The Earth formed ~4.6 billion years ago and the earliest forms of life are thought to have appeared ~3.5 billion years ago
- To describe the geology and history of life on Earth, scientists have developed the geological time scale
The geological time scale measures time on a scale involving four units:
- An epoch is the smallest unit of time on the scale, but still encompasses a period of millions of years
- Chronologically, epochs are grouped together into larger units called periods
- Periods are combined to make a subdivision called an era
- An eon is the largest division of geological time
The division of time units in the scale is typically based on the occurrence of a significant geological event (e.g. mass extinction)
- Consequently, geological time categories are variable in length and do not usually consist of a uniform length of time
Geological Time Scale
