The Earth's geologic time scale is broken down into different periods based on the occurrence of significant events or changes in the Earth's history. These periods are defined by the fossil record and the geological evidence found in rock formations.
The largest divisions in the geologic time scale are eons, which typically span billions of years. The current eon is the Phanerozoic, which began around 541 million years ago. Within the Phanerozoic eon, there are three eras: the Paleozoic, Mesozoic, and Cenozoic.
Each era is further divided into periods, which reflect major geological and biological shifts. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. These periods are characterized by the appearance and extinction of different groups of organisms, as well as significant changes in global climate and geologic processes.
The Mesozoic era is divided into the Triassic, Jurassic, and Cretaceous periods, known for the dominance of dinosaurs and the emergence of many modern plant and animal groups. The Cenozoic era is divided into the Paleogene, Neogene, and Quaternary periods, which cover the time from the extinction of dinosaurs to the present day and feature the rise of mammals and the development of modern ecosystems.
Within each period, there are also smaller subdivisions called epochs, which capture more specific changes in ecosystems and climate. These geological time divisions are continuously refined and updated as new evidence is discovered and our understanding of the Earth's history improves.
How is Earths geologic time scale broken down into different periods?
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