Geologists have subdivided geologic time into periods based on significant changes in the Earth's fossil record, rock formations, and major geologic events. These changes are often associated with:

* Mass extinctions: The disappearance of a large number of species in a relatively short period of time.

* Major evolutionary developments: The appearance of new, significant life forms.

* Changes in Earth's climate and environment: Fluctuations in sea level, ice ages, and shifts in tectonic plates.

* Significant geological events: Large-scale volcanic eruptions, asteroid impacts, and mountain-building events.

By studying these changes, geologists can create a timeline of Earth's history and understand the processes that have shaped our planet.