The Archean is the period in the earth's history from about 3.8 to 2.5 billion years ago (Ga). The term was derived from the Latin word for first because the beginning of the Archean is defined as the age of the oldest rocks identified on Earth. As the study of these rocks continues and older rocks are discovered, some scientists now expand the Archean back to 4 billion years to include recently dated rocks. The Archean is part of the Precambrian Era, the entire time span between the formation of the earth 4.56 billion years ago and the beginning of the Cambrian Era, 544 million years ago. The Archean is preceded by the Hadean Eon, a little used term for the period from which no rocks are preserved (4.56–3.8 Ga), and is followed by the Proterozoic (2.5–0.54 Ga).

Archean aged rocks are found mostly in the interior of continents. They provide evidence that Earth during...