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Hadean
:''Prebiotic world redirects here but there is as yet no subdivision of this Eon to reflect recent developments''
The Hadean is a geologic eon of the Earth, and lies before the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago.〔(【引用サイトリンク】url=http://stratigraphy.org/column.php?id=Chart/Time%20Scale )〕 The name "Hadean" comes from Hades, the ancient Greek god of the underworld, in reference to the hellish conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term: the "Priscoan period". Other, older texts simply refer to the eon as the Pre-Archean. Nonetheless, in 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia.〔 Early edition, published online before print.〕 According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."〔
==Etymology==
The Hadean was a violent period in the history of Earth. Hence, it receives its name from Hades (; from Ancient Greek , ''Hāidēs''; Doric Ἀΐδας ''Aidas''), the ancient Greek god of the underworld.〔(Terence McCarthy, Bruce Rubisge. "Story of Earth and Life". Ed. University of the Witwatersrand. School of Geosciences. ISBN 9781770071483. P. 70 )〕
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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