Geologic time is organized on the geologic time scale into a hierarchical structure with several major divisions and subdivisions. The geologic time scale is a system that allows geologists to categorize and understand the vast expanse of Earth's history. Here is the basic organizational structure of the geologic time scale:
1. **Eon:** The largest and longest division of time. The geologic time scale currently recognizes four eons: Hadean, Archean, Proterozoic, and Phanerozoic. The Phanerozoic Eon is the most recent and includes the time from about 541 million years ago to the present.
2. **Era:** Each eon is divided into eras. For example, the Phanerozoic Eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic. Eras are characterized by significant changes in the Earth's geology and life forms.
3. **Period:** Eras are further subdivided into periods. The Paleozoic Era, for instance, includes periods such as the Cambrian, Ordovician, and Permian. These divisions are based on the occurrence of significant geologic events and the presence or absence of specific fossils.
4. **Epoch:** Periods are subdivided into epochs. For example, the Cenozoic Era includes epochs like the Paleogene and Neogene. Epochs represent shorter intervals of geologic time and are often characterized by distinct climatic and ecological changes.
5. **Age:** The finest level of subdivision on the geologic time scale is the age. Ages are defined by specific markers, often involving the appearance or disappearance of certain species of fossils. They represent relatively short periods within an epoch.
It's important to note that the geologic time scale is not a fixed calendar with specific durations, and the boundaries between divisions are determined by the geological and paleontological evidence available. Over time, the time scale has been refined and adjusted as our understanding of Earth's history has improved through scientific research.