1. The Geologic Time Scale

2. The Geologic Time Scale divides Earth’s history into different time units based on major biological and geological events

3. Eons Eons – longest units of time
There are four eons in Earth’s history
The first three eons are group together and referred to as Precambrian Time
The most recent eon (the Phanerozoic) is divided into eras and periods

4. Eras The Phanerozoic EON is divided into 3 ERAS:
Paleozioc – 544 to 245 mya (million years ago)
Mesozoic – 245 – 65 mya
Cenozoic – 65 mya – present
The length is determined by a major EVENT, not by a certain amount of time

5. Periods Eras can be divided into smaller units of time called Periods.
Just like eras, the length of a period is determined by a major EVENT, not by a certain amount of time
Each era may have a different number of periods based on what major events occurred.