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Geologic time scale

From Academic Kids

The geologic 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 Geologic Survey.

Current geologic evidence holds that the age of the Earth is about 4570 million years old. The geologic 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 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.

Contents

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 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 -- e.g. "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 refered 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.

History of the time scale

The principles underlying geologic time scales were laid down by Nicholas Steno in the late 17th 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; 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" 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 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, 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 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, there was no way to determine what time scale they represented. Creationists proposed dates of only a few thousand years, while others suggested large (and even infinite) ages. For over 100 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 20th 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 Sections and Points) for geologic periods and faunal stages. Their most recent work is described in the 2004 geologic time scale of Gradstein et al. (ISBN 0521786738), and used as the foundation of this page.

Table of geologic time

Template:Mergefrom

Eon Era Period1 Series/
Epoch
Major Events Start, Million
Years Ago2
Phane-
rozoic
Cenozoic Neogene3 Holocene End of recent glaciation and rise of modern civilization 0.011430 ± 0.00013 9
Pleistocene Flourishing and then extinction of many large mammals (Pleistocene megafauna); Evolution 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. Horses and mastodonts diverse. Grasses become ubiquitous. First hominoids appear. 23.03 ± 0.05 *
Paleogene
3
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 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. 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, as do many modern insect clades. 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 *
Carbon-
iferous
4 /
Pennsyl-
vanian
Upper/Late Winged insects appear and are abundant, some (esp. Protodonata and Palaeodictyoptera) 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 *
Carbon-
iferous
4 /
Missis-
sippian
Upper/Late Large primitive trees, first land vertebratess, 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). First (wingless) insects. 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. Armoured agnaths 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 planktonic primitive vertebrates 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 *
Proter-
ozoic

5
Neo-
proterozoic
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 6
Tonian First acritarch radiation 1000 6
Meso-
proterozoic
Stenian Narrow highly metamorphic belts due to orogeny as Rodinia formed. 1200 6
Ectasian Platform covers continue to expand 1400 6
Calymmian Platform covers expand 1600 6
Paleo-
proterozoic
Statherian First complex single-celled life. Columbia (supercontinent). 1800 6
Orosirian The atmosphere became oxygenic. Vredefort and Sudbury Basin asteroid impacts. Much orogeny. 2050 6
Rhyacian Bushveld Formation formed. Huronian glaciation. 2300 6
Siderian banded iron formations formed 2500 6
Archean
5
Neoarchean Stabilization of most modern cratons, possible mantle overturn event 2800 6
Mesoarchean First stromatolites 3200 6
Paleoarchean First known oxygen producing bacteria 3600 6
Eoarchean Simple single-celled life 3800
Hadean
5,7
Lower Imbrian   c.3850
Nectarian   c.3920
Basin groups 4100 Ma - Oldest known rock c.4150
Cryptic8 4400 Ma - Oldest known mineral; 4570 Ma - Formation of Earth c.4570
  1. Paleontologists often refer to faunal stages rather than geologic periods. The stage nomenclature is quite complex. See Harland (http://flatpebble.nceas.ucsb.edu/cgi-bin/bridge.pl?action=startScale) for an excellent time ordered list of faunal stages.
  2. Dates 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.
  3. Historically, 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.
  4. In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods.
  5. The Proterozoic, Archean and Hadean are often collectively referred to as Precambrian Time, and sometimes also as the Cryptozoic.
  6. Defined by absolute age (Global Standard Stratigraphic Age).
  7. 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.
  8. These four era names were taken from Moon geology. Their use for Earth geology is unofficial.
  9. The start time for the Holocene epoch is here given as 11,430 years ago ± 130 years. For further discussion of the dating of this epoch, see Holocene.

Graphical timelines

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</timeline>

Millions of Years

References

See also

External link

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