The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronology (a scientific branch of geology that aims to determine the age of rocks). It is used primarily by Earth scientists (including geologists, paleontologists, geophysicists, geochemists, and paleoclimatologists) to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties, and fossils. The definition of standardised international units of geologic time is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective^[1] is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC)^[2] that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units.^[2]

While some regional terms are still in use,^[3] the table of geologic time presented in this article conforms to the nomenclature, ages, and colour codes set forth by the ICS.^[1][4]

Principles

The geologic time scale is a way of representing deep time based on events that have occurred throughout Earth's history, a time span of about 4.54 ± 0.05 Ga (4.54 billion years).^[5] It chronologically organises strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. For example, the Cretaceous–Paleogene extinction event, marks the lower boundary of the Paleogene System/Period and thus the boundary between the Cretaceous and Paleogene systems/periods. For divisions prior to the Cryogenian, arbitrary numeric boundary definitions (Global Standard Stratigraphic Ages, GSSAs) are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record.^[6][3]

Historically, regional geologic time scales were used^[3] due to the litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardising globally significant and identifiable stratigraphic horizons that can be used to define the lower boundaries of chronostratigraphic units. Defining chronostratigraphic units in such a manner allows for the use of global, standardised nomenclature. The ICC represents this ongoing effort.

The relative relationships of rocks for determining their chronostratigraphic positions use the overriding principles of:^{[7][8][9][10]}

• Superposition – Newer rock beds will lie on top of older rock beds unless the succession has been overturned.
• Horizontality – All rock layers were originally deposited horizontally.^{[note 1]}
• Lateral continuity – Originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer.
• Biologic succession (where applicable) – This states that each stratum in a succession contains a distinctive set of fossils. This allows for a correlation of the stratum even when the horizon between them is not continuous.
• Cross-cutting relationships – A rock feature that cuts across another feature must be younger than the rock it cuts across.
• Inclusion – Small fragments of one type of rock but embedded in a second type of rock must have formed first, and were included when the second rock was forming.
• Relationships of unconformities – Geologic features representing periods of erosion or non-deposition, indicating non-continuous sediment deposition.

Terminology

The GTS is divided into chronostratigraphic units and their corresponding geochronologic units. These are represented on the ICC published by the ICS; however, regional terms are still in use in some areas.

Chronostratigraphy is the element of stratigraphy that deals with the relation between rock bodies and the relative measurement of geological time.^[11] It is the process where distinct strata between defined stratigraphic horizons are assigned to represent a relative interval of geologic time.

A chronostratigraphic unit is a body of rock, layered or unlayered, that is defined between specified stratigraphic horizons which represent specified intervals of geologic time. They include all rocks representative of a specific interval of geologic time, and only this time span.^[11] Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units.^[11] Geochronology is the scientific branch of geology that aims to determine the age of rocks, fossils, and sediments either through absolute (e.g., radiometric dating) or relative means (e.g., stratigraphic position, paleomagnetism, stable isotope ratios).^[12]

A geochronologic unit is a subdivision of geologic time. It is a numeric representation of an intangible property (time).^[12] Eon, era, period, epoch, subepoch, age, and subage are the hierarchical geochronologic units.^[11] Geochronometry is the field of geochronology that numerically quantifies geologic time.^[12]

A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundaries of stages on the geologic time scale.^[13] (Recently this has been used to define the base of a system)^[14]

A Global Standard Stratigraphic Age (GSSA)^[15] is a numeric only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined.^[11] They are used where GSSPs have not yet been established. Research is ongoing to define GSSPs for the base of all units that are currently defined by GSSAs.

The numeric (geochronometric) representation of a geochronologic unit can, and is more often subject to change when geochronology refines the geochronometry, while the equivalent chronostratigraphic unit remains the same, and their revision is less common. For example, in early 2022 the boundary between the Ediacaran and Cambrian periods (geochronologic units) was revised from 541 Ma to 538.8 Ma but the rock definition of the boundary (GSSP) at the base of the Cambrian, and thus the boundary between the Ediacaran and Cambrian systems (chronostratigraphic units) has not changed, merely the geochronometry has been refined.

The numeric values on the ICC are represented by the unit Ma (megaannum) meaning "million years", i.e., 201.4 ± 0.2 Ma, the lower boundary of the Jurassic Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other SI prefix units commonly used by geologists are Ga (gigaannum, billion years), and ka (kiloannum, thousand years), with the latter often represented in calibrated units (before present).

Divisions of geologic time

• An eon is the largest geochronologic time unit and is equivalent to a chronostratigraphic eonothem.^[16] There are four formally defined eons: the Hadean, Archean, Proterozoic and Phanerozoic.^[2]

• An era is the second largest geochronologic time unit and is equivalent to a chronostratigraphic erathem.^[11][16] There are ten defined eras: the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic and Cenozoic, with none from the Hadean eon.^[2]

• A period is equivalent to a chronostratigraphic system.^[11][16] There are 22 defined periods, with the current being the Quaternary period.^[2] As an exception two subperiods are used for the Carboniferous Period.^[11]

• An epoch is the second smallest geochronologic unit. It is equivalent to a chronostratigraphic series.^[11][16] There are 37 defined epochs and one informal one. There are also 11 subepochs which are all within the Neogene and Quaternary.^[2] The use of subepochs as formal units in international chronostratigraphy was ratified in 2022.^[17]

• An age is the smallest hierarchical geochronologic unit and is equivalent to a chronostratigraphic stage.^[11][16] There are 96 formal and five informal ages.^[2]

• A chron is a non-hierarchical formal geochronology unit of unspecified rank and is equivalent to a chronostratigraphic chronozone.^[11] These correlate with magnetostratigraphic, lithostratigraphic, or biostratigraphic units as they are based on previously defined stratigraphic units or geologic features.

The Early and Late subdivisions are used as the geochronologic equivalents of the chronostratigraphic Lower and Upper, e.g., Early Triassic Period (geochronologic unit) is used in place of Lower Triassic Series (chronostratigraphic unit).

Rocks representing a given chronostratigraphic unit are that chronostratigraphic unit, and the time they were laid down in is the geochronologic unit, i.e., the rocks that represent the Silurian Series are the Silurian Series and they were deposited during the Silurian Period.

Naming of geologic time

The names of geologic time units are defined for chronostratigraphic units with the corresponding geochronologic unit sharing the same name with a change to the latter (e.g. Phanerozoic Eonothem becomes the Phanerozoic Eon). Names of erathems in the Phanerozoic were chosen to reflect major changes in the history of life on Earth: Paleozoic (old life), Mesozoic (middle life), and Cenozoic (new life). Names of systems are diverse in origin, with some indicating chronologic position (e.g., Paleogene), while others are named for lithology (e.g., Cretaceous), geography (e.g., Permian), or are tribal (e.g., Ordovician) in origin. Most currently recognised series and subseries are named for their position within a system/series (early/middle/late); however, the ICS advocates for all new series and subseries to be named for a geographic feature in the vicinity of its stratotype or type locality. The name of stages should also be derived from a geographic feature in the locality of its stratotype or type locality.^[11]

Informally, the time before the Cambrian is often referred to as the Precambrian or pre-Cambrian (Supereon).^{[6][note 3]}