Periods of Time

Evidence from dating based on the decay of certain radioactive isotopes indicates that the Earth is about 4.54 billion years old. The geology or deep time of Earth’s past has been organized into various units according to events which took place in each period. Different spans of time are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or fossil events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Cambrian) are defined by the absolute age.

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The following chart is taken from a poster prepared by the Irish Geological Survey.