Final answer:
The subdivisions of the geologic time scale that represent the greatest expanse of time are called Eons. Geologic time is first divided into eons, which include the Hadean, Archean, Proterozoic, and Phanerozoic.
Step-by-step explanation:
The subdivisions of the geologic time scale that represent the greatest expanse of time are called Eons. Geologic time is first divided into eons, which include the Hadean, Archean, Proterozoic, and Phanerozoic. Eons are then subdivided into eras, eras into periods, periods into epochs, and epochs into ages.