The geologic time scale: eons, eras, periods and epochs
Earth is about 4600 million years old. Scientists divide this long history into eons. Counting in millions of years ago (Ma): the Hadean eon 4600–4000, the Archean 4000–2500, the Proterozoic 2500–541 and the Phanerozoic 541 to today. The Phanerozoic eon has three eras: the Paleozoic 541–252, the Mesozoic 252–66 and the Cenozoic from 66 to today. The Mesozoic is the age of the dinosaurs; the non-bird dinosaurs died out in the mass extinction about 66 million years ago. The dates are the best current estimates, and they can change a little when new evidence appears. On a model timeline where 1 cm stands for 500 million years, the whole of Earth’s history is 4600 ÷ 500 = 9.2 cm. The Phanerozoic is only 541 ÷ 500 ≈ 1.1 cm of it, and the time since the dinosaurs’ extinction is 66 ÷ 500 = 0.13 cm, about 1.3 mm.
| eon | from – to (million years ago) | length on 1 cm = 500 million |
|---|---|---|
| Hadean | 4600 – 4000 | 1.2 cm |
| Archean | 4000 – 2500 | 3.0 cm |
| Proterozoic | 2500 – 541 | 3.9 cm |
| Phanerozoic | 541 – today | 1.1 cm |
Timeline strip, with a grown-up. Cut a strip of paper 9.2 cm long (1 cm = 500 million years). Mark where each eon begins using the table and colour the four eons. Mark 66 million years ago with a pin: how close to the end is it? Write one sentence about how short human history is on this strip.