Earth's history is divided using the geological time scale, which organizes approximately 4.5 billion years into eons, eras, periods, epochs and ages. Boundaries are identified from major changes preserved in rocks, particularly changes in the fossil record.
This is HL-only content in IB ESS subtopic 3.1, Biodiversity and evolution.
How the divisions work
Scientists study rock strata and correlate layers from different locations. A significant, widespread change in fossils, rock chemistry or other geological evidence may be used to define a boundary between units.
| Division | Relationship and example |
|---|---|
| Eon | Largest division, such as the Phanerozoic Eon |
| Era | Subdivision of an eon, such as the Cenozoic Era |
| Period | Subdivision of an era, such as the Quaternary Period |
| Epoch | Subdivision of a period, such as the Pleistocene and Holocene epochs |
| Age | Smallest formal subdivision of geological time |
For example, the boundary between the Cretaceous and Paleogene periods is associated with a major mass extinction approximately 66 million years ago. The fossil record shows the abrupt disappearance of non-avian dinosaurs and many marine organisms, while a globally distributed iridium-rich layer provides additional evidence of an asteroid impact.
Epochs therefore do not represent equal lengths of time. Their boundaries reflect detectable environmental or biological transitions rather than regular numerical intervals.
The Anthropocene is a proposed epoch intended to recognize rapid environmental change caused by human activity, including altered sedimentation, pollution and accelerated species extinction. A common misconception is that the Anthropocene is already a formally accepted geological epoch. It remains a proposed term, although it is important in ESS for understanding humanity's planetary influence.
Exam technique
For an IB ESS explain question, identify the hierarchy and connect geological boundaries to evidence from rock strata and fossils. Use a named example, such as the Cretaceous to Paleogene mass extinction, and avoid stating that epochs have equal durations or that the Anthropocene is formally established.