Why are time periods on the geologic time scale not even in length
Time periods on the geologic time scale are not even in length because they are determined by significant events or changes in the Earth's history rather than by a consistent measurement of time. The geologic time scale is divided based on major geological, biological, and climatic events that have occurred throughout history, such as the formation of major mountain ranges, the appearance or extinction of certain organisms, or changes in global climate.
These events and changes do not occur at regular intervals or with consistent durations, so the time periods on the geologic time scale do not have equal lengths. For example, the Precambrian eon, which encompasses about 88% of Earth's history, is divided into three periods - the Hadean, Archean, and Proterozoic - that span a vast amount of time, while the Phanerozoic eon, which represents the most recent 12% of Earth's history, is divided into three eras - the Paleozoic, Mesozoic, and Cenozoic - that are relatively shorter in comparison.
Additionally, the geologic time scale is continuously updated and refined as new discoveries and research provide better understanding of Earth's history. This dynamic nature of the time scale further contributes to the uneven lengths of its time periods as new information and events are incorporated into the geological timeline.