What separates one eon from another?

Eons are the largest divisions of geological time, separated by significant changes in Earth's geology and life forms. Major transitions, such as the appearance of complex life or major extinction events, typically mark the boundaries between eons. For example, the transition from the Archean to the Proterozoic eon is characterized by the emergence of atmospheric oxygen and multicellular life. These shifts reflect profound changes in Earth's environment, biological diversity, and tectonic activity.