the seas, primitive sharks common and very diverse, echinoderms (especially crinoids and blastoids) abundant, Corals, bryozoa, and brachiopods (Productida, Spriferida, etc) very common; Goniatites common, trilobites and nautiloids in 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 and horsetails appear, progymnosperms (first seed bearing plants) appear, first trees (Archaeopteris). In the sea, strophomenid and atrypid brachiopods, rugose and tabulate corals, and crinoids are abundant. Goniatite ammonoids are common, and coleoids appear. Trilobites reduced in numbers. Ostracoderms decline; Jawed fish (Placoderms, lobe-finned and ray-finned fish, and early sharks) important life in the sea. First amphibians (but still aquatic). "Old Red Continent" (Euramerica).||385.3 ± 2.6 *|
|Middle||397.5 ± 2.7 *|
|Lower/Early||416.0 ± 2.8 *|
|Silurian||Pridoli||First vascular land plants, millipedes and arthropleurids, first jawed fish, as well as many types of armoured jawless forms. Sea-scorpions reach large size. Tabulate and rugose corals, brachiopods (Pentamerida, Rhynchonellida, etc), and crinoids all abundant; trilobites and molluscs diverse. Graptolites not as varied.||418.7 ± 2.7 *|
|Ludlow||422.9 ± 2.5 *|
|Wenlock||428.2 ± 2.3 *|
|Llandovery||443.7 ± 1.5 *|
|Ordovician||Upper/Late||Invertebrates very diverse and include many new types. Early corals, Brachiopods (Orthida, Strophomenida, etc), bivalves, nautiloids, trilobites, ostracods, bryozoa, many types of echinoderms (cystoids, crinoids, starfish, etc), branched graptolites, and other taxa all common. Conodonts were a group of eel-like vertebrates characterized by multiple pairs of bony toothplates that appear at the start of the Ordovician. Ice age at the end of the period. First very primitive land plants.||460.9 ± 1.6 *|
|Middle||471.8 ± 1.6|
|Lower/Early||488.3 ± 1.7 *|
|Cambrian||Furongian||Major diversification of life in the Cambrian Explosion; more than half of modern animal phyla appear, along with a number of extinct and problematic forms. Archeocyatha abundant in the early Cambrian. Trilobites, Priapulida, sponges, inarticulate brachiopods, and many other forms all common. First chordates appear. Anomalocarids are top predators. Edicarian animals rare, then die out.||501.0 ± 2.0 *|
|Middle||513.0 ± 2.0|
|Lower/Early||542.0 ± 1.0 *|
|Ediacaran||First multi-celled animals. Edicarian fauna (vendobionta) flourish worldwide. Simple trace fossils from worm-like animals. First sponges.||630 +5/-30 *|
|Cryogenian||Possible snowball Earth period, Rodinia begins to break up.||850 7|
|Tonian||First acritarch radiation||1000 7|
|Stenian||Narrow highly metamorphic belts due to orogeny as Rodinia formed.||1200 7|
|Ectasian||Platform covers continue to expand.||1400 7|
|Calymmian||Platform covers expand.||1600 7|
|Statherian||First complex single-celled life (eukaryotes). Columbia (supercontinent).||1800 7|
|Orosirian||The atmosphere became oxygenic. Vredefort and Sudbury Basin asteroid impacts. Much orogeny (the processes that occur during mountain-building).||2050 7|
|Rhyacian||Bushveld Formation formed. Huronian glaciation.||2300 7|
|Siderian||Banded iron formations formed.||2500 7|
|Neoarchean||Stabilization possible of most modern cratons (old, stable part of the continental crust that has survived merging and splitting of continents and supercontinents).mantle overturn event.||2800 7|
|Mesoarchean||First stromatolites.||3200 7|
|Paleoarchean||First known oxygen producing bacteria.||3600 7|
|Eoarchean||Simple single-celled life (prokaryote).||3800|
|Basin groups9||4100 Ma—Oldest known rock||c.4150|
|Cryptic9||4400 Ma—Oldest known mineral; 4570 Ma—Formation of Earth||c.4570|
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