An eon is the second largest unit of time on the geologic time scale. It does not have a fixed duration, but eons start and end with major changes in Earth's geological evolution. There have been four eons since the formation of Earth: * Hadean (4.5 to 3.8 billion years ago) * Archean (3.8 to 2.5 billion years ago) * Proterozoic (2.5 to 0.5 billion years ago) * Phanerozoic (0.5 billion years ago to the present)