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Thread: why is the basalt beneath the granite?

  1. #1 why is the basalt beneath the granite? 
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    the basalt is formed by the eruption of the volcano,so it should lie on the surface of the earth.
    the granite is a plutonic igneous rock having visibly crystalline texture; so it should be formed underground.
    so why is the basalt beneath the granite?


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  3. #2  
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    What basalt, where?
    Are you talking about the lowermost basaltic portions of the continental crust? If so, the rock is only basaltic, not actually basalt. It is the same in composition as basalt but it is a coarse-grained intrusive rock.


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    the basalt is formed by the eruption of the volcano,so it should lie on the surface of the earth.
    And that's exactly where it is around here. If you hike up to either North or South Table Mountain near Denver the "table top" is basalt. This a a piece I picked up. I'm no geologist but I believe it's porphyritic basalt.

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  5. #4 Re: why is the basalt beneath the granite? 
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    Quote Originally Posted by heaventian
    the basalt is formed by the eruption of the volcano,so it should lie on the surface of the earth.
    the granite is a plutonic igneous rock having visibly crystalline texture; so it should be formed underground.
    so why is the basalt beneath the granite?
    Your thinking in general terms is correct, but we're gonna need a bit more context on your particular situation/question to figure out what's going on.
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    Thank you!
    I really appreciate it for all your help!
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    just like what The Matt thought,I doubt why the granite forms the most of the upper crust while the basalt forms the lower crust.
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    Quote Originally Posted by heaventian
    just like what The Matt thought,I doubt why the granite forms the most of the upper crust while the basalt forms the lower crust.
    Ah, yes, I see.

    Less confusing terms that you may already know about generally fall along felsic (granitic), mafic (basaltic), and ultramafic (mantle rocks like peridotite) characterized by relative abundances of SiO2 vs Mg/Fe/Ca cations.
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    Quote Originally Posted by Bunbury
    This a a piece I picked up. I'm no geologist but I believe it's porphyritic basalt.
    It's tricky to be certain from a single photo, but I'm not sure it's a basalt. Reasons:
    Basalts are typically dark rocks, black or grey.
    The colouration suggests an orthoclase feldspar is making up a lot of the groundmass. This would not be present, in these quantities, in basalt.
    The large dark crystals do not look like olivines, which we would expect in basalts. They might be pyroxenes, which would be OK, but they could be hornblendes, which is not. (Looking at it again, I think they are pyroxenes.)

    On balance I think it is most likely a syenite.

    It's a nice sample, whatever it is.
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    John, thanks for that. I was going on the basis that South Table Mountain, where I picked it up, is capped by basalt according to the local geological lore, but there are both gray and reddish layers as this picture shows. The red overlies the gray and is everywhere, in large boulders and smaller pieces as well as flat slabs. I thought this layer of shale-like material between a gray and a red layer was curious. (I was actually up there looking for the KT Boundary but didn't find it. It's supposedly the first place it was identified in North America.)



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    Some googling tells me that the capping lavas (there are three flows in total) consist of "shoshonite, a potassium-rich basalt."
    http://www.periclespress.com/geology_Table.html

    Shoshonite is not a rock type I have encountered before. That's not unusual. There a host of variants of the fundamental rock types with local names. The potassium rich character would account for the colouration - orthoclase is a potassium feldspar. So your sample almost certainly is a basalt, but of an unusual composition.

    I was interested to learn that the mountains overlook Golden, home to the Colorado School of Mines, which is world famous - at least in the Earth Science community. A dear colleague, who persists in buying up mineral rights in obscure places in hope of striking it rich, was educated there.

    The shale like material you found between the flows is likely a fossil soil. The feldspar in the rock readily breaks down into clay minerals. I am slightly puzzled by the apparent freshness of the upper surface of the flow, but again working from a photograph is not the ideal way to go. :?

    You can get a paper on "The Cretaceous-Tertiary boundary interval at south table mountain, near Golden, Colorado" here: http://www.springerlink.com/content/d21th5921041342m/

    It would be neat if that clay layer between the flows is the KT boundary itself. If you can just pop back up with your portable mass spectrometer and check the iridum content we would know for sure. :wink:

    Alas, not. It seems, if this paper is correct, that the lavas are much younger.
    http://www.buttecounty.net/dds/Plann...aFormation.pdf
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  12. #11 Re: why is the basalt beneath the granite? 
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    Quote Originally Posted by heaventian
    the basalt is formed by the eruption of the volcano,so it should lie on the surface of the earth.
    the granite is a plutonic igneous rock having visibly crystalline texture; so it should be formed underground.
    so why is the basalt beneath the granite?
    Basalt is denser, hence heavier than granite.
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  13. #12  
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    Quote Originally Posted by John Galt
    Some googling tells me that the capping lavas (there are three flows in total) consist of "shoshonite, a potassium-rich basalt."
    http://www.periclespress.com/geology_Table.html

    Shoshonite is not a rock type I have encountered before. That's not unusual. There a host of variants of the fundamental rock types with local names. The potassium rich character would account for the colouration - orthoclase is a potassium feldspar. So your sample almost certainly is a basalt, but of an unusual composition.

    I was interested to learn that the mountains overlook Golden, home to the Colorado School of Mines, which is world famous - at least in the Earth Science community. A dear colleague, who persists in buying up mineral rights in obscure places in hope of striking it rich, was educated there.

    The shale like material you found between the flows is likely a fossil soil. The feldspar in the rock readily breaks down into clay minerals. I am slightly puzzled by the apparent freshness of the upper surface of the flow, but again working from a photograph is not the ideal way to go. :?

    You can get a paper on "The Cretaceous-Tertiary boundary interval at south table mountain, near Golden, Colorado" here: http://www.springerlink.com/content/d21th5921041342m/

    It would be neat if that clay layer between the flows is the KT boundary itself. If you can just pop back up with your portable mass spectrometer and check the iridum content we would know for sure. :wink:

    Alas, not. It seems, if this paper is correct, that the lavas are much younger.
    http://www.buttecounty.net/dds/Plann...aFormation.pdf
    School of Mines is a great school. We recruit most of our chemical engineers from there. The other famous landmark in Golden is the Coors brewery.

    When I took that picture I was hoping it was the KT boundary but alas it is not so obvious. It is apprently somehwere halfway up the mountain visible in a disused quary. (This is only a small mountain BTW). Anytime you're out this way I'll be glad to take you up there.
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