Geologic Time Scale


Staff member
Jun 3, 2020
The Earth was formed Billions of years ago and the human calendars won't be enough to measure the Geological time periods which span several Millions of years. The Geologic Time Scale (GTS) is a system for chronological dating of strata. The Geologic Time Scale serves as a reference system to identify the age of strata and relate them to the Geological events of that time. There are so many applications of this scale, that this is used across multiple disciplines of Earth Science.

The time intervals in a Geologic Time Scale
The time intervals in a Geologic Time Scale are not uniform like days and weeks in a human calendar, but are variable in length. The Geologic time is divided into these intervals based on the significant events in the Geological history of the Earth. The boundary between 2 Geologic intervals is marked by a Geologic event such as the separation of a continent, or extinction of an organism.

The units in a Geologic Time Scale
The Geologic Time Scale contains several units of measurement just like our human calendars are divided into months, weeks, days etc. However, the time units in Geology are not uniform like in our calendar. For example, a week in our calendar will always be 7 days, whereas a Period in Geological time scale may considerably vary. The following is a comparison between Gregorian calendar and Geologic Time Scale:
S. No.Gregorian CalendarGeologic Time Scale

The Eons are largest intervals of Geologic Time Scale which have several hundreds of millions of years duration. There are 4 Eons in a Geologic Time Scale: Hadean, Archean, Proterozoic, and Phanerozoic. The most studied Eon is Phanerozoic, which started over 500 million years ago and is marked by the first appearance of organisms with hard parts. In other words, Phanerozoic marks the evidence of preliminary life on Earth.

The Eons are divided into smaller intervals known as Eras, which mark the significant events in Earth's history.

The Eras are divided into Periods, which mark the relatively less significant events in Earth's history (in comparison to Eras). Sometimes, the Periods are subdivided into Sub-periods.

The Periods are divided into Epochs, which are smaller intervals that mark Geological events. The subdivision of Periods into Epochs is only possible in recent strata as the older strata doesn't contain enough information to identify/compare these minor events. The weathering or deformation destroys much of these minor events.
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