Below is the full-blown description of the Geological Periods. Rather than breaking up the Periods as even mathematical bins, each Era, Period, Epoch, etc., is always attached to a certain characteristic of the prevalent species at that time. The exact dates of these eras then becomes an academic matter, consequential, but open for debate and refinement without altering the original definition of the Periods.
| EON | ERA | PERIOD | EPOCH | DATES | AGE of | |||
| Phanerozoic | Cenozoic | Quaternary | Holocene | 0-2 | Mammals | Humans | ||
| Pleistocene | ||||||||
| Tertiary | Neogene | Pliocene | 2-5 | |||||
| Miocene | 5-24 | |||||||
| Paleogene | Oligocene | 24-37 | ||||||
| Eocene | 37-58 | |||||||
| Paleocene | 58-66 | Extinction of dinosaurs | ||||||
| Mesozoic | Cretaceous | 66-144 | Reptiles | Flowering plants | ||||
| Jurassic | 144-208 | 1st birds/mammals | ||||||
| Triassic | 208-245 | First Dinosaurs | ||||||
| Paleozoic | Permian | 245-286 | Amphibians | End of trilobites | ||||
| Carboniferous | Pennsylvanian | 286-320 | First reptiles | |||||
| Mississippian | 320-360 | Large primitive trees | ||||||
| Devonian | 360-408 | Fishes | First amphibians | |||||
| Silurian | 408-438 | First land plant fossils | ||||||
| Ordovician | 438-505 | Invertebrates | First Fish | |||||
| Cambrian | 505-570 | 1st shells, trilobites dominant | ||||||
| Proterozoic | Also known as Precambrian | 570-2,500 | 1st Multicelled organisms | |||||
| Archean | 2,500-3,800 | 1st one-celled organisms | ||||||
| Hadean | 3,800-4,600 | Approx age of oldest rocks 3,800 | ||||||
|
Note: Dates are in millions of years
Reference: Graham R. Thompson, Jonathan Turk, Modern Physical Geology, Saunders College Publishing |
Radioactive Dating
When a mineral grain forms, the elements contained are locked in. If it contains radioactive
elements, these will have slowly decayed with time. We can see their 'waste' accumulating in
the rock the longer they have been locked in, giving us an estimate of when the rock formed.
This method works best on igneous rocks. Important radioactive elements (parent-daughter)
include: Argon-Potassium, Rubidium-Strontium, and Uranium-Lead.
For more info click HERE and HERE