A year is divided into months, weeks, and days. The geologic time scale is also divided into time categories. What are the time divisions that are used for the geologic time scale and what determines how they are divided?
The geologic time scale is divided into several time divisions, which are determined based on major events in Earth's history, such as changes in the fossil record, the formation of specific rock layers, or significant geological events. The primary divisions of the geologic time scale, from largest to smallest, are:
1. Eons: The largest division of time, encompassing billions of years. The geologic time scale recognizes four primary eons: Hadean, Archean, Proterozoic, and Phanerozoic.
2. Eras: Eons are further subdivided into eras, which represent hundreds of millions of years. The geologic time scale recognizes three primary eras within the Phanerozoic eon: Paleozoic, Mesozoic, and Cenozoic.
3. Periods: Eras are divided into periods, representing tens of millions of years. There are a total of 11 recognized periods within the Phanerozoic eon, for example, the Permian, Jurassic, or Pleistocene.
4. Epochs: Periods are further divided into epochs, representing several million years. The current epoch within the Cenozoic era is the Holocene epoch.
The divisions within the geologic time scale are determined by analyzing geological and paleontological evidence found in the rock record. This evidence includes the presence or absence of certain types of fossils, changes in sedimentary layers, and the occurrence of significant geological events such as mass extinctions or volcanic eruptions. Scientists use these markers to establish boundaries and subdivisions within the geologic time scale, helping to organize Earth's history into distinct time intervals.