Geological Era

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees 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 billion years old. 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 Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.
Each era on the scale is separated from the next by a major event or change.

Terminology

The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided intoperiods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time.

Geologists qualify these units as Early, Mid, and Late when referring to time, and Lower, Middle, and Upper when referring to the corresponding rocks. For example, the Lower Jurassic Series in chronostratigraphy corresponds to the Early Jurassic Epoch ingeochronology.The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus “early Miocene” but “Early Jurassic.”

Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on succession of trilobites. In East Asia and Siberia, the same unit is split into Alexian, Atdabanian, and Botomian stages. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.

If the entire history of the Earth was to be compressed into one day, then the Earth will be formed at midnight, the Moon will be formed at 5:00 am, the atmosphere will form at 2:00 pm, the first life will appear at 6:00 pm, dinosaurs will appear at 9:00 pm and become extinct at 10:00 pm, and modern humans will not appear until 11:59 pm and 59 seconds.

It is generally assumed that planets are formed by the accretion of gas and dust in a cosmic cloud, but there is no way of estimating the length of this process. Our Earth acquired its present size, more or less, between 4 billion and 5 billion years ago. Life on Earth originated about 2 billion years ago, but there are no good fossil remains from periods earlier than the Cambrian, which began about 490 million years ago.

The known geological history of Earth since the Precambrian Time is subdivided into three eras, each of which includes a number of periods. They, in turn, are subdivided into epochs and stage ages. In an epoch, a certain section may be especially well known because of rich fossil finds.

New Geological Period

In March 2004, geologists added a new time period to Earth’s chronology—the Ediacaran Period. The Ediacaran Period lasted about 50 million years, from 600 million years ago to about 542 million years ago. It was the last period of the Precambrian’s Neoproterozoic Era. Multicelled organisms first appeared during this time. This period is the first new one added in 120 years.

Precambrian Time

The Precambrian’s lower limit is not defined, but ended about 542 million years ago. The Precambrian encompasses about 90% of Earth’s history.

Eonothem eon	Duration¹	Eras	Events
Archaean (Greek archaios = ancient )	2,500?	Eoarchean (Greek eos = dawn + archaios = ancient) Paleoarchean (Greek palaios = old) Mesoarchean (Greek mesos = middle) Neoarchean (Greek neo = new)	Formation of oceans, atmosphere, and continents; bacteria
Proterozoic (Greek proteros = earlier + zoön = animal)	c. 2,000	Paleoproterozoic (Greek palaios = old) Mesoproterozoic (Greek mesos = middle) Neoproterozoic (Greek neo = new)	Oxygen build-up; multicelled organisms

1.In millions of years.

Paleozoic Era

This era began 542 million years ago and lasted about 291 million years. The name was compounded from Greek palaios (old) and zoön (animal).

Period	Duration¹	Epochs	Events
Cambrian (Cambria, Latin name for Wales)	54	Lower Cambrian Middle Cambrian Upper Cambrian	Invertebrate sea life proliferating during this and the following period
Ordovician (Latin Ordovices, people of early Britain)	45	Lower Ordovician Upper Ordovician	Diverse marine life, including vertebrates; vascular plants
Silurian (Latin Silures, people of early Wales)	28	Lower Silurian Upper Silurian	Coral reefs; giant scorpions; first jawed fish
Devonian (Devonshire in England)	57	Lower Devonian Upper Devonian	Numerous fishes, other sea life; many plants, first trees; wingless insects
Carboniferous (Latin carbo = coal + fero = to bear)	60	Upper, Middle, and Lower Mississippian² Upper, Middle, and Lower Pennsylvanian²	Maximum coal formation in swampy forests; insects, amphibians, reptiles; fishes, clams, crustaceans
Permian (district of Perm in Russia)	48	Lower Permian Upper Permian	Large reptiles, amphibians; most species become extinct

1. In millions of years.

2. Mississippian and Pennsylvanian names are used only in the U.S.

Mesozoic Era

This era began 251 million years ago and lasted about 186 million years. The name was compounded from Greek mesos (middle) and zoön (animal). Popular name: Age of Reptiles.

Period	Duration¹	Epochs	Events
Triassic (trias = triad)	51	Lower Triassic Middle Triassic Upper Triassic	Early dinosaurs, crocodiles, turtles; first mammals
Jurassic (Jura Mountains)	54	Lower Jurassic Middle Jurassic Upper Jurassic	Many seagoing reptiles; early large dinosaurs; later, flying reptiles (pterosaurs), earliest known birds
Cretaceous (Latin creta = chalk)	80	Lower Cretaceous Upper Cretaceous	Dinosaurs and other reptiles dominate; seed-bearing plants appear

1. In millions of years.

Cenozoic Era¹

This era began 66 million years ago and includes the geological present. The name was compounded from Greek kainos (new) and zoön (animal). Popular name: Age of Mammals.

Period	Duration²	Epochs	Events
Paleogene (Greek palaios = old + genes = born)	42	Paleocene (Greek palaios = old + kainos = new). Eocene (Greek eos = dawn). Oligocene (Greek oligos = few).	Rich insect fauna, early bats, increasingly diverse varieties of mammals and birds
Neogene (Greek neo = new + genes = born)	23	Miocene (Greek meios = less + kainos = new). Pliocene (Greek pleios = more). Pleistocene (Greek pleistos = most) (popular name: Ice Age). Holocene (Greek holos = entire), the last 10,000 years to the present.	Further development of mammals and birds. Various forms of humans, including Homo sapiens

1. This table reflects the divisions used by the International Commission on Stratigraphy. The U.S. Geological Survey divides the Cenezoic Era into the Tertiary Period (with the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs) and the Quaternary Period (with the Pleistocene and Holocene Epochs).
2. In millions of years.

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