Geologic time refers to the history of Earth represented by rock strata. It began around 4 billion years ago and continues to the present. Geologic time is divided into units including eons, eras, periods, and epochs based on stratigraphy and fossil evidence. The geologic time scale provides a framework to reconstruct Earth's history by correlating rock layers containing certain fossil types across regions. Absolute dating methods have provided numerical ages to the relative geologic timescale developed from fossils.

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geologic time
The stratigraphic chart of geologic time.
Encyclopædia Britannica, Inc. Source:
International Commission on Stratigraphy
(ICS)
Geologic time
Geologic time, the extensive interval of time occupied by the geologic history of Earth.
Formal geologic time begins at the start of the Archean Eon (4.0 billion to 2.5 billion years
ago) and continues to the present day. Modern geologic time scales additionally often
include the Hadean Eon, which is an informal interval that extends from about 4.6 billion
years ago (corresponding to Earth’s initial formation) to 4.0 billion years ago. Geologic time is,
in effect, that segment of Earth history that is represented by and recorded in the planet’s
rock strata.
The geologic time scale is the “calendar” for events in
Earth history. It subdivides all time into named units of
abstract time called—in descending order of duration—
eons, eras, periods, epochs, and ages. The enumeration
of those geologic time units is based on stratigraphy,
which is the correlation and classi cation of rock strata.
The fossil forms that occur in the rocks provide the chief
means of establishing a geologic time scale. One of the
most widely used standard charts showing the
relationships between the various intervals of geologic
time is the International Chronostratigraphic Chart,
which is maintained by the International Commission on
Stratigraphy (ICS).
Because living things have undergone evolutionary changes over geologic time, particular
kinds of organisms are characteristic of particular parts of the geologic record. By correlating
the strata in which certain types of fossils are found, the geologic history of various regions—
and of Earth as a whole—can be reconstructed. The relative geologic time scale developed
from the fossil record has been numerically quanti ed by means of absolute dates obtained
with radiometric dating methods. See also geochronology.
The Editors of Encyclopaedia Britannica This article was
most recently revised and updated by John P. Rafferty,
Editor.

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The diversity of marine animal families since
late Precambrian time. The data for the
curve comprise only those families that are
reliably preserved in the fossil record; the
1,900 value for living families also includes
those families rarely preserved as fossils.
The several pronounced dips in the curve
correspond to major mass-extinction events.
The most catastrophic extinction took place
at the end of the Permian Period.
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