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! :)

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"

I'm not too sure about the first fact. Can someone elaborate? I know felsic lava has high viscosity though, and in any case it is overwhelmingly due to its high silica content.


-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...


Not too sure either.There was pretty free-flowing lava from Mt. Sicily when it erupted a few years ago, and I'm not sure what Sicily's content is regarded as.


-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?


Not necessarily. Lava that cools slowly will not form such a thin crust since there's plenty of time for the heat to dissipate (conduct) out.


"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?


I don't see why not. Felsic lava tends to be more chunky anyway, I would have more a problem with the opposite.


"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?


As the information states, aa is lava that is cooled beyond the extent where it can flow like pahoehoe. So lava tubes would be found where pahoehoe lava solidified.

It occurred to me a while ago that as Olympus Mons on Mars (the largest volcano in the solar system) is a 700 km (or 700 mile?) wide basalt volcano, it should be full of lava tubes, any one of which could be hundreds of miles long. There'd be no need to build a surface city on Mars; it'd be much smarter just to build a city underground, by sealing off the lava tubes and making them habitable.