Geologic time scale

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The geological time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic periods presented here is 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.

Evidence from radiometric dating indicates that the Earth is about 4,570 million years old (expressed with m.y.a., i.e. million years ago, or "Ma" as in "it dates from 4570 Ma"). The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.

Contents 1 Graphical timelines 2 Terminology 3 History of the time scale 4 Table of geologic time 5 References and footnotes 6 See also 7 External links

[edit] Graphical timelines

The second and third timelines are each subsections of their preceding timeline as indicated by asterisks.

Millions of Years

The Holocene (the latest epoch) is too small to be shown clearly on this timeline.

[edit] Terminology

The largest defined unit of time is the Eon. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time paleontologists define 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 geological nomenclature, though in general there are far more recognized faunal stages than defined geological time units.

Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper Jurassic", and "Middle Cambrian". Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." 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 Lower Cambrian is referred to as the Waucoban series that is then subdivided into zones based on trilobites. The same timespan 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.

[edit] History of the time scale

The principles underlying geologic (geological) time scales were laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or 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. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th 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;

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 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" (now Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) 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, Jean d'Omalius d'Halloy 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 between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today.

The process was dominated by British geologists, 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" was named after Perm, Russia, because it was defined using strata in that region by a Scottish geologist Roderick Murchison. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) —red beds, capped by chalk, followed by black shales— that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous" (from Latin creta meaning 'chalk') as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin^[1] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates).

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 (pioneered by such geologists as Arthur Holmes) 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 on Stratigraphy) 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.^[2] and is used as the basis of this page.

More information on the development of the geologic time scale can be found in the articles history of geology and history of paleontology.

[edit] Table of geologic time

The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy (see lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon). It should be noted that the height of each table entry does not scale in any way with the duration of each subdivision of time.

Eon		Era	Period[3]		Series/ Epoch	Major events	Start, million years ago[4]
Phanerozoic		Cenozoic	Neogene [5]		Holocene	End of recent glaciation and rise of human civilization.	0.011430 ± 0.00013[6]
					Pleistocene	Flourishing and then extinction of many large mammals (Pleistocene megafauna). Evolution of anatomically modern humans. Present Ice Age starts.	1.806 ± 0.005 *
					Pliocene	Cool and dry climate. Australopithecines, many of the existing genera of mammals, and recent mollusks appear. Homo habilis appears.	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 [5]		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. Ice cap develops on Antarctica.	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 Pangaea 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 Pangaea, 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- iferous[7]/ 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[7]/ 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 Ediacaran 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 *
Preca- mbrian [8]	Proter- ozoic [9]	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 [10]
			Tonian		Rodinia supercontinent persists. Trace fossils of simple multi-celled eukaryotes. First radiation of dinoflagellate-like acritarchs.		1000 [10]
		Meso- proterozoic	Stenian		Narrow highly metamorphic belts due to orogeny as supercontinent Rodinia is formed.		1200 [10]
			Ectasian		Platform covers continue to expand. Green algae colonies in the seas.		1400 [10]
			Calymmian		Platform covers expand.		1600 [10]
		Paleo- proterozoic	Statherian		First complex single-celled life: protists with nuclei. Columbia is the primordial supercontinent.		1800 [10]
			Orosirian		The atmosphere became oxygenic. Vredefort and Sudbury Basin asteroid impacts. Much orogeny.		2050 [10]
			Rhyacian		Bushveld Formation occurs. Huronian glaciation.		2300 [10]
			Siderian		Oxygen Catastrophe: banded iron formations result.		2500 [10]
	Archean [9]	Neoarchean		Stabilization of most modern cratons; possible mantle overturn event.			2800 [10]
		Mesoarchean		First stromatolites (probably colonial cyanobacteria). Oldest macrofossils.			3200 [10]
		Paleoarchean		First known oxygen-producing bacteria. Oldest definitive microfossils.			3600 [10]
		Eoarchean		Simple single-celled life (probably bacteria and perhaps archaea). Oldest probable microfossils.			3800
	Hadean [9][11]	Formation of earth (4570 mya). Oldest known mineral, zircon (4400 mya).					c.4570

[edit] References and footnotes

[edit] See also

• Age of the Earth
• History of Earth
• Anthropocene
• Cosmological timeline
• List of fossil sites (with link directory)
• Logarithmic timeline
• Lunar geologic timescale
• Martian geologic timescale
• Timeline of evolution
• Timeline of human evolution
• Time line of the geologic history of the United States
• Timetable of the Precambrian

[edit] External links

• NASA: Geologic Time
• GSA: Geologic Time Scale
• British Geological Survey: Geological Timechart
• GeoWhen Database
• International Commission on Stratigraphy Time Scale
• CHRONOS
• National Museum of Natural History - Geologic Time