Geological Time Scale Calculator
Look up any geological era, period, or epoch from millions of years ago.
Find out what was happening on Earth at any point in its 4.5-billion-year history.
The Geological Time Scale Earth is approximately 4.54 billion years old. Geologists divide this vast span of time into a hierarchy: Eons (largest), Eras, Periods, and Epochs (smallest). Each division is defined by major events in Earth’s history — mass extinctions, tectonic changes, climate shifts, and the evolution or disappearance of key life forms.
The Four Eons Hadean (4,540-4,000 Ma): Earth’s formation, no life, molten surface. Archean (4,000-2,500 Ma): First single-celled life appears, atmosphere has no oxygen. Proterozoic (2,500-541 Ma): Oxygen accumulates, first multicellular life. Phanerozoic (541 Ma-present): All complex visible life, the eon we live in.
Eras of the Phanerozoic Paleozoic (541-252 Ma): “Age of Fishes and Amphibians.” Ends with the Permian Extinction — the largest mass extinction in history, killing 96% of marine species. Mesozoic (252-66 Ma): “Age of Reptiles.” Dinosaurs dominate. Ends with the Cretaceous-Paleogene extinction event (asteroid impact, 66 Ma). Cenozoic (66 Ma-present): “Age of Mammals.” Humans appear in the last 0.3 Ma.
Key Periods Cambrian (541-485 Ma): Explosion of complex life — most major animal phyla appear. Carboniferous (359-299 Ma): Giant forests, formation of coal deposits. Jurassic (201-145 Ma): Peak of dinosaur diversity. Quaternary (2.6 Ma-present): Ice ages, human evolution.
Ma = Millions of Years Ago Geologists use “Ma” (mega-annum) as the standard abbreviation for millions of years ago. “Ka” means thousands of years ago. “Ga” means billions (giga-annum).
How Geological Time Is Determined Radiometric dating (measuring the decay of radioactive isotopes) provides absolute ages. Stratigraphy (studying rock layers) establishes relative ages. Fossil correlation links rock layers across different locations. Index fossils — species that existed for a short time but spread widely — are particularly useful for dating.