GENERAL GEOLOGY

Detailed explanation-1: -The further subdivision of the eras into 12 “periods” is based on identifiable but less profound changes in life-forms. In the most recent era, the Cenozoic, there is a further subdivision of time into epochs.

Detailed explanation-2: -Eras are subdivided into periods. The events that bound the periods are wide-spread in their extent but are not as significant as those which bound the eras.

Detailed explanation-3: -Eras are divisions within eons that span hundreds of millions of years. Periods are the subdivisions within eras that span tens of millions of years. There are a different number of periods within each era. For example, the Paleozoic has seven different periods, while the Mesozoic has three.

Detailed explanation-4: -The geologic history of the Earth is broken up into hierarchical chunks of time. From largest to smallest, this hierarchy includes eons, eras, periods, epochs, and ages.