Archean

The Archean Eon (pronunciation: /ɑːrˈkiːən/, also spelled Archaean) is a geologic eon, 4,000 to 2,500 million years ago (4 to 2.5 billion years), that followed the Hadean Eon and preceded the Proterozoic Eon. During the Archean, the Earth's crust had cooled enough to allow the formation of continents.

Archean (or Archaean) comes from the ancient Greek Αρχή (Arkhē), meaning "beginning, origin". It's earliest use is from 1872, when it meant "of the earliest geological age." In earlier literature the Hadean Eon was included as part of the Archean.

Instead of being based on stratigraphy, the beginning and end of the Archean Eon are defined chronometrically. The eon's lower boundary or starting point of 4 Gya (4 billion years ago) is officially recognized by the International Commission on Stratigraphy.

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, from the formation of the Earth's core, and produced by radioactive elements.

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