The eras of geologic time
Ten eras in three eons (the Hadean has none). Start dates are from the International Commission on Stratigraphy chart, version 2024/12, in millions of years ago (Ma). Older charts and textbooks give slightly different numbers, because the dates are refined as rocks are re-dated. Precambrian boundaries such as 2,500, 1,600 and 1,000 Ma are set by agreed round numbers, not by a layer of rock.
Eras are the second level of the time scale. Most people know the last three (Paleozoic, Mesozoic, Cenozoic); the seven older ones fill in the long, quiet stretch when life was small.
The names follow one pattern, so learn the pattern, not ten words: Eo (dawn), Paleo (old), Meso (middle), Neo (new), attached to the eon's name. The Archean gets all four, the Proterozoic gets three (no Eo), and the Phanerozoic's three use -zoic alone. Chunk by eon, and anchor the last three with pictures: a trilobite, a dinosaur, a mammoth.
Encode make it vivid → Space bring it back on day 1, 3, 7, 21 → Retrieve pull it from your head. From the infographic Remember It on Purpose.
The eras of geologic time
| Order | Era | Eon | Began (million years ago) |
|---|---|---|---|
| 1 | Eoarchean | Archean | 4031 |
| 2 | Paleoarchean | Archean | 3600 |
| 3 | Mesoarchean | Archean | 3200 |
| 4 | Neoarchean | Archean | 2800 |
| 5 | Paleoproterozoic | Proterozoic | 2500 |
| 6 | Mesoproterozoic | Proterozoic | 1600 |
| 7 | Neoproterozoic | Proterozoic | 1000 |
| 8 | Paleozoic | Phanerozoic | 538.8 |
| 9 | Mesozoic | Phanerozoic | 251.902 |
| 10 | Cenozoic | Phanerozoic | 66.0 |
The names follow one pattern, so learn the pattern, not ten words: Eo (dawn), Paleo (old), Meso (middle), Neo (new), attached to the eon's name. The Archean gets all four, the Proterozoic gets three (no Eo), and the Phanerozoic's three use -zoic alone. Chunk by eon, and anchor the last three with pictures: a trilobite, a dinosaur, a mammoth. · teddyrookbook.com/memory/challenges/geologic-eras/
Sources
- International Commission on Stratigraphy: International Chronostratigraphic Chart v2024/12
- U.S. Geological Survey: Divisions of Geologic Time (Fact Sheet 2018-3054)
Checked October 30, 2026. Progress is kept on this device only (per learner). More lists: all memory challenges · memory games.