Timescale Geological
Time Units

Geological Time Units

Rarely used divisions are defined in fainter text

Rank Geological Time Units
Geological Time Units
1st Order Eonotherm Eon The largest division of geologic time, embracing several eras and continuing for hundreds of millions or billions of years. Originally there were only two eons, the Precambrian and the Phanerozoic. However, owing to the huge duration of the former, there is a tendency now to elevate the former Precambrian eras such as the Archean (previously Archeozoic) and the Proterozoic into eons.
"Eon" also means any span of one billion years.
2nd Order Erathem Era A geologic division including several periods, but smaller than an eon. Generally lasts for many tens or hundreds of millions of years, and often characterized by distinct life-forms - e.g. the Cenozoic is the "age of mammals". Commonly recognized eras are the Paleozoic, Mesozoic, and Cenozoic. Precambrian divisions such as the Proterozoic and the Archean were conventionally eras but are now often referred to as eons. New Precambrian eras include the Sinian, the Riphean, the Huronian, etc.
3rd Order System Period The most commonly used unit of geologic time, representing one subdivision of an era. Eras may have from two to six or seven periods each. Each period generally lasts for some thirty to eighty million years. (the only exception, the mere 1.6 million year long Quaternary, is actually a hold-over from the 19th century formulation, as its numeric name indicates)
4th Order Series Epoch A subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Only in the Cenozoic era (Tertiary and Quaternary periods) is the category of epochs widely used in a popular context. In pre-Tertiary divisions, epochs seem mostly to corresponds to what I would consider to be subperiods (e.g. the Lower Jurassic period is called the Lias Epoch), and it would seem that the ages of these periods are more like Tertiary epochs in duration!
"Epoch" is also used for a division of time corresponding to a paleomagnetic interval.
5th Order Stage Age A subdivision of a geologic epoch, usually lasting between five and ten million years period, although there are also shorter and longer Ages. As most species only last for several million years, this is the best time division for looking at the evolution of life through time. Also, radiometric dating, while often accurate enough to give a resolution down to Age, cannot yet give a reliable resolution down to chron. A stage or age, being a group of successive chronozones or chrons, is the basic working unit of chronostratigraphy, is the finest unit that is generally used.
6th Order Chronozone Chron The smallest subdivision of geologic time. Generally lasting only a million years or even less, and usually determined by biostratigraphic intervals. e.g. the Psiloceras planorbis chron is the time period when that ammonite was alive, as indicated by the presence of its fossil shells in rock strata of that time. It is often very hard to determine chrons and chronozones accurately on a worldwide basis, because particular organisms were and are generally confined to particular geographic regions or provinces. Planktonic organisms like graptolites or conodonts, or foraminifers, which often have a very large (but still generally not worldwide) distribution are useful in this case, as are terrestrial microfossils like pollen. Even so, it is often extremely difficult to match up terrestrial and marine chrons. Owing to the rapid evolution of Jurassic Ammonites, it seems that some chrons of this period were as brief as 75,000 years!

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content by M. Alan Kazlev
checked ATW050726, edited RFVS111107
page uploaded 8 May 2001, most recent update 25 April 2002
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