Earth’s history is too long to narrate without a framework. The geologic time scale divides the 4.54-billion-year record into hierarchical named units—eons, eras, periods, epochs, and ages—based on the rock and fossil record. Absolute ages from radiometric dating calibrate the relative sequence.

Major boundaries often coincide with mass extinctions or other global events. The scale is continually refined as new dates and correlations appear. International commissions formalize the definitions of stage boundaries using global stratotype sections.

Students first encounter the familiar eras and periods; specialists work at the level of stages and zones. The time scale allows any dated rock or fossil to be placed in a common narrative of Earth history. It is both a practical tool and a distillation of two centuries of stratigraphic research.

• Hierarchical system for Earth’s history
• Combines relative and absolute dating
• Boundaries often mark major biotic events
• Continuously updated by international agreement