A Framework for Deep Time
The geologic time scale is the framework geologists and paleontologists use to describe and communicate the timing and relationships of events in Earth's history. It is calibrated using radiometric dates, reference stratigraphic sections, and globally recognized fossils. The current version is maintained by the International Commission on Stratigraphy (ICS) and updated regularly as new data emerge.
Time in geology is measured in millions of years (Ma) and billions of years (Ga). The entire human written record occupies the last approximately 5,000 years — less than one ten-thousandth of one percent of Earth's history. The Cambrian explosion, often described as one of the most significant events in the history of animal life, occurred 541 million years ago — nearly 100,000 times further back than the earliest human writing.
Eons & Major Transitions
Hadean
4.5 – 4.0 GaNo confirmed fossil record. Earth largely molten initially.
- Formation of Earth
- Moon-forming impact
- Late Heavy Bombardment
Archean
4.0 – 2.5 GaLife exists as prokaryotes; atmosphere lacks free oxygen.
- Oldest microbial fossils (~3.5 Ga)
- Stromatolites
- Emergence of oxygenic photosynthesis
Proterozoic
2.5 – 541 MaAtmospheric oxygen rises; multicellular life appears in the Ediacaran.
- Great Oxidation Event (~2.4 Ga)
- First eukaryotes (~1.8 Ga)
- Snowball Earth events
- Ediacaran biota (~575–541 Ma)
Phanerozoic — Paleozoic
541 – 252 MaExplosion of animal phyla; marine ecosystems dominate early; land ecosystems develop.
- Cambrian explosion (~541 Ma)
- First vertebrates
- Colonization of land by plants and animals
- First forests
- End-Ordovician extinction (~444 Ma)
- Late Devonian extinction (~375 Ma)
- End-Permian extinction (~252 Ma) — largest ever
Phanerozoic — Mesozoic
252 – 66 MaThe age of dinosaurs, marine reptiles, and flying pterosaurs.
- Recovery from end-Permian
- Rise of dinosaurs (~230 Ma)
- First mammals
- First flowering plants (~130 Ma)
- First birds
- K-Pg mass extinction (~66 Ma)
Phanerozoic — Cenozoic
66 – 0 MaThe age of mammals and modern-style ecosystems.
- Diversification of mammals
- Rise of primates
- Grasslands spread globally
- Antarctic ice sheet forms
- Pleistocene ice ages
- Hominin evolution
- Holocene (~11.7 ka–Present)
A Note on Scale
The timeline above is not drawn proportionally to time. If it were, the Hadean and Archean eons — spanning nearly 2 billion years — would dwarf all subsequent entries. The Phanerozoic eon (541 Ma–present), which contains all animal life, represents only about 12% of Earth's total history. Most familiar paleontological content — dinosaurs, ammonites, trilobites — fits into this final 12%.
Related Topics
Frequently Asked Questions
The geologic time scale is defined by international stratigraphic committees — primarily the International Commission on Stratigraphy (ICS) — based on globally recognized rock sequences called GSSPs (Global Boundary Stratotype Sections and Points), or "golden spikes." Each boundary is defined by a specific chemical, biological, or physical marker in a rock outcrop that can be recognized worldwide.
Geologists use several methods. Relative dating — using the principle of superposition and index fossils — establishes the sequence of events. Absolute (radiometric) dating uses the known decay rates of radioactive isotopes (such as uranium-238, potassium-40, and carbon-14) to determine numerical ages. Different isotope systems are used for different time ranges: uranium-lead dating works for ancient rocks (billions of years); potassium-argon for tens of millions of years; radiocarbon for only the last ~50,000 years.
These are hierarchical subdivisions of geological time. Eons are the largest units (e.g., Phanerozoic). Each eon is divided into eras (e.g., Mesozoic). Each era is divided into periods (e.g., Cretaceous). Periods are divided into epochs (e.g., Maastrichtian). The divisions are not equal in duration — they are defined by biological and geological events, not by uniform time intervals.
"Ma" stands for mega-annum, meaning one million years. "Ga" stands for giga-annum, meaning one billion years. "ka" stands for kilo-annum, meaning one thousand years. These abbreviations are standard in geological literature and refer to years before present.