GEOLOGIC TIME

Stratigraphic terms—Lower, Middle, and Upper—are capped. Descriptive time terms—early, middle, and late—are lowercased and are not hyphenated as compound modifiers:

            Lower Jurassic, early Devonian times, Triassic-period rocks, Niagara epoch, Cenozoic era

            Pleistocene or Ice Age or Glacial epoch

When citing an era, use the earliest date first: The Cretaceous period lasted from 145 million to 66 million years ago.

Geologic periods are defined by stratigraphic boundaries. Geologists reference global standard stratotype sections and points (GSSPs), internationally accepted representative formations that are dated to establish boundaries between periods. The International Commission on Stratigraphy (ICS) has maintained the GSSP register since 1977.

A GSSP ratified by the ICS will generally remain valid for at least a decade. As methods of determining rock formations’ ages continually improve, numerical age estimates for geologic periods may be revised more often. The earliest time periods are the least defined and most likely to change with further research.

National Geographic’s numeric age estimates for geologic periods rely on the International Chronostratigraphic Chart, maintained by the ICS. As of 2021, many—but not all—of the chart’s numerical age estimates came from The Geologic Time Scale 2012, by Gradstein, Ogg, Schmitz, and Ogg, Cambridge University Press. 

The following table is based on the ICS’s International Chronostratigraphic Chart, v2021/07:

Phanerozoic eon

(541.0 mya - present)

(mya = million years ago)

Cenozoic era

(66.0 mya - present)

Quaternary period (2.58 mya - present)

            Holocene epoch (11,700 yrs ago - present)

            Pleistocene epoch (2.58 mya - 11,700 yrs ago)

Neogene period (23.03 - 2.58 mya)

Pliocene epoch (5.33 - 2.58 mya)

Miocene epoch (23.03 - 5.33 mya)

Paleogene period (66.0 - 23.03 mya)

Oligocene epoch (33.9 - 23.03 mya)

Eocene epoch (56.0 - 33.9 mya)

Paleocene epoch (66.0 to 56.0 mya)

Mesozoic era

(251.9 - 66.0 mya)

Cretaceous period (145.0 - 66.0 mya)

Jurassic period (201.3 - 145.0 mya)

Triassic period (251.9 - 201.3 mya)

Paleozoic era

(541.0 - 251.9 mya)

Permian period (298.9 - 251.9 mya)

Carboniferous period (358.9 - 298.9 mya)

Pennsylvanian subperiod (323.2 - 298.9 mya)

Mississippian subperiod (358.9 - 323.2 mya)

Devonian period (419.2 - 358.9 mya)

Silurian period (443.8 - 419.2 mya)

Ordovician period (485.4 - 443.8 mya)

Cambrian period (541.0 - 485.4 mya)

Precambrian time

(~4,600 - 541.0 mya)

Proterozoic eon

(2,500 - 541.0 mya)

            Neoproterozoic era (1000 - 541.0 mya)

                        Ediacaran period (~635 - 541.0 mya)

                        Cryogenian period (~720 - ~635 mya)

                        Tonian period (1,000 - ~720 mya)

Mesoproterozoic era (1,600 - 1,000 mya)

            Stenian period (1,200 - 1,000 mya)

            Ectasian period (1,400 - 1,200 mya)

            Calymmian period (1,600 - 1,400 mya)

Paleoproterozoic era (2,500 - 1,600 mya)

            Statherian period (1,800 - 1,600 mya)

            Orosirian period (2,050 - 1,800 mya)

            Rhyacian period (2,300 - 2,050 mya)

            Siderian period (2,500 - 2,300 mya)

Archean eon

(4,000 - 2,500 mya)

            Neoarchean era (2,800 - 2,500 mya)

            Mesoarchean era (3,200 - 2,800 mya)

            Paleoarchean era (3,600 - 3,200 mya)

            Eoarchean era (4,000 - 3,600 mya)

Hadean eon

(~4,600 - 4,000 mya)

The ICS treats the Hadean eon as informal. The Hadean’s onset corresponds with the formation of Earth, which the ICS rounds to 4.6 billion years ago.

Earth’s age is commonly cited as 4.54±0.05 billion years (±1%). This range comes from the 1991 book The Age of the Earth, by Dalrymple, Stanford University Press. For an overview, see Dalrymple 2001 and Tera 1980. Today’s best age estimate for the solar system’s oldest objects—and the age of the sun and protoplanetary disk—is 4.5673±0.00016 billion years, published by Connelly, et al., 2017.

The Anthropocene epoch—a proposed geologic period defined by the environmental impact of human activity—remains unofficial. An ICS working group voted to endorse the Anthropocene concept; work on that proposal will last through at least early 2023, when the ICS will decide whether to adopt the Anthropocene as a formal geologic period. Meanwhile, the term can be applied per the definition in Merriam-Webster.