* Eons are the largest divisions of geologic time, representing immense spans of time. There are four eons: the Hadean, Archean, Proterozoic, and Phanerozoic.
* Eras are subdivisions of eons, representing significant changes in the Earth's history and life forms. For example, the Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.
These two units, eons and eras, provide a framework for understanding the vast history of our planet and the evolution of life on Earth.
