Question: what are the four major divisions of geologic history?
Answer: The largest unit of geologic time is an eon. Earth's 4.6-billion-year history is divided into four eons: the Hadean, Archean, Proterozoic, and Phanerozoic. The Hadean, Archean, and Proterozoic eons together are called Precambrian time. Precambrian time makes up almost 90 percent of Earth's history.