By comparing and combining several methods of measuring [a measurement is a high-precision estimate] ages of rocks, minerals, fossils, and organic remains, scientists have derived a time scale spanning Earth's 4.5-billion-year age. Inevitably, recent time is divided more finely and measured more precisely than ancient time. We argue about exact boundaries among geologic eras, periods, and epochs, but overall duration and seqence are in little doubt. Viewed against a span of billions of years, a few million years here and there don't seem very important.
We humans find the span of geologic time nearly incomprehensible. We consider 100 years a long human lifespan, but a million years is 10,000 times longer, and a million years is a trivial period in deep geologic time. Most geologic processes operate too slowly for us to see them work. Such slow, irresistible immensity scares us. Rather than think about that, let's have another beer . . .
| Divisions of Geologic Time | |||||
|---|---|---|---|---|---|
| Begin | End | Era | Period | Epoch | Events |
| -10,000 years | today | Cenozoic | Quaternary | Holocene | Recession of glaciers; extinction of megafauna; dominance of mankind |
| -2 My | -10,000 years | Pleistocene | Glacial episodes | ||
| -5 My | -2 My | Tertiary | Pliocene | Monkeys | |
| -24 My | -5 My | Miocene | Rise of Colorado Plateau; formation of Basin and Range; beginning of mankind | ||
| -37 My | -24 My | Oligocene | (?) | ||
| -58 My | -37 My | Eocene | (?) | ||
| -65 My | -58 My | Paleocene | Recovery from Cretaceous terminal event; ascendance of mammals; rise of Rocky Mountains | ||
| -140 My | -65 My | Mesozoic | Cretaceous | Dominance of dinosaurs; flowering plants; primates; evolution of birds; Rocky Mountains begin to rise; sea level stands at Fall Line; demise of dinosaurs and other large animals at end, probably resulting from Chicxulub impact (Cretaceous terminal event) | |
| -200 My | -140 My | Jurassic | Dinosaurs; beginning of birds; Pangea splits; Atlantic Ocean widens | ||
| -250 My | -200 My | Triassic | Dinosaurs; Pangea begins to split; Atlantic Ocean opens | ||
| -290 My | -250 My | Paleozoic | Permian | Pangea intact; no Atlantic Ocean; demise of many marine animals; evolution of reptiles | |
| -320 My | -290 My | Pennsylvanian | Iapetus Ocean closes; Pangea assembles; coal swamps; reptiles | ||
| -360 My | -320 My | Mississippian | Fern forests; sharks | ||
| -410 My | -360 My | Devonian | Insects; amphibians | ||
| -440 My | -410 My | Silurian | Vascular land plants; air-breating animals | ||
| -500 My | -440 My | Ordovician | Fish; corals | ||
| -600 My | -500 My | Cambrian | Proliferation of multi-cellular life | ||
| -4500 My | -600 My | Pre-Cambrian | Formation of Earth; multiple tectonic cycles; beginnings of life; Iapetus Ocean begins to open near end | ||
This time scale is adapted from several sources, most of which disagree slightly.