Geologic Time Scale

The construction of the Geologic Time Scale was one of the great intellectual achievements of the earth sciences, spanning several centuries and progressing from purely qualitative observations to the precisely calibrated quantitative framework used today.

Early stratigraphic principles (17th–18th centuries). In 1669, the Danish naturalist Nicolaus Steno articulated two foundational principles: the principle of original horizontality (sedimentary layers are deposited in a roughly horizontal orientation) and the principle of superposition (in an undisturbed sequence, older layers lie beneath younger ones). These insights allowed workers to begin recognizing the relative order of rock successions in local areas. James Hutton, writing in the late 18th century, contributed the principle of uniformitarianism—the idea that natural processes operating in the present also operated in the past at broadly comparable rates—providing a philosophical foundation for interpreting deep time.

Fossil succession and the relative time scale (early 19th century). William Smith, an English surveyor and canal builder, demonstrated in 1815 that fossils occur in rocks in a consistent, predictable order. His monumental geological map of England and Wales, and the principle of faunal succession it embodied, gave geologists a powerful tool for correlating rock units across geographically separated areas. Throughout the 19th century, workers in Europe named the major eras and periods largely on the basis of characteristic fossil assemblages: the Cambrian (Adam Sedgwick, 1835), Silurian (Roderick Murchison, 1835), Devonian (Sedgwick and Murchison, 1839), Permian (Murchison, 1841), Triassic (Friedrich August von Alberti, 1834), Jurassic (Alexander von Humboldt, 1795/Alexandre Brongniart, 1829), Cretaceous (Jean Baptiste Julien d'Omalius d'Halloy, 1822), and others. Giovanni Arduino, as early as the 1760s in Italy, had proposed a four-fold division of rocks into Primary, Secondary, Tertiary, and Quaternary—terms some of which persist in modified form today.

Radiometric calibration (20th century). The discovery of radioactivity at the end of the 19th century transformed geology by providing an absolute clock. In 1913, the British geologist Arthur Holmes published the first geologic time scale that included numerical ages derived from radiometric dating of uranium-bearing minerals. Holmes continued to refine his scale through subsequent editions (notably 1937 and 1960). In the 1950s, Clair Patterson used uranium-lead dating of meteorites to determine the age of the Earth at approximately 4.55 billion years, a figure that has since been refined only slightly to 4.54 ± 0.05 Ga.

Modern standardization. The International Commission on Stratigraphy, established in 1961 under the IUGS, took on the task of producing a single, globally accepted time scale. The concept of the Global Boundary Stratotype Section and Point (GSSP) was formalized in the 1970s–1980s, providing an objective mechanism for defining the lower boundaries of chronostratigraphic units. Successive editions of comprehensive reference works—Harland et al. (1982, 1989), Gradstein et al. (2004, 2012), and most recently Gradstein, Ogg, Schmitz & Ogg (2020), known as GTS2020—have progressively refined boundary ages using high-precision radiometric techniques and astronomically tuned cyclostratigraphy.

The GTS is organized into a dual hierarchy of time units and rock units:

Geochronologic (time) units: Eon → Era → Period → Epoch → Subepoch → Age → Subage.

Chronostratigraphic (rock) units: Eonothem → Erathem → System → Series → Subseries → Stage → Substage.

The four recognized eons, from oldest to youngest, are the Hadean (~4.6–4.0 Ga), Archean (~4.0–2.5 Ga), Proterozoic (2.5 Ga–538.8 Ma), and Phanerozoic (538.8 Ma–present). The first three are informally grouped as the Precambrian, representing roughly 88% of Earth's history. The Phanerozoic Eon—the eon of "visible life"—is subdivided into three eras: the Paleozoic (538.8–251.9 Ma), Mesozoic (251.9–66.0 Ma), and Cenozoic (66.0 Ma–present). Each era is further divided into periods (e.g., Triassic, Jurassic, Cretaceous within the Mesozoic), epochs, and ages.

Naming conventions carry information: Early/Middle/Late designate geochronologic (time) subdivisions, while Lower/Middle/Upper designate the corresponding chronostratigraphic (rock) subdivisions. Thus one speaks of "Early Jurassic" time but "Lower Jurassic" strata.

The modern GTS defines the base of each chronostratigraphic unit by a GSSP—a specific physical point within an accessible, well-preserved rock section. A GSSP must fulfill strict criteria established by the ICS: the boundary must be marked by an observable, unambiguous change in the rock record (commonly the first appearance datum of a fossil taxon, a magnetic polarity reversal, or a geochemical excursion); the section must be continuous, tectonically undisturbed, and accessible to researchers worldwide; and secondary markers should be present to allow correlation across different paleogeographic settings.

As of the most recent ICS accounting (Cohen et al., 2025), 81 GSSPs have been formally ratified for 102 Phanerozoic stages. Stages that still lack ratified GSSPs are assigned approximate ages (indicated by the tilde symbol '~' on the official chart). For the Precambrian eons and their subdivisions, boundaries were historically defined by GSSAs—fixed numerical ages (e.g., the Archean-Proterozoic boundary at 2,500 Ma)—but the ICS is actively working to replace these with GSSPs where suitable rock sections can be identified.

Once ratified, a GSSP is commemorated with a physical marker (colloquially called a "golden spike") at the type locality. A moratorium of at least 10 years protects newly established GSSPs from being changed, ensuring stability.

Numerical ages on the GTS are not part of the formal definition of Phanerozoic units (only GSSPs define them), but they are essential for practical use. The current standard reference, GTS2020, integrates ages from multiple radiometric systems (U-Pb zircon dating, 40Ar/39Ar dating, Re-Os dating) and from astrochronology—the tuning of sedimentary cycles to calculated variations in Earth's orbital parameters (Milankovitch cycles). Astrochronology has been particularly transformative for the Cenozoic and parts of the Mesozoic, achieving precision on the order of tens of thousands of years.

Age units follow SI conventions: ka (kilo-annum, 10³ years), Ma (mega-annum, 10⁶ years), and Ga (giga-annum, 10⁹ years). Durations are expressed in millions of years (m.y.) to distinguish them from points in time.

The visual representation of the GTS is published as the International Chronostratigraphic Chart (ICC), maintained online at stratigraphy.org. The chart is updated every few years to incorporate newly ratified GSSPs and revised numerical ages. The latest version as of late 2024 is v2024/12. It uses a standardized color scheme established by the Commission for the Geological Map of the World (CGMW). The chart is available under the Creative Commons BY 4.0 license. A Korean-language edition of the chart has been produced and approved by the Geological Society of Korea (대한지질학회), adopting the Korean stratigraphic terminology: 누대 (eon), 대 (era), 기 (period), 세 (epoch), 절 (age).

In Korean geological terminology, the formal translation of the International Chronostratigraphic Chart is "국제지질연대층서표" (literally, International Geologic Time-Stratigraphic Chart). The hierarchy of geochronologic units in Korean is: 누대(累代) for eon, 대(代) for era, 기(紀) for period, 세(世) for epoch, and 절(節) for age. The corresponding chronostratigraphic (rock) units append 층(層, 'stratum') to form compound terms: 누대층 (eonothem), 대층 (erathem), 계(系, system), 통(統, series), and 조(組, stage). This terminology was standardized by the Geological Society of Korea's Committee on Geological Nomenclature and is endorsed by the ICS.

The Geologic Time Scale serves as the universal language for communicating when events in Earth's history occurred. Every fossil description, geological map, paleoclimate reconstruction, and evolutionary study references the GTS to place observations in temporal context. The boundaries between major units frequently coincide with transformative biological and geological events: the Archean-Proterozoic boundary marks the rise of oxygen in the atmosphere; the base of the Phanerozoic (Cambrian) corresponds to the explosive diversification of complex animal life; the Paleozoic-Mesozoic boundary (Permian-Triassic) records Earth's most devastating mass extinction; and the Mesozoic-Cenozoic boundary (Cretaceous-Paleogene) is defined at the iridium anomaly associated with the end-Cretaceous mass extinction that eliminated non-avian dinosaurs. These boundaries are not arbitrary—they reflect genuine, globally recognizable changes in the rock and fossil records.

The GTS remains a living document. Several active debates and ongoing revisions are noteworthy. The proposed "Anthropocene" epoch—marking the time when human activities began to leave a globally detectable geological signal—has been extensively debated since the early 2000s. As of the latest ICS actions, the Anthropocene has not been formally ratified as a unit; the Subcommission on Quaternary Stratigraphy rejected the initial GSSP proposal in 2024, and the term currently has no formal stratigraphic status, though it remains widely used informally. Numerous Mesozoic and Paleozoic stage boundaries still await formal GSSPs (e.g., the base of the Jurassic Oxfordian, the Triassic Norian, and the Cretaceous Berriasian, among others). Additionally, the push to replace Precambrian GSSAs with GSSPs represents a major ongoing endeavor that will require identifying suitable rock sections in very ancient, often heavily metamorphosed terrains.

The periodic updates to the ICC—most recently in December 2024—reflect the continuous refinement of numerical ages, the ratification of new GSSPs, and evolving consensus on unit names and boundaries. The reference publication for the current chart is Cohen, Harper, Gibbard & Car (2025), published in the IUGS journal Episodes.