The geologic time scale
The geologic time scale is the division of the Earth's 4.54 billion years of history into units of five ranks. Each eon contains eras, each era periods, and so on down to epochs and ages1. The scale has four eons. The three oldest, the Hadean, the Archean and the Proterozoic, cover 4 billion years. The fourth, the Phanerozoic, began 539 million years ago and is still going on1. Its name means "visible life": its rocks are rich in fossils of animals and plants visible to the naked eye.
Stratigraphy is the study of rock layers and their order. The International Commission on Stratigraphy sets the boundaries of the scale's units2. In a stack of rock layers the oldest lie at the bottom, so the start of a unit of time corresponds to the base of a layer. To define that start, the Commission's specialists choose an outcrop, a place where rock is exposed at the surface, and pick a single layer there, often the lowest in which the fossils of a given species are found3. Geologists everywhere then place the start of the unit by comparing the layers of their own region with this reference layer. They call it the "golden spike", and a metal spike marks it in the rock3. Stratigraphers began defining boundaries this way in the 1960s2.
In September 2018, 71 of the 102 ages of the Phanerozoic had their golden spike2. Lucas, who made this count, faults the specialists for often placing the reference layer at an arbitrary point in the gradual evolution of a microfossil, where one form shades imperceptibly into the next2. He also faults the method for starting each era or period with its first age, which in his view diminishes the significance of the boundaries of the large units2.
The Holocene is the current epoch. The International Union of Geological Sciences, which confirms the Commission's decisions, ratified its division into three ages in 20184. The start of each corresponds to an abrupt change of climate, which geologists identified in cylinders of ice drilled in Greenland for the first two, and in a stalagmite from a cave in India for the third4.
The word Anthropocene refers to the period in which human activity transforms the planet. A working group of the Commission proposed making it a new epoch of the scale5. In 2023 Finney and Gibbard argued for treating it instead as a geological event, that is, a transformation spread out in time with no single starting date6. In 2024 the Subcommission on Quaternary Stratigraphy voted down the proposal to make the Anthropocene an epoch5.
In 2025 Harper and colleagues defended the golden-spike method against the criticisms it has received, including those of Lucas3. They point out that the reference layer is chosen where several markers coincide, fossil or not, and that thanks to it geologists worldwide share the same boundaries, precise and stable3.
References for this introduction
- Cohen et al. (2013), Episodes · The ICS International Chronostratigraphic Chartdoi:10.18814/epiiugs/2013/v36i3/002
- Lucas (2018), Frontiers in Earth Science · The GSSP Method of Chronostratigraphy: A Critical Reviewdoi:10.3389/feart.2018.00191
- Harper et al. (2025), Palaeogeography, Palaeoclimatology, Palaeoecology · Stability and precision in chronostratigraphic definition: The Global Boundary Stratotype Section and Point (GSSP) is the solutiondoi:10.1016/j.palaeo.2025.113515
- Walker et al. (2018), Episodes · Formal ratification of the subdivision of the Holocene Series/Epoch (Quaternary System/Period): two new Global Boundary Stratotype Sections and Points (GSSPs) and three new stages/subseriesdoi:10.18814/epiiugs/2018/018016
- Witze (2024), Nature · It’s final: the Anthropocene is not an epoch, despite protest over votedoi:10.1038/d41586-024-00868-1
- Finney & Gibbard (2023), Journal of Quaternary Science · The Humanities are invited to the Anthropocene Event but not to the Anthropocene Series/Epochdoi:10.1002/jqs.3520
50 units, from the widest division to the narrowest
A period boundary is a decision before it is a date. Where the decision and the measurement do not agree, the entry for the unit gives the reason.
Two ways of fixing a boundary
The first method consists in designating a place. Geologists choose an outcrop somewhere in the world, then a determined layer within that outcrop, and establish that the period begins at that precise point. That layer becomes the reference for the whole planet, and at many such sites a metal marker is sealed into the rock, whence the name golden spike. The consequence is less immediate: the boundary is a place and not a value. If a new dating corrects the age of the layer, the figure displayed changes and the boundary remains where it is.
The second method exists because the first presupposes a condition: the same layer must be identifiable at the other end of the globe. Fossils make that correlation possible; in their absence there is no shared marker and therefore no layer to designate. For the oldest times a value is accordingly adopted by convention: one billion years, two billion five hundred million. Those values are round because no event imposes them, and the relation is reversed: here it is the number that constitutes the boundary, so that a new vote moves it permanently. A boundary fixed in rock may itself be revised: in 2009 the base of the Quaternary was transferred from one outcrop to another, and the Pleistocene gained three quarters of a million years with no discovery involved.
- golden spike20
- A layer of rock chosen as the reference, at one precise place on the globe.
- spike still to drive2
- The division is accepted, but the reference cliff has not been chosen. The figure will change.
- chosen number12
- An age settled in common, for want of a recognisable layer. Nothing in the rock marks it.
- measured age2
- A date obtained by measurement, not an official boundary. A better measurement will move it.
- today1
- The chart leaves this boundary open.
20 boundaries have their golden spike, 12 are chosen numbers, 2 are still waiting for their cliff. 50 units and 37 boundaries: that is expected, since each boundary serves the two units it separates.
Hadean
Eon
Initial eon of the history of the Earth, covering five hundred million years of which no rock is preserved. Its name is formed on Hades, and it is the only division of the geological time scale defined by the absence of the archives that should record it.
- 4.54 Gabasemeasured age
Age of Earth's accretion, measured on meteorites: no terrestrial rock is that old.
- 4.03 Gatopmeasured age
Follows the oldest dated rock, and moves back with every older find.
Archean
Eon
Eon covering a billion and a half years during which the planet acquires its continents, its oceans, its magnetic field and life, and whose documentation is extremely sparse.
- 4.03 Gabasemeasured age
- 2.5 Gatopchosen number
A round number, agreed because nothing in the record singles out the passage.
01
Eoarchean
Era
Era of the dawn, covering the first four hundred million years from which any rock survives. Its documentation is sparse and contested, and concerns the oldest traces of life so far proposed.
- 4.03 Gabasemeasured age
- 3.6 Gatopchosen number
A round number cutting the Archean into four roughly equal parts. No event marks it.
02
Paleoarchean
Era
Era of four hundred million years in which life ceases to be a contested trace and becomes a macroscopic structure accessible to direct observation.
- 3.6 Gabasechosen number
- 3.2 Gatopchosen number
A convenience: nothing in the rock marks it.
03
Mesoarchean
Era
Era in which the crust of the Earth may begin to operate under the present regime, one plate descending beneath another and driving the processes that build continents.
- 3.2 Gabasechosen number
- 2.8 Gatopchosen number
A convenience too: the change it is meant to frame took hundreds of millions of years.
04
Neoarchean
Era
Era of three hundred million years during which oxygen is produced by photosynthesis but consumed as fast as it appears, the ocean absorbing the entire output.
- 2.8 Gabasechosen number
- 2.5 Gatopchosen number
Proterozoic
Eon
Longest eon of the geological time scale, covering two billion years during which oxygen establishes itself in the atmosphere, the eukaryotic cell appears and the planet undergoes several global glaciations.
- 2.5 Gabasechosen number
- 539 Matopgolden spike
In Newfoundland, placed on the first appearance of a burrow rather than of a body.
05
Paleoproterozoic
Era
Era of nine hundred million years opened by the most important chemical change in the history of the planet: the accumulation of oxygen in the atmosphere, toxic to almost every organism then present.
- 2.5 Gabasechosen number
- 1.6 Gatopchosen number
A decision: no event falls here.
Siderian
Period
Period in which oxygen becomes durably established in the atmosphere and the planet, deprived of its methane greenhouse, undergoes its first global glaciations.
- 2.5 Gabasechosen number
- 2.3 Gatopchosen number
A round number: the oxygenation it frames is a process, not an instant.
Rhyacian
Period
Period marked by a major excursion of the carbon cycle, by the largest known magmatic intrusion and by the first macroscopic fossils whose biological origin is debated.
- 2.3 Gabasechosen number
- 2.05 Gatopchosen number
Convention. The events on either side are dated by their rocks, not by this line.
Orosirian
Period
Period of major continental collisions, in which the first mappable supercontinent forms and one cell acquires another permanently.
- 2.05 Gabasechosen number
- 1.8 Gatopchosen number
Convention, and it marks nothing.
Statherian
Period
Period from which the long stability that follows proceeds: continents consolidate, oxygen settles at a low level, and the deep sea passes into a sulfidic regime for a billion years.
- 1.8 Gabasechosen number
- 1.6 Gatopchosen number
06
Mesoproterozoic
Era
Era of six hundred million years characterised by the stability of its geochemical archives. It occupies the centre of the interval the literature designates as the boring billion.
- 1.6 Gabasechosen number
- 1 Gatopchosen number
The round one billion years.
Calymmian
Period
Period in which organisms attain a size visible to the naked eye, under an atmosphere containing at most a thousandth of present-day oxygen.
- 1.6 Gabasechosen number
- 1.4 Gatopchosen number
A decision. Behind this round number there is nothing at all.
Ectasian
Period
Period of two hundred million years almost devoid of recorded events, to which the appearance of sexual reproduction is generally referred.
- 1.4 Gabasechosen number
- 1.2 Gatopchosen number
Fixed on the chart, not read in the rock.
Stenian
Period
Period of the narrow orogenic belts that welded Rodinia together, and of the oldest fossil assignable to a group still living.
- 1.2 Gabasechosen number
- 1 Gatopchosen number
07
Neoproterozoic
Era
Era of four hundred and sixty million years in which ice reaches the tropics more than once, a supercontinent breaks apart and the first animals appear.
- 1 Gabasechosen number
- 539 Matopgolden spike
Tonian
Period
Opening period of the Neoproterozoic, marked by the break-up of Rodinia, by the second rise of oxygen and by the first direct evidence of predation between cells.
- 1 Gabasechosen number
- 720 Matopchosen number
An age chosen to catch the onset of the Sturtian glaciation: it moves as that dating improves.
Cryogenian
Period
Period of the two most extensive glaciations in the history of the planet, in which eukaryotic algae displace bacteria in the primary production of the open ocean.
- 720 Mabasechosen number
- 635 Matopgolden spike
A defined section, at the top of the Marinoan glacial deposits.
Ediacaran
Period
First period added to the chart in a hundred and twenty-five years, and the only Precambrian period whose base is defined by a section rather than a number. It carries the appearance and then the disappearance of large soft-bodied organisms.
- 635 Mabasegolden spike
- 539 Matopgolden spike
Phanerozoic
Eon
Eon of visible life, covering the last twelve per cent of the history of the Earth. Every organism commonly displayed in collections belongs to it, which explains the concentration of entries on this timeline at its right-hand end.
- 539 Mabasegolden spike
- 2026toptoday
The present, left open by the chart: whether the last few centuries have earned a division of their own is still argued.
08
Paleozoic
Era
Era of two hundred and eighty-seven million years, running from the first shelled animals to the most severe extinction in the fossil record. The move onto land occurs within it and is not reversed.
- 539 Mabasegolden spike
- 252 Matopgolden spike
Among the most precisely dated boundaries of the Phanerozoic: the extinction interval is bracketed by uranium and lead to within a few tens of thousands of years.
Cambrian
Period
Period in which nearly all animal body plans appear, later radiations having explored those plans without adding new ones.
- 539 Mabasegolden spike
- 487 Matopgolden spike
Ordovician
Period
Period of major marine diversification, in which plants reach the continents, closed by the first of the five great extinctions.
- 487 Mabasegolden spike
- 443 Matopgolden spike
Silurian
Period
Short period marked by the acquisition of jaws in vertebrates, by the first fossils of terrestrial animals and by the establishment of the ozone layer.
- 443 Mabasegolden spike
- 420 Matopgolden spike
Devonian
Period
Period in which plants acquire wood and tree stature, vertebrates reach dry land, and reefs attain their greatest extent.
- 420 Mabasegolden spike
- 359 Matopgolden spike
Carboniferous
Period
Period in which carbon burial raises atmospheric oxygen to the highest level of the last half billion years, and in which the amniotic egg frees vertebrates from the aquatic environment.
- 359 Mabasegolden spike
- 299 Matopgolden spike
Permian
Period
Period of the single continent, dominated on land by the near relatives of mammals, and closed by the most severe extinction in the history of life.
- 299 Mabasegolden spike
- 252 Matopgolden spike
09
Mesozoic
Era
Era of one hundred and eighty-six million years lying between two mass extinctions, the only division on the chart both of whose limits correspond to a biological crisis.
- 252 Mabasegolden spike
- 66 Matopgolden spike
The Chicxulub line, an iridium layer traceable worldwide: one of the sharpest boundaries on the chart.
Triassic
Period
Period of recovery after the end-Permian extinction, in which dinosaurs and mammals appear, and closed by a further volcanic province.
- 252 Mabasegolden spike
- 201 Matopgolden spike
Dated by uranium and lead in the basalts of the Central Atlantic province, which is why it is known to better than a hundred thousand years.
Jurassic
Period
Period of the break-up of the supercontinent, in which the largest land animals in history reach their maximum size and the first birds appear.
- 201 Mabasegolden spike
- 143 Matopspike still to drive
One of the few Phanerozoic boundaries still without a spike; its estimated age has moved by several million years.
Cretaceous
Period
Longest period of the Phanerozoic, in which flowering plants spread, climate reaches its thermal maximum and an impact closes the Mesozoic era.
- 143 Mabasespike still to drive
- 66 Matopgolden spike
10
Cenozoic
Era
Era covering the last sixty-six million years: mammals occupy a world emptied by an impact, grasslands spread, ice sheets form, and a lineage of primates appears within it.
- 66 Mabasegolden spike
- 2026toptoday
Paleogene
Period
Period of forty-three million years in which the planet passes from a greenhouse regime almost without ice to one with a polar ice sheet, while mammals occupy the niches vacated by the impact.
- 66 Mabasegolden spike
- 23 Matopgolden spike
Paleocene
Epoch
Epoch of ten million years in a world emptied by the impact, in which surviving lineages increase rapidly in size, and closed by the most abrupt carbon injection of the Cenozoic.
- 66 Mabasegolden spike
- 56 Matopgolden spike
Placed on the carbon isotope spike that marks the Palaeocene-Eocene warming.
Eocene
Epoch
Warmest epoch of the Cenozoic, in which bats, cetaceans and primates appear, and closed by the glaciation of Antarctica.
- 56 Mabasegolden spike
- 33.9 Matopgolden spike
Oligocene
Epoch
Epoch in which Antarctica carries an ice sheet close to its present size while the rest of the world remains warm, a configuration that climate models reproduce only in part.
- 33.9 Mabasegolden spike
- 23 Matopgolden spike
Neogene
Period
Period in which grasslands spread across the continents, the Mediterranean dries out and refills, and a lineage of African apes acquires bipedalism.
- 23 Mabasegolden spike
- 2.6 Matopgolden spike
A spike in Sicily, aligned with Marine Isotope Stage 103. Lowered from 1.8 Ma by a ratification in 2009.
Miocene
Epoch
Longest epoch of the Neogene, marked by the uplift of the Tibetan plateau and the strengthening of the monsoon, by the diversity of apes, and closed by the drying of the Mediterranean.
- 23 Mabasegolden spike
- 5.3 Matopgolden spike
Pliocene
Epoch
Last warm epoch before the ice ages, in which bipedalism is attested by footprints, the oldest stone tools are knapped, and the two Americas are joined.
- 5.3 Mabasegolden spike
- 2.6 Matopgolden spike
Quaternary
Period
Period of the ice ages and the only period in which Homo sapiens exists: ice sheets advance and retreat dozens of times in step with orbital variation.
- 2.6 Mabasegolden spike
- 2026toptoday
Pleistocene
Epoch
Epoch of the ice ages, covering two and a half million years of advance and retreat of the ice sheets, and traversed by at least three distinct human populations.
- 2.6 Mabasegolden spike
- 11.7 katopgolden spike
A layer counted out in the NGRIP2 Greenland ice core, 11,700 years before 2000, dated to the year, which no rock section can offer.
Gelasian
Age
Opening age of the Quaternary, attached to that period by a vote in 2009, in which the northern hemisphere ice sheets become permanent and hominins already leave Africa.
- 2.6 Mabasegolden spike
- 1.8 Matopgolden spike
The Vrica section in Calabria, which carried the base of the Quaternary from 1985 until 2009.
Calabrian
Age
Age defined by a golden spike previously assigned to the base of the Quaternary, and reassigned in 2011 after that base was lowered.
- 1.8 Mabasegolden spike
- 774 katopgolden spike
The Chiba section, on Japan's Boso Peninsula, ratified in 2020 after a long contest with European candidates.
Chibanian
Age
Age defined by a section on the Boso peninsula, in Japan, the only unit of the geological time scale named after a Japanese locality, and in which Homo sapiens appears.
- 774 kabasegolden spike
- 129 katopspike still to drive
Alone among the Pleistocene subseries, this one has no spike: the figure is fixed by convention at the onset of the last interglacial.
Late Pleistocene
Age
Last subdivision of the Pleistocene, covering the final hundred and twenty millennia of the glaciations, in which Homo sapiens leaves Africa, reaches the last habitable continents and produces the first narrative images.
- 129 kabasespike still to drive
- 11.7 katopgolden spike
Holocene
Epoch
Current epoch, opened 11,700 years ago, characterised by unusual climatic stability and containing the appearance of agriculture, cities and writing.
- 11.7 kabasegolden spike
- 2026toptoday
Greenlandian
Age
First age of the Holocene, defined in a Greenland ice core, in which warming becomes established, agriculture begins and the Sahara carries grassland vegetation.
- 11.7 kabasegolden spike
- 6,237 BCEtopgolden spike
A layer in the NGRIP1 core, 8,236 years before 2000; the 8.2 ka cooling under it is a regional signal read as global.
Northgrippian
Age
Second age of the Holocene, opened by the sharpest climatic oscillation of the last ten millennia, and in which lactase persistence spreads.
- 6,237 BCEbasegolden spike
- 2,251 BCEtopgolden spike
A stalagmite in Mawmluh Cave, Meghalaya, 4,250 years before 2000. Ratified in 2018 and contested at once.
Meghalayan
Age
Third age of the Holocene, the briefest on the geological time scale and the only one defined in a stalagmite. It begins with a drought episode and extends to the present.
- 2,251 BCEbasegolden spike
- 2026toptoday
Where these figures come from
- Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
- Head, M. J. (2019). Formal subdivision of the Quaternary System/Period: Present status and future directions. Quaternary International 500, 32–51.
- Walker, M., Head, M. J., Berkelhammer, M., et al. (2018). Formal ratification of the subdivision of the Holocene Series/Epoch: two new GSSPs and three new stages/subseries. Episodes 41(4), 213–223.
These three publications underwrite the chart itself. The references proper to each unit appear on its own entry, and all fifty entries are linked above.