History of Earth

This section covers:

International standards and resources

The International Commission on Stratigraphy (ICS) sets the global standards for the scale for expressing the history of Earth, including the International Chronostratigraphic Chart (ICC).

Australian conventions and resources

Geoscience Australia manages the Australian Stratigraphy Commission, which develops stratigraphic principles and classification for Australia. Geoscience Australia also maintains the Australian Stratigraphic Units Database.

Chronostratigraphic and geochronologic terms

The ICS defines global units (eg systems, series and stages) for when rock bodies were formed; these are called chronostratigraphic units and are shown on the ICC. The ICC is, in turn, the basis for the geochronologic units (eon, era, period, epoch and age) of the International Geologic Time Scale.

A chronostratigraphic unit has a binomial name – a proper noun plus a term word. Use initial capitals for both words:

Pleistocene Series     Upper Pleistocene Stage

Geochronologic units define the geological time intervals of the ICC. A geochronologic unit name uses the same proper noun as the equivalent chonostratigraphic unit, with the corresponding geochronologic term (eon, era, period, epoch or age):

Pleistocene Series [chronostratigraphic]     Pleistocene Epoch [geochronologic]     Mesozoic Era     Late Jurassic     Cretaceous Period

Upper and Lower in formal chronostratigraphic names are replaced by Late and Early, respectively, in the geochronologic timescale:

Upper Triassic Series [chronostratigraphic]     Late Triassic Epoch [geochronologic]

See the table below for a comparison of chronostratigraphic and geochronologic unit names.

Main hierarchical levels of formal chronostratigraphic and geochronologic terms

Chronostratigraphic Geochronologic











Miocene Series

Miocene Epoch

Upper/Lower Jurassic [formal]

Early/Late Jurassic [formal]

upper/lower Permian [informal]

early/late Permian [informal]

The term words are not needed if there is no ambiguity:

... from the Cretaceous to the …     ... until the end of the Permian

Adjectival words are capitalised only if they are part of the formal name:

in the Middle Jurassic     from the Upper to the Middle Pleistocene

Use lower case for informal terms (eg early, late) that are used to modify formal names:

from the early Mesozoic to the late Paleozoic

Caution! The ICS recommends paleo- (not palaeo-).
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Expressing geological time

Absolute dates can be expressed as follows:

by the Middle to Late Devonian (ca 385 Ma), fish had ...

between 55 ka and 21 ka

formed by 3,500 Ma

older than 4,000 Ga

where ka = kiloannum (a thousand years ago), Ma = mega-annum (a million years ago) and Ga = giga-annum (a billion years ago).

Note that, in expressing a range of time, the largest (oldest) number occurs first.

Duration of time can also expressed in kiloyears (kyr), megayears (Myr) and gigayears (Gyr):

at rates of 20–30 m per Myr

for the past 300 Myr

The Jurassic Period lasted approximately 50 Myr [duration] from ca 201 Ma to ca 152 Ma [absolute dates].

The before present (BP) timescale is often used in geology; see Date and time.

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