What are the primary subdivisions of geologic time from largest to smallest?

• A. Eons, Eras, Periods
• B. Ages, Epochs, Ages
• C. Eras, Periods, Epochs
• D. Eons, Ages, Epochs

Answer: (A)

Geologic time is organized hierarchically into broad blocks that run from very long to shorter intervals. The largest unit is the eon, which spans enormous spans of Earth's history. Within each eon are eras, representing major phases of this history. Inside each era are periods, which mark more specific intervals with distinct rock layers and fossil groups. After periods come even finer divisions—epochs and ages—but the primary subdivisions from largest to smallest are eons, eras, and periods. So the best answer lists those three in that order. The other options mix in smaller units too early or place them in an order that doesn’t reflect the broad-to-narrow hierarchy.