Topic
Earth History & Geology
Deep time, changing continents, mass extinctions, dating evidence, and the chemical history of Earth.
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- Geological Time OverviewEarth history is 4.54 billion years long, but the part most familiar from fossils occupies only a thin recent slice. The Phanerozoic Eon — abundant visible animal life — begins only 538.8 million years ago, roughly the last 12% of Earth's history.
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- How We Know the Age of EarthEarth is about 4.54 billion years old. The important part is not memorizing the number; it is understanding why we trust it.
- Radiometric DatingRadiometric dating turns radioactive decay into a clock. If a parent isotope decays with constant decay constant \(\lambda\), then
- Great Oxidation EventFor much of early Earth history, the atmosphere contained almost no free oxygen. Photosynthetic microbes were producing oxygen long before it accumulated in the air, because oxygen was rapidly consumed by sinks — reduced volcanic gases, minerals, and especially dissolved iron in the oceans.
- Cambrian ExplosionThe Cambrian explosion is the interval early in the Cambrian when the animal fossil record suddenly becomes much richer and many major bodyplan lineages become recognizable. The current international boundary for the start of the Cambrian is 538.8 ± 0.6 million years ago.
- The Great DyingThe Permian–Triassic extinction, about 251.9 million years ago, was the most severe known mass extinction. Exact percentages depend on how the fossil record is analyzed, but estimates commonly put marine species loss above 80%, with catastrophic disruption on land as well.
- Pangaea and Continental DriftPangaea was the supercontinent that assembled by the late Paleozoic and contained most of Earth's continental land. It began breaking apart roughly 200 million years ago, eventually producing today's Atlantic Ocean and continental arrangement.
- End-Cretaceous ExtinctionAbout 66 million years ago, the Cretaceous ended in a mass extinction that eliminated all nonavian dinosaurs along with ammonites and many marine plankton groups. Roughly threequarters of species are estimated to have disappeared.