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Thread: Lava viscosity and more...

  1. #1 Lava viscosity and more... 
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    While reading the sections of volcanic eruptions in my text book, I am puzzled by 2 problems...

    1) "One of the three factors of lava viscosity is "volatile (gas) content", and the more gases in the lava, the less viscous it is"

    "Felsic lava has large amount of dissovled gas and the lava has high viscosity"

    These two quotes are both from my text book, and it seems to me that they are contradicting each other! Is one of the above quotes wrong? Can someone explain?

    2) "When mafic lava cools rapidly, the surface of the flow forms a crust. If deformed slowly by further movement of the flow, this crust wrinkles to form a ropy volcanic rock known as PAHOEHOE. If the crust is deformed rapidly or grows too thick to form wrinkles, the surface breaks into jagged chunks to form AA, which has a sharp and blocky texture. Sometimes the outer part of a mafic lava flow cools so rapidly that it forms a hardened shell around a liquid interior. The still-molten lava flows out, leaving tunnels called LAVA TUBE. When lava flows out of fissures on the ocean floor, it cools rapidly, often separating into rounded blobs called PILLOW LAVA."

    This paragraph really confuses me...

    -Are PAHOEHOE, AA, LAVA TUBE, and PILLOW LAVA all (or mostly) formed by mafic lava, and never felsic lava? This is what I get from the paragraph, because throughout the paragraph, it keeps mentioning mafic lava, but never says felsic lava...

    "When mafic lava cools rapidly, the surface of the flow forms a crust"
    -I thought that ALL lava will cool to form crust, not only mafic lava and not only those that cools rapidly, lava that cools slowly will also form crust, true?

    "If deformed slowly by further movement of the flow, this crust wrinkles to form a ropy volcanic rock known as PAHOEHOE."
    -What does it mean by deformed slowly, like slow further flowing of the solidified lava?

    "If the crust is deformed rapidly or grows too thick to form wrinkles, the surface breaks into jagged chunks to form AA, which has a sharp and blocky texture."
    -Can lava that cools slowly form AA?

    "Sometimes the outer part of a mafic lava flow cools so rapidly that it forms a hardened shell around a liquid interior. The still-molten lava flows out, leaving tunnels called LAVA TUBE."
    -Where can LAVA TUBE be found, near PAHOEHOE or near AA?

    Can somebody explain? I would appreciate!


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  3. #2  
    Universal Mind John Galt's Avatar
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    I'm very rusty on lava viscosities, but I think the following is the explanation you are looking for.

    Felsic lavas are more viscous than mafic lavas.
    High gas content does reduce the visocity of the lava.

    These are two independent factors. i.e. A mafic lava is naturally of low viscosity. If there is a high gas content its viscosity will be lowered further. A felsic lava starts out with a high viscosity. If it has a high gas content its viscosity will be lower than the same lava with little or no gas, but still significantly higher than the mafic lava, with or without gas.

    Recognise also that it is the high viscosity of the felsic lava that contributes to the high gas content by trapping the gas.


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  4. #3  
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    Quote Originally Posted by Ophiolite
    I'm very rusty on lava viscosities, but I think the following is the explanation you are looking for.

    Felsic lavas are more viscous than mafic lavas.
    High gas content does reduce the visocity of the lava.

    These are two independent factors. i.e. A mafic lava is naturally of low viscosity. If there is a high gas content its viscosity will be lowered further. A felsic lava starts out with a high viscosity. If it has a high gas content its viscosity will be lower than the same lava with little or no gas, but still significantly higher than the mafic lava, with or without gas.

    Recognise also that it is the high viscosity of the felsic lava that contributes to the high gas content by trapping the gas.
    Thanks for your time! :wink:

    From your reply, the 2 quotes above are both correct.

    Felsic lava almost always has more gas content in it because they produce explosive eruptions...

    Although felsic lava has more gas content in it, it is still more viscous than mafic lava, so that means that "gas content" isn't a strong factor that affect viscosity, rather it is the composition (amount) of silica that strongly affects viscosity...??

    "Recognise also that it is the high viscosity of the felsic lava that contributes to the high gas content by trapping the gas."
    Why??
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  5. #4  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by kingwinner
    Thanks for your time! :wink:
    My pleasure. I wish I had more time to do your questions more justice. The force me to think about issues I haven't looked at recently, so its a form of revision for me.
    Quote Originally Posted by kingwinner
    From your reply, the 2 quotes above are both correct.
    Exactly so.
    Quote Originally Posted by kingwinner
    Felsic lava almost always has more gas content in it because they produce explosive eruptions...
    It would be better (more accurate) to say this the other way round. Felsic lavas generally produce explosive lavas, in part because they have more gas trapped in them.
    Quote Originally Posted by kingwinner
    Although felsic lava has more gas content in it, it is still more viscous than mafic lava, so that means that "gas content" isn't a strong factor that affect viscosity, rather it is the composition (amount) of silica that strongly affects viscosity...??
    Correct. I don't know the relative effects of the two, but I suspect there is an order of magnitude difference. i.e. The composition effects the viscosity roughly ten times as much as the gas content does.
    Quote Originally Posted by kingwinner
    "Recognise also that it is the high viscosity of the felsic lava that contributes to the high gas content by trapping the gas."
    Why??
    If the gas is to escape it has to rise within the magma. To do that is must push aside the lava to make room for itself. If the magma has a low viscosity it will be easier to push aside, and vice versa.
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  6. #5  
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    Quote Originally Posted by Ophiolite
    Quote Originally Posted by kingwinner
    Thanks for your time! :wink:
    My pleasure. I wish I had more time to do your questions more justice. The force me to think about issues I haven't looked at recently, so its a form of revision for me.
    Quote Originally Posted by kingwinner
    From your reply, the 2 quotes above are both correct.
    Exactly so.
    Quote Originally Posted by kingwinner
    Felsic lava almost always has more gas content in it because they produce explosive eruptions...
    It would be better (more accurate) to say this the other way round. Felsic lavas generally produce explosive lavas, in part because they have more gas trapped in them.
    Quote Originally Posted by kingwinner
    Although felsic lava has more gas content in it, it is still more viscous than mafic lava, so that means that "gas content" isn't a strong factor that affect viscosity, rather it is the composition (amount) of silica that strongly affects viscosity...??
    Correct. I don't know the relative effects of the two, but I suspect there is an order of magnitude difference. i.e. The composition effects the viscosity roughly ten times as much as the gas content does.
    Quote Originally Posted by kingwinner
    "Recognise also that it is the high viscosity of the felsic lava that contributes to the high gas content by trapping the gas."
    Why??
    If the gas is to escape it has to rise within the magma. To do that is must push aside the lava to make room for itself. If the magma has a low viscosity it will be easier to push aside, and vice versa.
    So are cinder cone volcanoes formed mainly as a result of felsic lava (highly viscous lava) that breaks into solid fragments of pyroclastic materials? (since only viscous lava with gas in it can form pyroclastic materials?)
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