Final answer:
The geologic time scale is divided into eons, eras, periods, epochs, and ages, reflecting major geological, climatic, and biological changes on Earth. Eons such as the Hadean, Archean, and Proterozoic comprise the Precambrian. The time scale divisions and their numerical ages are determined by radiometric dating, with the International Commission on Stratigraphy updating it periodically.
Step-by-step explanation:
The geologic time scale was originally devised based on major changes in geology, climate, and the evolution of life. Earth's history is first divided into eons, such as the Hadean, Archean, Proterozoic, and Phanerozoic, with the first three being grouped together as the Precambrian. These eons are subdivided into eras, which are further divided into periods, epochs, and ages. The system of color-coding each period on the geologic time scale helps to easily distinguish them. The establishment of these divisions relied on significant events and the geological, biological, or climatical changes associated with these events.
Radiometric dating of rocks, minerals, and fossils provided the numerical ages that help to accurately place these events on the time scale. The International Commission on Stratigraphy contributes to continuously updating this information based on new research.