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USGS CMG InfoBank: Composition of the Continental Crust

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Comment: 05:05 - 05:53 (00:48)
Source: Annenberg/CPB Resources - Earth Revealed - 7. Mountain Building
Keywords: "granitic rock", greenstone, magma, crust, volcanism, lithosphere, sediment

Our transcription: Continental crust, however, consists mostly of granitic rocks and not greenstone lavas, so a mechanism must have existed for transforming the composition of the crust.
Some geologists speculate that the greenstone lavas weathered to form sediments rich in potassium, aluminum and silica.
These sediments could have been incorporated by magmas erupting at a later date altering the molten rock to a more granitic composition.
Or perhaps repeated partial melting of the lithosphere underneath the greenstone islands produced a more granitic crust.
Whatever the reason, the world of greenstone belt volcanism did not last.

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