1. of, relating to, or denoting the earliest eon of Earth's history, lasting from the formation of the Earth to about 2.5 billion years ago
Usage: geology; also spelled Archean

1. the Archaean eon; the earliest division of geological time
Usage: geology; also spelled Archean

Examples

• Archaean rocks are among the oldest known materials on Earth.
• The Archaean eon saw the formation of the earliest continents.
• Geologists study Archaean fossils to understand early life on Earth.
• During the Archaean, the atmosphere contained very little oxygen.
• Archaean greenstone belts contain important mineral deposits.