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archean
The Archean Eon (, also spelled Archaean) is a geologic eon, , 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, 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 ('), meaning "beginning, origin".
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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