What Is An Eon

An eon, often spelled as “aeon,” is a measurement of time in the geologic time scale. It basically expresses a long period of time, usually much longer than an era.


Eons are used to look at time in periods longer than centuries and millennia. Unlike a year, century or other similar measurement, eons have no set length. They are classified instead by geological events.


The earth is in its fourth eon, the Phanerozoic, which began about 1 million years ago.

Eras, Periods and Epochs

The Phanerozoic eon is divided into smaller segments called eras. This eon is made up of 12 eras (so far). The eras are further divided into periods. Periods are divided into even smaller epochs. Like eons, these smaller time units are not divided based on their chronological length, but on geological events.

Holmes, the Father of Geologic Time

The first geologic time scale was proposed by Arthur Holmes, a British geologist, in 1913. Just decades earlier, many geologists believed the planet was only a few thousand years old. Using radiological dating techniques new at the time, Holmes dated some rock materials to over 1.5 billion years old. He proposed the ideas of plate tectonics and continental drift.

Other Usages

The Greek version of “eon” means “forever.” In the ancient religion of Zoroastrianism, an aeon was the period of time before the world began, seen as god’s domain, similar to the space where God created earth in the Bible, according to the Encyclopedia Britannica.

READ  Classroom Activities For The Geologic Time Scale