Archean History 2

Archean History 2 - Archean Remember Precambrian time...

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1 Archean Remember… • Precambrian time includes 80% of the earth's history, from 4.5 byr - 700 myr. • So for the first 500 myr there is no geologic record. • Original crust was thin and composed mainly of komatite & basalt. • Granites have formed from basalts from partial melting with the crust. – -mafic to felsic • This process causes enrichment of lighter elements such as Si, Al, K, Na. Granites formed by accreted terrains onto a larger land mass. • The chemistry of granitic rocks changed through time. This seems to indicate increasing chemical differentiation of the earth through time. Partial melting Early crust characteristics Early atmosphere
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2 How did the oxygen get into the atmosphere? • Photochemical dissociation – Breakup of water molecules by ultraviolet light. Also produces ozone. – Significant in the past. • Photosynthesis – More important. Plants evolved to split carbon dioxide into carbon and oxygen. Archean rock types
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Archean History 2 - Archean Remember Precambrian time...

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