Inline videos. See also:Category: Articles with embedded Videos..

Geologic time scale

From Biocrawler, the free encyclopedia.

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.

The Earth is thought by geologists to be about 4,570 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

1 Terminology

2 History of the timescale

3 Table of geologic time

[edit]

Terminology

The largest defined unit 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 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 occuring 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 of time 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.

[edit]

History of the timescale

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 (GSSPs) 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.

[edit]

Table of geologic time

Eon	Era	Period¹		Series/ Epoch	Major Events	End, Million Years Ago²
Phanerozoic	Cenozoic	Neogene³		Holocene	End of recent glaciation and rise of modern civilization	Ongoing
				Pleistocene	Flourishing and then extinction of many large mammals (Pleistocene megafauna); Evolution of fully modern humans	0.011430 ± 0.00013
				Pliocene	Intensification of present ice age. Cool and dry climate; Australopithecines appear, many of the existing genera of mammals, and recent molluscs appear	1.806 ± 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.	5.332 ± 0.005 ^*
		Paleogene³		Oligocene	Warm climate; Rapid evolution and diversification of fauna, especially mammals. Major evolution and dispersal of modern types of angiosperms	23.03 ± 0.05 ^*
				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.	33.9 ± 0.1 ^*
				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)	55.8 ± 0.2 ^*
	Mesozoic	Cretaceous		Upper/Late	Flowering plants appear, along with new types of insects. More modern teleost fish begin to appear. Ammonites, belemnites, rudists, and sponges 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.	65.5 ± 0.3 ^*
		Cretaceous		Lower/Early		99.6 ± 0.9 ^*
		Jurassic		Upper/Late	Gymnosperms 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. Ammonites and belemnites abundant. Breakup of Pangea into Gondwana and Laurasia	145.5 ± 4.0
				Middle		161.2 ± 4.0
				Lower/Early		175.6 ± 2.0 ^*
		Triassic		Upper/Late	Archosaurs dominant and diverse on land; cynodonts become smaller and more mammal-like. Many large aquatic amphibians. Ichthyosaurs and ceratites common in the seas. First dinosaurs, mammals, teleosts, and crocodylia.	199.6 ± 0.6
				Middle		228.0 ± 2.0
				Lower/Early		245.0 ± 1.5
	Paleozoic	Permian		Lopingian	Landmass unites in the supercontinent of Pangea. Synapsid reptiles become common (Pelycosaurs and Therapsids), 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. End of Permo-carboniferous ice age. At the end of the period the Permian extinction event- 95% of life on Earth becomes extinct	251.0 ± 0.4 ^*
				Guadalupian		260.4 ± 0.7 ^*
				Cisuralian		270.6 ± 0.7 ^*
		Carbon- iferous⁴	Pennsyl- vanian	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.	299.0 ± 0.8 ^*
				Middle		306.5 ± 1.0
				Lower/Early		311.7 ± 1.1
			Missis- sippian	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, brachiopods (Productida, Spriferida, etc) very common; trilobites and nautiloids in decline. Glaciation in East Gondwana.	318.1 ± 1.3 ^*
				Middle		326.4 ± 1.6
				Lower/Early		345.3 ± 2.1
		Devonian		Upper/Late	First clubmosses and horsetails appear, progymnosperms (first seed bearing plants) appear, first trees (Archaeopteris). First Amphibians (but still aquatic). In the sea strophomenid and atrypid brachiopods, rugose and tabulate corals, and crinoids abundant; ammonoids and coleoids appear; jawed fish (Placoderms, lobe-finned and ray-finned fish, and early sharks) important life in the sea. "Old Red Continent" (Euramerica)	359.2 ± 2.5 ^*
				Middle		385.3 ± 2.6 ^*
				Lower/Early		397.5 ± 2.7 ^*
		Silurian		Pridoli	First vascular land plants, millipedes and arthropleurids, first jawed fish, sea-scorpions reach large size, tabulate and rugose corals and brachiopods very common; trilobites and molluscs diverse. Graptolites not as varied.	416.0 ± 2.8 ^*
				Ludlow		418.7 ± 2.7 ^*
				Wenlock		422.9 ± 2.5 ^*
				Llandovery		428.2 ± 2.3 ^*
		Ordovician		Upper/Late	Invertebrates dominant; they include brachiopods (Orthida, Strophomenida, etc), bivalves, nautiloids, trilobites, bryozoa, many types of echinoderms (cystoids, crinoids, starfish, etc), graptolites, and other taxa. Conodonts were primitive planktonic vertebrates that appear at the start of the Ordovician. Ice age at the end of the period. First land plants appear.	443.7 ± 1.5 ^*
				Middle		460.9 ± 1.6 ^*
				Lower/Early		471.8 ± 1.6
		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. Trilobites, Priapulida, sponges, inarticulate brachiopods, anomalocarids, and many other forms common.	488.3 ± 1.7 ^*
				Middle		501.0 ± 2.0 ^*
				Lower/Early		513.0 ± 2.0
Proterozoic⁵	Neo- proterozoic	Ediacaran		First multi-celled animals		542.0 ± 1.0 ^*
		Cryogenian		Possible snowball Earth period, Rodinia begins to break up		630 +5/-30 ^*
		Tonian		First acritarch radiation		850 ⁶
	Meso- proterozoic	Stennian		Formation of Rodinia		1000 ⁶
		Ectasian				1200 ⁶
		Calymmian				1400 ⁶
	Paleo- proterozoic	Statherian		First complex single-celled life		1600 ⁶
		Orosirian		Transition to oxygen atmosphere		1800 ⁶
		Rhyacian				2050 ⁶
		Siderian				2300 ⁶
Archaean⁵	Neoarchean	Stabilization of most modern cratons, possible mantle overturn event				2500 ⁶
	Mesoarchean	First stromatolites				2800 ⁶
	Paleoarchean	First known oxygen producing bacteria				3200 ⁶
	Eoarchean	Simple single-celled life				3600 ⁶
Hadean^5,7	4100 MYA - Oldest known rock; 4400 MYA - Oldest known mineral; 4570 MYA - Formation of Earth					3800

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.
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.
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.
In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods.
The Proterozoic, Archean and Hadean are often collectively referred to as Precambrian Time, and sometimes also as the Cryptozoic.
Defined by absolute age (Global Standard Stratigraphic Age).
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.

[edit]

Graphical timeline

Millions of Years</center>

[edit]

References

GeoWhen Database (http://www.stratigraphy.org/geowhen/)
International Commission on Stratigraphy Time Scale (http://www.stratigraphy.org/gssp.htm)
BBC Interactive Time Line (http://www.bbc.co.uk/history/games/rocky/indextime.html)

[edit]