1. Eons: The largest and broadest divisions of geologic time.
* Precambrian Eon: The earliest and longest eon, encompassing the vast majority of Earth's history, from about 4.5 billion years ago to 541 million years ago.
* Phanerozoic Eon: The current eon, starting at the end of the Precambrian and continuing to the present.
2. Eras: Within each eon, there are multiple eras.
* Precambrian Eon: This eon doesn't have formal eras, but it is sometimes informally divided into three parts: Hadean, Archean, and Proterozoic.
* Phanerozoic Eon:
* Paleozoic Era: (541-252 million years ago) "Old Life" - rise of complex life forms.
* Mesozoic Era: (252-66 million years ago) "Middle Life" - age of dinosaurs.
* Cenozoic Era: (66 million years ago to present) "Recent Life" - age of mammals.
3. Periods: Eras are further subdivided into periods.
* Paleozoic Era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
* Mesozoic Era: Triassic, Jurassic, Cretaceous
* Cenozoic Era: Paleogene (Paleocene, Eocene, Oligocene), Neogene (Miocene, Pliocene), Quaternary (Pleistocene, Holocene)
4. Epochs: Periods are further subdivided into epochs (except for the Quaternary period, which is often divided into two epochs).
* Paleogene: Paleocene, Eocene, Oligocene
* Neogene: Miocene, Pliocene
* Quaternary: Pleistocene, Holocene
Note: The geologic time scale is a constantly evolving system, and the specific boundaries between units may be adjusted as new information becomes available.
