Geologic time scale

The geologic time scale or geological time scale is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy and geochronology. It is used primarily by Earth scientists 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 geological time is the responsibility of the International Commission on Stratigraphy, a constituent body of the International Union of Geological Sciences, whose primary objective is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart that are used to define divisions of geological time. The chronostratigraphic divisions are in turn used to define geochronologic units.

Principles

The geologic time scale is a way of representing deep time based on events that have occurred through Earth's history, a time span of about 4.54 ± 0.05 billion years. 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 are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record.
Historically, regional geologic time scales were used 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 International Chronostratigraphic Chart represents this ongoing effort.
Several key principles are used to determine the relative relationships of rocks and thus their chronostratigraphic position.

The law of superposition that states that in undeformed stratigraphic sequences the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface. In practice, this means a younger rock will lie on top of an older rock unless there is evidence to suggest otherwise.
The principle of original horizontality that states layers of sediments will originally be deposited horizontally under the action of gravity. However, it is now known that not all sedimentary layers are deposited purely horizontally, but this principle is still a useful concept.
The principle of lateral continuity that states layers of sediments extend laterally in all directions until either thinning out or being cut off by a different rock layer, i.e. they are laterally continuous. Layers do not extend indefinitely; their limits are controlled by the amount and type of sediment in a sedimentary basin, and the geometry of that basin.
The principle of cross-cutting relationships that states a rock that cuts across another rock must be younger than the rock it cuts across.
The law of included fragments that states small fragments of one type of rock that are embedded in a second type of rock must have formed first, and were included when the second rock was forming.
The relationships of unconformities which are geologic features representing a gap in the geologic record. Unconformities are formed during periods of erosion or non-deposition, indicating non-continuous sediment deposition. Observing the type and relationships of unconformities in strata allows geologist to understand the relative timing of the strata.
The principle of faunal succession that states rock strata contain distinctive sets of fossils that succeed each other vertically in a specific and reliable order. This allows for a correlation of strata even when the horizon between them is not continuous.
Divisions of geologic time

The geologic time scale is divided into chronostratigraphic units and their corresponding geochronologic units.

An ' is the largest geochronologic time unit and is equivalent to a chronostratigraphic eonothem. There are four formally defined eons: the Hadean, Archean, Proterozoic and Phanerozoic.
An ' is the second largest geochronologic time unit and is equivalent to a chronostratigraphic erathem. There are ten defined eras: the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic and Cenozoic, with none from the Hadean eon.
A ' is equivalent to a chronostratigraphic system. There are 22 defined periods, with the current being the Quaternary period. As an exception, two subperiods are used for the Carboniferous Period.
An ' is the second smallest geochronologic unit. It is equivalent to a chronostratigraphic series. There are 37 defined epochs and one informal one. The current epoch is the Holocene. There are also 11 subepochs which are all within the Neogene and Quaternary. The use of subepochs as formal units in international chronostratigraphy was ratified in 2022.
An is the smallest hierarchical geochronologic unit. It is equivalent to a chronostratigraphic stage. There are 96 formal and five informal ages. The current age is the Meghalayan.
A is a non-hierarchical formal geochronology unit of unspecified rank and is equivalent to a chronostratigraphic chronozone. These correlate with magnetostratigraphic, lithostratigraphic, or biostratigraphic units as they are based on previously defined stratigraphic units or geologic features.

Chronostratigraphic unit	Geochronologic unit	Time span
Eonothem	Eon	Several hundred million years to two billion years
Erathem	Era	Tens to hundreds of millions of years
System	Period	Millions of years to tens of millions of years
Series	Epoch	Hundreds of thousands of years to tens of millions of years
Subseries	Subepoch	Thousands of years to millions of years
Stage	Age	Thousands of years to millions of years

The subdivisions and are used as the geochronologic equivalents of the chronostratigraphic and, e.g., Early Triassic Period is used in place of Lower Triassic System.
Rocks representing a given chronostratigraphic unit are that chronostratigraphic unit, and the time they were laid down in is the geochronologic unit, e.g., the rocks that represent the Silurian System the Silurian System and they were deposited the Silurian Period. This definition means the numeric age of a geochronologic unit can be changed when refined by geochronometry while the equivalent chronostratigraphic unit remains unchanged. For example, in early 2022, the boundary between the Ediacaran and Cambrian periods was revised from 541 Ma to 538.8 Ma but the rock definition of the boundary at the base of the Cambrian, and thus the boundary between the Ediacaran and Cambrian systems has not been changed; rather, the absolute age has merely been refined.

Terminology

is the element of stratigraphy that deals with the relation between rock bodies and the relative measurement of geological time. It is the process where distinct strata between defined stratigraphic horizons are assigned to represent a relative interval of geologic time.
A ' 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. Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units.
A ' is a subdivision of geologic time. It is a numeric representation of an intangible property. These units are arranged in a hierarchy: eon, era, period, epoch, subepoch, age, and subage.
is the scientific branch of geology that aims to determine the age of rocks, fossils, and sediments either through absolute or relative means.
is the field of geochronology that numerically quantifies geologic time.
A is an internationally agreed-upon reference point on a stratigraphic section that defines the lower boundaries of stages on the geologic time scale.
A is a numeric-only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined. 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 standard international units of the geologic time scale are published by the International Commission on Stratigraphy on the International Chronostratigraphic Chart. However, regional terms are still in use in some areas. The numeric values on the International Chronostratigrahpic Chart are represented by the unit Ma. For example, 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, and ka, with the latter often represented in calibrated units.

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 suffix. Names of erathems in the Phanerozoic were chosen to reflect major changes in the history of life on Earth: Paleozoic, Mesozoic, and Cenozoic. Names of systems are diverse in origin, with some indicating chronologic position, while others are named for lithology, geography, or are tribal in origin. Most currently recognised series and subseries are named for their position within a system/series ; however, the International Commission on Stratigraphy 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.
Informally, the time before the Cambrian is often referred to as the Precambrian or pre-Cambrian.

Name	Time span	Duration	Etymology of name
Phanerozoic			From Greek φανερός 'visible' or 'abundant' and ζωή 'life'.
Proterozoic			From Greek πρότερος 'former' or 'earlier' and ζωή 'life'.
Archean			From Greek ἀρχή 'beginning, origin'.
Hadean			From Hades,, the god of the underworld in Greek mythology.

Name	Time span	Duration	Etymology of name
Quaternary			First introduced by Jules Desnoyers in 1829 for sediments in France's Seine Basin that appeared to be younger than Tertiary rocks.
Neogene			Derived from Greek νέος 'new' and γενεά 'genesis' or 'birth'.
Paleogene			Derived from Greek παλιός 'old' and γενεά 'genesis' or 'birth'.
Cretaceous	~	~	Derived from Terrain Crétacé used in 1822 by Jean d'Omalius d'Halloy in reference to extensive beds of chalk within the Paris Basin. Ultimately derived from Latin crēta 'chalk'.
Jurassic		~	Named after the Jura Mountains. Originally used by Alexander von Humboldt as 'Jura Kalkstein' in 1799. Alexandre Brongniart was the first to publish the term Jurassic in 1829.
Triassic			From the Trias of Friedrich August von Alberti in reference to a trio of formations widespread in southern Germany.
Permian			Named after the historical region of Perm, Russian Empire.
Carboniferous			Means 'coal-bearing', from the Latin carbō and ferō.
Devonian			Named after Devon, England.
Silurian			Named after the Celtic tribe, the Silures.
Ordovician			Named after the Celtic tribe, Ordovices.
Cambrian			Named for Cambria, a Latinised form of the Welsh name for Wales, Cymru.
Ediacaran		~	Named for the Ediacara Hills. Ediacara is possibly a corruption of Kuyani 'Yata Takarra' 'hard or stony ground'.
Cryogenian		~	From Greek κρύος 'cold' and γένεσις 'birth'.
Tonian		~	From Greek τόνος 'stretch'.
Stenian			From Greek στενός 'narrow'.
Ectasian			From Greek ἔκτᾰσῐς 'extension'.
Calymmian			From Greek κάλυμμᾰ 'cover'.
Statherian			From Greek σταθερός 'stable'.
Orosirian			From Greek ὀροσειρά 'mountain range'.
Rhyacian			From Greek ῥύαξ 'stream of lava'.
Siderian			From Greek σίδηρος 'iron'.

Geologic time scale

Principles

Divisions of geologic time

Terminology

Naming of geologic time