Earth system
Geological time
The time scale has two vocabularies — one for rock, one for time — and its boundaries are defined by physical reference points, not by the numbers attached to them.
Chronostratigraphic units are bodies of rock; geochronologic units are the intervals of time they represent. System pairs with period, series with epoch, stage with age.[1, 2]
A stage is defined by Global Boundary Stratotype Sections and Points — real sections in real rock — and those boundary stratotypes fix the boundaries of every higher rank built from them. Numerical ages calibrate the scale; they do not define it.[1, 2]
Those numbers come from radiometric dating, which records the last time a sample was reset rather than the age of its material. That limitation is precisely why the definitions live in rock sections.[1, 2]
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Reading deep time
The geological time scale has two parallel vocabularies, and confusing them is the commonest error in writing about Earth history. One names bodies of rock; the other names lengths of time.
Where the Holocene begins
The epoch we live in has a formally defined base — and the way that boundary is defined says a great deal about how geological time is standardised.
Dating the Earth
Radiometric dating is precise, well understood, and does not measure what people usually think it measures. Understanding what it actually records explains why the age of the Earth had to come from meteorites.
Eons
The largest divisions, from the formation of the Solar System to the present.
Geological time unit
Hadean
The first eon of Earth history, from the formation of the Solar System to the age of the oldest rocks yet found. Almost no rock record survives from it, which is precisely why its lower boundary is set by meteorites rather than by strata.
Geological time unit
Archean
The eon following the Hadean, whose base is fixed at the age of the oldest well-dated terrestrial rocks. Ancient rocks of this age are found on every continent.
Geological time unit
Proterozoic
The long eon preceding the Phanerozoic, spanning roughly two billion years of Earth history.
Geological time unit
Phanerozoic
The eon of abundant visible life, from the base of the Cambrian to the present. Its subdivisions are defined by physical reference points in rock, not by numerical ages.
Eras of the Phanerozoic
The three divisions of the eon of abundant visible life.
Geological time unit
Paleozoic
The first era of the Phanerozoic, beginning with the Cambrian and ending at the largest mass extinction in the rock record.
Geological time unit
Mesozoic
The middle era of the Phanerozoic, comprising the Triassic, Jurassic and Cretaceous periods.
Geological time unit
Cenozoic
The most recent era of the Phanerozoic, beginning 66 million years ago and continuing to the present.
Periods and epochs
A selection sufficient to demonstrate nesting and adjacency; the corpus deliberately does not reproduce the whole chart.
Geological time unit
Cambrian
The first period of the Paleozoic and of the Phanerozoic. Its base defines the base of the Phanerozoic Eon.
Geological time unit
Cretaceous
The last period of the Mesozoic, ending at the boundary that also ends the era.
Geological time unit
Paleogene
The first period of the Cenozoic, beginning at the boundary that closes the Mesozoic and comprising the Paleocene, Eocene and Oligocene epochs.
Geological time unit
Neogene
The second period of the Cenozoic, spanning the Miocene and Pliocene epochs, between the Paleogene and the Quaternary.
Geological time unit
Quaternary
The current period, beginning 2.58 million years ago; it comprises the Pleistocene and Holocene epochs.
Geological time unit
Pleistocene
The epoch preceding the Holocene, spanning most of the Quaternary.
Geological time unit
Holocene
The current epoch of the geological time scale, spanning roughly the last 11,700 years since the end of the last glacial period.
How time is fixed
The standard, its steward, and the method that calibrates it.
Dataset
International Chronostratigraphic Chart
The standard reference chart of the geological time scale, maintained and periodically revised by the International Commission on Stratigraphy; Lithoen cites specific dated versions.
Institution
International Commission on Stratigraphy
The body of the International Union of Geological Sciences responsible for standardising the international geological time scale, including the International Chronostratigraphic Chart.
Scientific method
Radiometric dating
Determination of numerical ages from the decay of long-lived radioactive isotopes in rocks and minerals. It dates the last time a sample was melted or sufficiently disturbed, which is why the age of a rock and the age of the Earth are different questions.
Hub last updated 2026-09-06. Every entry above is a structured record; the bracketed references link to the entities whose claims support each statement.