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Long span of geologic time.
In formal usage, eons are the longest portions of geologic time ( era s are the second-longest). Three eons are recognized: the Phanerozoic Eon (dating from the present back to the beginning of the Cambrian Period), the Proterozoic Eon , and the Archean Eon . Less formally, eon often refers to a span of one billion years.
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[c mediumvioletred] (as used in expressions)
Archean Eon
Archaean Eon
{{link=Phanerozoic Eon">Phanerozoic Eon
Proterozoic Eon