The oldest period in Earth's history is the Hadean Eon, which lasted from approximately 4600 to 4000 million years ago (MYA). This is based on the geologic time scale.
Understanding Geologic Time
The geologic time scale divides Earth's history into eons, eras, periods, and epochs. The Precambrian Supereon encompasses the earliest part of Earth's history, and is further divided into three eons:
- Hadean (4600-4000 MYA): The oldest eon, characterized by the formation of Earth and its initial cooling.
- Archean (4000-2500 MYA): Saw the emergence of the first life forms.
- Proterozoic (2500-541 MYA): A period of significant continental growth and the evolution of more complex life.
The reference provided states that "The oldest subdivision of the time scale is the Precambrian (symbolized by PC, X, Y, or Z in the GRI GIS data). The Precambrian is split into three eons: Hadean (4600-4000 MYA), Archean (4000-2500 MYA), and Proterozoic (2500-541 MYA)." This clearly identifies the Hadean as the oldest. Other references discuss various ages related to human history, biological processes, and other topics, but are not relevant to the geological age of Earth's periods.