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Thread: Crust

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    How old is the Earth's crust??


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    WYSIWYG Moderator marnixR's Avatar
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    the oldest continental crust ranges from 3.7 to 4.2 billion years, i.e. close to the earth's age
    oceanic crust does not exceed 200 million years, due to older oceanic crust having subducted


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    Quote Originally Posted by marnixR View Post
    the oldest continental crust ranges from 3.7 to 4.2 billion years, i.e. close to the earth's age
    oceanic crust does not exceed 200 million years, due to older oceanic crust having subducted
    Forgive me if I'm wrong but the oldest rocks on the Moon are older that the Earth's oldest rocks yet the theories of Moon formation require both to be present at the same time.

    So I fathomed the explanation for that apparent contradiction was that the rocks represent the time the body solidified and the therefore I'd agree that the Moon solidified before the Earth did.
    Does that make sense?
    Apollo 17 sample helps date Moon - space - 26 January 2009 - New Scientist
    gives a taste of what I'm talking about.
    Zircons from Earth tell the story of a fast-cooling planet that developed a solid crust within 200 million years of formation from the solar nebula, says John Valley at the University of Wisconsin-Madison, whose group dated the oldest terrestrial samples. "It's reasonable that there would be something older from the moon than on Earth," he says, because the smaller moon cooled more quickly after the colossal impact.
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    Quote Originally Posted by Robittybob1 View Post
    Forgive me if I'm wrong but the oldest rocks on the Moon are older that the Earth's oldest rocks yet the theories of Moon formation require both to be present at the same time.

    So I fathomed the explanation for that apparent contradiction was that the rocks represent the time the body solidified and the therefore I'd agree that the Moon solidified before the Earth did.
    no - you forget about erosion and various other ways that rocks (even continental rocks) get reworked

    meaning that it's likely that the earth and the moon were formed at the same time (which would have to be the case if the current theories on their formation are correct : see giant impact hypothesis), continental rocks get deformed during mountain building, metamorphosed at great depth and worn away through wind, ice and rain - hence only a small portion of the oldest rocks (and none for the very oldest ones) have remained relatively untouched until the present day

    in contrast the moon has no atmosphere and no plate tectonics and hence its oldest rocks have remained largely untouched
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    Quote Originally Posted by marnixR View Post
    Quote Originally Posted by Robittybob1 View Post
    Forgive me if I'm wrong but the oldest rocks on the Moon are older that the Earth's oldest rocks yet the theories of Moon formation require both to be present at the same time.

    So I fathomed the explanation for that apparent contradiction was that the rocks represent the time the body solidified and the therefore I'd agree that the Moon solidified before the Earth did.
    no - you forget about erosion and various other ways that rocks (even continental rocks) get reworked

    meaning that it's likely that the earth and the moon were formed at the same time (which would have to be the case if the current theories on their formation are correct : see giant impact hypothesis), continental rocks get deformed during mountain building, metamorphosed at great depth and worn away through wind, ice and rain - hence only a small portion of the oldest rocks (and none for the very oldest ones) have remained relatively untouched until the present day

    in contrast the moon has no atmosphere and no plate tectonics and hence its oldest rocks have remained largely untouched
    Exactly - it had me baffled for both bodies (the Earth and Theia) had to somewhat pre-exist to prior to the impact or capture (if you are like me and prefer the Moon Capture theory) for to collide or be captured they must have existed prior to that event, but the dating of the rocks is from or after that event because of the melting and re-solidification.

    Well that was how I ended up understanding it!
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    WYSIWYG Moderator marnixR's Avatar
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    Quote Originally Posted by Robittybob1 View Post
    ... but the dating of the rocks is from or after that event because of the melting and re-solidification.

    Well that was how I ended up understanding it!
    that's more or less my understanding too - the impact completely reset the geological evolution of the earth + formed the moon in the process
    subsequent to that i assume that both formed crusts at more or less the same time, although the late heavy bombardment may have affected the earth's crust more because of the earth's greater mass

    following this episode the oldest rocks on earth closely match the end of the bombardment, whereas on the moon, more old crust remained
    after that nothing much more happened on the moon, whereas a lot of the early evidence for the first continents on earth has been the victim of subsequent geological processes
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