For the Gnostic word, see aeon. For other uses, see eon (disambiguation).

In general usage, an eon (sometimes spelled aeon) is a very long period of time. Geologists refer to an eon as the largest period of geologic time. For example, the Phanerozoic Eon, which is about 550 million years long, covers the period of time during which animals with hard shells that fossilize well have been abundant.

An eon is composed of several eras, which in turn are composed of periods, which are composed of epochs. We are currently in the Phanerozoic Eon, the Cenozoic Era, the Neogene Period, and the Holocene epoch.

One definition of an eon puts it as "a unit of geologic time equal to one billion years" , although it is rarely used in this manner.

Only one eon exists besides the Panerozoic: the Precambrian. However, the term eon not always used, as the Precambrian was traditionally classified as an era. If one accepts the interpretation of it as an eon, the geologic timetable is divided as follows:

The Precambrian eon covers the four billion years of Earth history prior to the appearance of hard-shelled animals, while the Phanerozoic essentially covers everything else. The former is currently about eight times as long as the latter, but assuming no new divisions are formed, this will change in the future. Note that such divisions are arbitrary in that they are designated by man, so in other words, the Phanerozoic will be longer than the Precambrian in three and a half billion years assuming an Earth-shattering geological event does not occur, necessitating such a shift.

References

According to the UCMP site (), there are only 2 time periods classified as "EON": 1) The Precambrian, and 2) the Phanerozoic. The Hadean, Archaean, and Proterozoic time periods are classified as "Eras."

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