Geologic time

Geologic time is the immense span of Earth’s history, organized into dated intervals and events recorded in rocks, fossils, and other evidence.

Geologic time scale: A chronological framework dividing Earth’s history into named eons, eras, periods, epochs, and ages. Its hierarchy turns geologic time into a standardized sequence of intervals.
Relative dating: The determination of whether rocks or events are older or younger than others, without assigning numerical ages. It establishes event order even where absolute dates are unavailable.
Precambrian: The informal span of Earth history before the Cambrian Period, covering most of the planet’s existence. It contains the longest stretch of geologic time, from Earth’s formation to complex abundant fossils.
James Hutton: A Scottish geologist whose work established deep time and helped found modern geology. His arguments for slow geological processes challenged short chronologies of Earth’s past.
Chronostratigraphy: The branch of stratigraphy that organizes rock bodies by the intervals of time they represent. It links divisions of geologic time to identifiable rock sequences.
Absolute dating: The assignment of a numerical age to a material or event using measured physical or chemical processes. It converts parts of Earth’s sequence into estimates in years.
Cambrian explosion: A geologically rapid diversification of animal life during the early Cambrian Period. It marks a major biological transition near the beginning of the Phanerozoic Eon.
William Smith: An English surveyor and geologist who produced an early nationwide geological map using fossil succession. His fossil-based ordering showed that strata could be correlated and arranged through time.
International Commission on Stratigraphy: An international scientific body that standardizes stratigraphic units and maintains the geologic time scale. It coordinates the official boundaries and names used on the time scale.
Index fossil: A fossil useful for identifying and correlating a relatively narrow interval of geological time. Its limited time range helps match strata and locate interval boundaries.

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