Period (geology)

Classification and examples

The hierarchy of geologic time is commonly expressed as eon > era > period > epoch > age. Well-known eras, such as the Mesozoic, are subdivided into periods; for example the Mesozoic includes the Triassic, Jurassic and Cretaceous periods. Other periods include the Cambrian, Ordovician, Devonian, Carboniferous and Permian in the Paleozoic and the Paleogene and Neogene in the Cenozoic.

How periods are defined

Periods are defined by stratigraphic evidence: layers of rock and the fossils they preserve. International bodies, notably the International Commission on Stratigraphy, formalize boundaries by selecting reference sections and points in the rock record (GSSPs). Boundaries may coincide with major events such as mass extinctions, rapid climate shifts or the first appearance of key fossil groups.

Uses and importance

Geologic periods provide a shared framework for correlating rocks and fossils across regions and for organizing Earth history into manageable intervals. They are essential in studies of evolution, paleoclimate, tectonics, and natural resources because they tie biological and physical changes to time. Maps, textbooks and research papers routinely use period names to communicate age and context.

Notable distinctions

• Length: Period durations vary from tens to hundreds of millions of years and are not uniform.
• Naming: Many periods are named for geographic regions, characteristic rock formations or historically used terms.
• Subdivision: Periods are divided into epochs and further into ages to allow finer temporal resolution.

For an introduction to chronostratigraphy and formal definitions of boundaries see specialized resources and chronologies maintained by stratigraphic organizations.