Archean
The Archean Eon (, also spelled Archaean) is a geologic eon, 4,000 to 2,500 million years ago, following the Hadean Eon and preceding the Proterozoic Eon. During the Archean, the earth's crust and layers had just formed, making the Earth much cooler than it was during the Hadean and allowing the formation of continents.
Classification issues
Instead of being based on stratigraphy, the beginning and end of the Archean are defined chronometrically. The lower boundary (starting point) of 4 billion years is officially recognized by the International Commission on Stratigraphy.
The Archean customarily starts at 4 Ga—at the end of the Hadean Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek Αρχή (Arkhē), meaning "beginning, origin".
Earth at the beginning of the Archean
The Archean is one of the four principal eons of Earth history. When the Archean began, the Earth's heat flow was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 million years ago). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements.
