What subdivisions make up the geologic time scale? What is the primary basis for differentiating the eras?
The geologic time scale is a system used to categorize and understand the history of Earth. It is divided into several subdivisions, each representing a significant period of time, and provides a chronological framework for studying Earth's past.
The major subdivisions of the geologic time scale, from largest to smallest, are eons, eras, periods, epochs, and ages.
1. Eon: The largest division of time in the geologic time scale is the eon, which represents billions of years. Currently, there are three recognized eons: the Archean, Proterozoic, and Phanerozoic (the most recent and the one we are currently in).
2. Era: The second largest division is the era, which typically represents hundreds of millions of years. Eras are defined based on major global geological events, changes in the fossil record, and the development of life forms. There are three eras within the Phanerozoic eon: the Paleozoic, Mesozoic, and Cenozoic.
3. Period: Below eras, periods represent tens of millions of years. They are characterized by distinct rock layers and fossil assemblages. Examples of periods within the Mesozoic era include the Jurassic and Cretaceous.
4. Epoch: Epochs represent subdivisions of periods and generally represent millions of years. They are defined by more specific geologic events and changes in the fossil record. An example is the Pleistocene epoch within the Cenozoic era.
5. Age: The smallest subdivision of the geologic time scale is the age, which typically represents a few million years or less. Ages are defined based on precise radiometric dating of rocks and minerals.
The primary basis for differentiating the eras in the geologic time scale is by major changes in Earth's environment and the evolution of life forms. Significant events such as mass extinctions, changes in climate, and the appearance of new species help to mark the boundaries between eras. Fossil evidence, such as the presence or absence of certain groups of organisms, is also used to identify and differentiate eras.