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Tuesday
Apr272010

eon v. era v. period v. epoch v. age

The Earth’s 4.5 billion year history has been divided and subdivided into time spans delimited by major geological or biological events, such as the development of multicellular life, or mass extinctions. From largest to smallest, the terms for these divisions are:

Eon

Eons can range from hundreds of millions to more than a billion years. Examples include the Archean, Proterozoic, and Phanerozoic Eons.

Era

Eons are subdivided into eras, which are intervals of tens to hundreds of millions of years. Examples: Paleozoic, Mesozoic, Cenozoic.

Period

A period may be tens of millions of years long. Examples: Cambrian, Devonian, Jurassic.

Epoch

More recent periods are further divided into epochs and ages. Epochs last from tens of thousands to millions of years. Often, periods are simply divided into Early, Middle, and Late Epochs. Other examples are Furongian, Oligocene, and Holocene.

Age

A typical age lasts for a few million years. Examples: Frasnian, Selandian, Ypresian.

We are now experiencing the Holocene Epoch in the Quaternary Period of the Cenozoic Era of the Phanerozoic Eon.

Geologic time chart. Image courtesy of the Science Education Resource Center at Carleton College.

Wikipedia on the geologic time scale

Enchanted Learning on the geologic time scale

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