Geological Timeline
Earth formed approximately 4.6 billion years ago. Geologists divide this immense span into nested time units: Eons → Eras → Periods → Epochs. Boundaries between units are defined by recognisable events in the rock record — major volcanic episodes, the origin of life, dramatic shifts in atmospheric chemistry, mass extinctions, and evolutionary radiations.
The vast majority of Earth's history belongs to the Precambrian (Hadean + Archean + Proterozoic). The fossil-rich Phanerozoic only begins ~538.8 Ma ago, yet it is the most finely subdivided eon.
Eon Overview
| Years after formation (Ma) | Eon | Start (Ma ago) | End (Ma ago) | Duration | Defining event |
|---|---|---|---|---|---|
| ~0–600 | Hadean | ~4600 | ~4000 | ~600 Myr | Earth's accretion, magma ocean, Moon-forming impact, first liquid water |
| ~600–2100 | Archean | ~4000 | 2500 | ~1500 Myr | Oldest rocks, first life (prokaryotes), early photosynthesis |
| ~2100–4061 | Proterozoic | 2500 | 538.8 | ~1960 Myr | Great Oxidation Event, eukaryotes, Snowball Earth, first multicellular animals |
| ~4061–4600 | Phanerozoic | 538.8 | 0 (present) | ~539 Myr | Cambrian explosion, repeated mass extinctions, land life, humans |
Hadean 4600–4000 Ma
The Hadean is the oldest and least-documented eon — nearly all rocks from this period have been destroyed by subsequent geological activity. The ICS does not yet define formal eras or periods within it. Our knowledge comes primarily from lunar samples, meteorites, and the rare Jack Hills zircon crystals that preserve geochemical signals of liquid water dating back ~4.4 Ga.
| Years after formation (Ma) | Time (Ma ago) | Key event |
|---|---|---|
| ~0 | ~4600 | Earth accretes from the solar nebula; planetesimal impacts build the proto-Earth |
| ~50–100 | ~4550–4500 | Giant Theia impact → Moon forms; iron core differentiation completes |
| ~100–300 | ~4500–4300 | Global magma ocean; thick steam and CO₂ atmosphere; no stable crust |
| ~200–300 | ~4400–4300 | Surface cools below boiling point; first liquid water (Jack Hills zircon δ¹⁸O evidence) |
| ~600–700 | ~4000–3900 | Late Heavy Bombardment (LHB): a contested hypothesis of intense asteroid and comet impacts across the inner Solar System |
Archean 4000–2500 Ma
The Archean is the first eon with a direct rock record. Earth's atmosphere lacked free oxygen; primitive oceans covered much of the surface. Single-celled prokaryotic life originated and thrived during this eon. The Archean is divided into four eras.
| Years after formation (Ma) | Era | Start (Ma ago) | End (Ma ago) | Key events |
|---|---|---|---|---|
| ~600–1000 | Eoarchean | 4000 | 3600 | Oldest surviving rocks (Acasta Gneiss, Canada, ~4.0 Ga); continued meteorite bombardment |
| ~1000–1400 | Paleoarchean | 3600 | 3200 | Earliest contested life evidence (stromatolites ~3.5 Ga); cratons begin forming |
| ~1400–1800 | Mesoarchean | 3200 | 2800 | Stromatolites widespread; early plate tectonics; continental crust growth |
| ~1800–2100 | Neoarchean | 2800 | 2500 | Oxygenic photosynthesis (cyanobacteria) begins accumulating free O₂; prelude to GOE |
Proterozoic 2500–538.8 Ma
The longest eon at ~2 billion years, the Proterozoic saw a dramatic rise in atmospheric oxygen (Great Oxidation Event), the evolution of eukaryotes, multiple global glaciations (Snowball Earth), and the first complex multicellular animals. It is divided into three eras and ten periods.
| Years after formation (Ma) | Era | Period | Start (Ma ago) | End (Ma ago) | Key events |
|---|---|---|---|---|---|
| ~2100–3000 | Paleoproterozoic | Siderian | 2500 | 2300 | Great Oxidation Event (GOE): atmospheric O₂ rises sharply |
| Rhyacian | 2300 | 2050 | Huronian glaciation; banded iron formations deposited worldwide | ||
| Orosirian | 2050 | 1800 | Global orogenies; supercontinent Columbia (Nuna) assembles | ||
| Statherian | 1800 | 1600 | Earliest contested eukaryote fossils; stable cratons widespread | ||
| ~3000–3600 | Mesoproterozoic | Calymmian | 1600 | 1400 | Precursor to supercontinent Rodinia; eukaryote diversification |
| Ectasian | 1400 | 1200 | Early evidence of sexual reproduction; algal diversification | ||
| Stenian | 1200 | 1000 | Supercontinent Rodinia assembles | ||
| ~3600–4061 | Neoproterozoic | Tonian | 1000 | 720 | Rodinia breaks apart; multicellular life diversifies |
| Cryogenian | 720 | 635 | Snowball Earth glaciations (Sturtian and Marinoan events) | ||
| Ediacaran | 635 | 538.8 | Ediacaran biota: first complex multicellular animals; Gondwana assembles |
Phanerozoic 538.8 Ma–present
The Phanerozoic ("visible life") eon is named for its abundant macroscopic fossil record. Though only ~12% of Earth's total history, it is the most finely subdivided eon. It comprises three eras and twelve periods.
| Years after formation (Ma) | Era | Period | Start (Ma ago) | End (Ma ago) | Key events |
|---|---|---|---|---|---|
| ~4061–4348 | Paleozoic | Cambrian | 538.8 | 485.4 | Cambrian Explosion: nearly all major animal body plans appear |
| Ordovician | 485.4 | 443.8 | Peak marine diversity; 1st mass extinction at end (~85% species lost) | ||
| Silurian | 443.8 | 419.2 | First vascular land plants; fish diversification | ||
| Devonian | 419.2 | 358.9 | Age of Fishes; tetrapods colonise land; 2nd mass extinction | ||
| Carboniferous | 358.9 | 298.9 | Vast coal forests (source of today's coal); first reptiles; peak atmospheric O₂ | ||
| Permian | 298.9 | 251.9 | Supercontinent Pangaea; end-Permian extinction: largest in Earth history (~96% marine species) | ||
| ~4348–4534 | Mesozoic | Triassic | 251.9 | 201.4 | Dinosaurs and first mammals appear; Pangaea begins breaking up |
| Jurassic | 201.4 | 145.0 | Dinosaurs dominate; first birds (Archaeopteryx); Atlantic Ocean opens | ||
| Cretaceous | 145.0 | 66.0 | Flowering plants diversify; end-Cretaceous impact → non-avian dinosaurs extinct | ||
| ~4534–4600 | Cenozoic | Paleogene | 66.0 | 23.03 | Rapid mammal radiation; primates appear; India–Asia collision (Himalayas) |
| Neogene | 23.03 | 2.58 | Grasslands spread; hominids evolve; global cooling trend | ||
| Quaternary | 2.58 | 0 (present) | Ice age cycles; Homo sapiens (~300 ka); human civilisation |
Time data based on the International Commission on Stratigraphy (ICS) International Chronostratigraphic Chart v2023/06. The Hadean start date (~4600 Ma) is an estimate; ICS has not formally defined its lower boundary. All times in millions of years ago (Ma).