The geologist has divided the earth tome into a series of time intervals, and they are not in equal length like the hours in a day as they are made through the historically significant events.
Thus they are divided into boundaries like the Permian and the Triassic boundaries are marked by the global extinction of species, another boundary of Precambrian and the Paleozoic which is marked by the first appearance of land plants and the animals.
Eon is divided into smaller times zones known as eras. And the eras are subdivided into periods. And the epochs are a finer subdivision of time as possible periods of Cenozoic.
The supereon known as Precambrian is divided onto Hadean, Archean and Proterozoic eons this is longest period of geologic history staring at 4600 million years ago. And quaternary period being the shortest under the Pleistocene epoch.
