Geology is a four-dimensional science, we think as much about time as about up and down, and north, south, east and west. The study of meteorites and the solar system has allowed the age of the Earth to be estimated at about 4.5 billion years old.
Years of studying rocks and collecting fossils have proven that the animals and plants that lived on the Earth have changed over time. In addition, some types of sedimentary rocks were formed more commonly during certain periods of geological time due to the climate and environment.
Measuring geological time
The geological time scale divides Earth’s history into several time slices. These time slices are united by common elements relating to the environments or organisms of that time.
Originally, divisions of the time scale were based entirely on the assemblages of fossils seen in rocks of that age (biostratigraphy). With technological advances, time scale divisions have been refined by precise radiometric age measurements (geochronology) and are continually being adjusted based on the latest research.
The largest subdivisions in geological time are called eons:
- Hadean: approximately 4,567 million to 4,031 million years ago.
- Archean: approximately 4,031 million to 2,500 million years ago.
- Proterozoic: approximately 2,500 million to 538.8 million years ago.
- Phanerozoic: approximately 538.8 million years ago to today.
Within the eons are several finer subdivisions called eras, periods, epochs, and ages.
The word ‘Phanerozoic’ means ‘visible life’ and originally, it was thought all life originated at the start of that eon. We now know that life extend far back into the Archean, although the forms of life prior to about 600 million years ago were mostly dominated by bacteria and algae.
Further reading on geological time
Learn more about geological time from trusted external sources.
Geological time and fossils (Geoscience Australia)— a set of downloadable posters, activities, and apps relating to geological time, with an Australia-wide focus.
International chronostratigraphic chart (International Commission on Stratigraphy)
