geologic time scale
geology The time scale, spanning millions of years, deduced from the records of rocks and their embraced fossils. The listing in Table 19 gives an indication of the current consensus, including estimates of starting and finishing times, in years before the present, for each named period and an indication of biological evolution (and resulting extinctions)
| Eon Era Period Epoch | Years BP | |
|---|---|---|
| 4 550 000 000 | ||
| Hadean | ||
| 3 500 000 000 | ||
| Pre-Cambrian | ||
| Eozoic | First cellular life - marine cyanobacteria | |
| 2 450 000 000 | ||
| Archaeozoic | Green algae, freshwater cyanobacteria | |
| 1 500 000 000 | ||
| Proterozoic | Terrestrial algae and fungi | |
| 570 000 000 | ||
| Phanerozoic | ||
| Palaeozoic | ||
| Cambrian | Fishes | |
| 508 000 000 | ||
| Ordovician | Spore-producing plants | |
| 436 000 000 | ||
| Silurian | Vascular plants, centi/millipedes, amphibia | |
| 404 000 000 | ||
| Devonian | Seed plants, non-flying insects, spiders | |
| 363 000 000 | ||
| Carboniferous | ||
| Mississippian | Flying insects | |
| 312 000 000 | ||
| Pennsylvanian | Reptiles | |
| 288 000 000 | ||
| Permian | ||
| 244 000 000 | Extinction of trilobites, rugose corals, etc. | |
| Mesozoic | ||
| Triassic | Mammals | |
| 208 000 000 | Extinction of much marine life | |
| Jurassic | Birds | |
| 145 000 000 | ||
| Cretaceous (‘K’) | Flowering plants | |
| 65 000 000 | Extinction of dinosaurs, ichthyosaurs, etc. | |
| Cenozoic | ||
| Tertiary (‘T’) | ||
| Palaeocene | ||
| 57 500 000 | ||
| Eocene | Bats, horses, and whales | |
| 36 000 000 | ||
| Oligocene | ||
| 23 500 000 | ||
| Miocene | Many mammals of modern appearance | |
| 5 000 000 | ||
| Pliocene | First Homo | |
| 1 800 000 | ||
| Quaternary | ||
| Pleistocene | First Homo sapiens | |
| 11 000 | ||
| Holocene | ||
| Today |
While well established and widely accepted, the scale undergoes continual revision and subdivision, as a result of on-going research and reinterpretation, and does not exist in a universally accepted detailed form.
The more recent the time, the more detailed the scale becomes, from a billion (109) years at the most distant subdivision to barely 10 000 at the most recent. The various subdivisions - eon, era, period, epoch, and age (then sub-age) - are the geologic time units. The eon is nominally a billion years, actually a half to two billion; virtually the whole of the detailed time scale lies in the one (current) eon, the Phanerozoic. The other units have no nominal size.
The period prior to the Cambrian, representing 80% of the total time of Earth's existence, is subject to much variety in its division and subdivision. Initially it was collectively the Pre-Cambrian, one undifferentiated period bereft of discernible markers of life or of geological activity. Increasing sophistication of techniques in recent years has changed matters, and progressively this huge span of time is being subdivided, but its subdivisional names are erratic; the above gives but one example, another example is shown in Table 20.
| Eon Era Period | Years BP |
|---|---|
| 4 550 000 000 | |
| Priscoan | |
| 4 000 000 000 | |
| Archaean | |
| 2 500 000 000 | |
| Proterozoic | |
| 570 000 000 | |
| Phanerozoic | |
| Palaeozoic | |
| Cambrian |
