Eons: The largest division, representing billions of years. There are four eons:
* Hadean Eon: 4.5 billion to 4 billion years ago (bya). The Earth's formation and initial cooling.
* Archean Eon: 4 bya to 2.5 bya. First evidence of life, early continents, and the first atmosphere.
* Proterozoic Eon: 2.5 bya to 541 million years ago (mya). Evolution of single-celled organisms into multicellular life, oxygenation of the atmosphere.
* Phanerozoic Eon: 541 mya to present. The time of complex life, with the emergence of animals, plants, and eventually humans.
Eras: Within each eon, there are eras, representing hundreds of millions of years.
* Paleozoic Era: 541 mya to 252 mya. The "Age of Fishes," with the emergence of vertebrates, invertebrates, and the colonization of land by plants and animals.
* Mesozoic Era: 252 mya to 66 mya. The "Age of Reptiles," featuring the rise and fall of the dinosaurs, the evolution of mammals, and the spread of flowering plants.
* Cenozoic Era: 66 mya to present. The "Age of Mammals," characterized by the diversification of mammals, the evolution of primates and humans, and the emergence of modern ecosystems.
Periods: Eras are further subdivided into periods, each lasting tens of millions of years. These represent significant changes in the Earth's climate, geography, and life.
Epochs: Finally, periods are divided into epochs, representing smaller units of time, typically several million years. They often mark distinct changes within a period, such as major extinction events or the appearance of new species.
Here's a table summarizing the major divisions of the geologic time scale:
| Eon | Era | Period | Epoch | Age (mya) |
|-----------------|--------------------|--------------------|-------------------------|-------------------|
| Phanerozoic | Cenozoic | Quaternary | Holocene | 0.0117 - Present |
| | | | Pleistocene | 2.6 - 0.0117 |
| | | Paleogene | Oligocene | 33.9 - 23.03 |
| | | | Eocene | 56 - 33.9 |
| | | | Paleocene | 66 - 56 |
| | Mesozoic | Cretaceous | Maastrichtian | 72.1 - 66 |
| | | | Campanian | 83.6 - 72.1 |
| | | | Santonian | 86.3 - 83.6 |
| | | | Coniacian | 89.8 - 86.3 |
| | | | Turonian | 93.9 - 89.8 |
| | | | Cenomanian | 100.5 - 93.9 |
| | | | Albian | 113 - 100.5 |
| | | | Aptian | 125 - 113 |
| | | | Barremian | 130 - 125 |
| | | | Hauterivian | 136.4 - 130 |
| | | | Valanginian | 145 - 136.4 |
| | | | Berriasian | 145 - 145 |
| | | Jurassic | Tithonian | 152.1 - 145 |
| | | | Kimmeridgian | 157.3 - 152.1 |
| | | | Oxfordian | 163.5 - 157.3 |
| | | | Callovian | 166.1 - 163.5 |
| | | | Bathonian | 168.3 - 166.1 |
| | | | Bajocian | 170.3 - 168.3 |
| | | | Aalenian | 174.1 - 170.3 |
| | | | Toarcian | 183 - 174.1 |
| | | | Pliensbachian | 190.8 - 183 |
| | | | Sinemurian | 199.3 - 190.8 |
| | | | Hettangian | 201.3 - 199.3 |
| | | Triassic | Rhaetian | 208.5 - 201.3 |
| | | | Norian | 227 - 208.5 |
| | | | Carnian | 237 - 227 |
| | | | Ladinian | 242 - 237 |
| | | | Anisian | 247.2 - 242 |
| | | | Olenekian | 251.9 - 247.2 |
| | | | Induan | 252.2 - 251.9 |
| | Paleozoic | Permian | Lopingian | 254.2 - 251.9 |
| | | | Guadalupian | 272.3 - 254.2 |
| | | | Cisuralian | 298.9 - 272.3 |
| | | Pennsylvanian | Gzhelian | 303.7 - 298.9 |
| | | | Kasimovian | 307 - 303.7 |
| | | | Moscovian | 315.2 - 307 |
| | | | Bashkirian | 323.2 - 315.2 |
| | | | Serpukhovian | 330.9 - 323.2 |
| | | | Visean | 346.7 - 330.9 |
| | | | Tournaisian | 358.9 - 346.7 |
| | | Carboniferous | Mississippian | 359.2 - 358.9 |
| | | | Chesterian | 330.9 - 323.2 |
| | | | Meramecian | 323.2 - 315.2 |
| | | | Osagean | 346.7 - 330.9 |
| | | | Kinderhookian | 359.2 - 346.7 |
| | | Devonian | Famennian | 372.2 - 359.2 |
| | | | Frasnian | 382.7 - 372.2 |
| | | | Givetian | 387.7 - 382.7 |
| | | | Eifelian | 393.3 - 387.7 |
| | | | Emsian | 407.6 - 393.3 |
| | | | Pragian | 410.8 - 407.6 |
| | | | Lochkovian | 419.2 - 410.8 |
| | | Silurian | Pridoli | 423 - 419.2 |
| | | | Ludlow | 425.6 - 423 |
| | | | Wenlock | 433.4 - 425.6 |
| | | | Llandovery | 443.8 - 433.4 |
| | | Ordovician | Hirnantian | 445.2 - 443.8 |
| | | | Katian | 453 - 445.2 |
| | | | Sandbian | 458.4 - 453 |
| | | | Darriwilian | 467.3 - 458.4 |
| | | | Floian | 477.7 - 467.3 |
| | | | Tremadocian | 485.4 - 477.7 |
| | | Cambrian | Furongian | 497 - 485.4 |
| | | | Series 2 | 500.5 - 497 |
| | | | Series 3 | 509 - 500.5 |
| | | | Series 4 | 514 - 509 |
| | | | Series 5 | 521 - 514 |
| | | | Series 6 | 538.8 - 521 |
| | | | Series 7 | 541 - 538.8 |
|-----------------|--------------------|--------------------|-------------------------|-------------------|
This table only lists the major periods and epochs. The geologic time scale is a vast and complex system with many further subdivisions.
