Eon (geology)
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In general usage, an eon (also 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. We are currently in the Phanerozoic eon, the Cenozoic era and the Neogene period.
| Eon | Era | Period (mya = million of years ago) |
|---|---|---|
| Phanerozoic | Cenozoic | 66 mya to present day |
| Mesozoic | 245 mya - 66 mya | |
| Paleozoic | 570 mya - 245 mya | |
| Proterozoic | 2500 mya - 570 mya | |
| Archaean | 3800 mya - 2500 mya | |
| Hadean | 4550 mya - 3800 mya | |
The Hadean, Archaean and Proterozoic form what is informally called the Precambrian. This covers the four billion years of Earth history prior to the appearance of hardshelled animals.
References
- USA National Park Service (http://www2.nature.nps.gov/geology/usgsnps/gtime/gtime1.html)
- Washington State University (http://astro.wsu.edu/worthey/earth/html/md08.html)