I think you have asked some questions that do have answers, but the science behind the answers is fairly specialized and complex. Others seem to have covered your questions fairly well, without going so far as to write out the vast details of the processes.
The one item seemingly not covered was:
1. Volcanic gasses are composed of CO2 and steam. Since carbon is rarely found in basement or igneous rocks, what could be the source of the CO2 found in the gas?
Now, this is getting into mantle chemistry considerations, which is a complex item in itself. My far from expert comments:
That may or may not be an accurate statement about what volcanic gasses are. Some types of volcanos are not very "gassy" - Example, the basaltic volcanos of Hawaii. Others may be more "gassy" - Example, the more silica rich volcanos such as Vesuvius and Mt. Saint Helens. To get into this further would be to get rather heavily into magmatic evolution theory, which is very off-topic here.
Anyhow, there is a CO
2 component in the mantle. It tends to get concentrated in the residual magma as the various minerals crystalize out because Carbon (C) does not fit into the crystal structures of common minerals (or even uncommon minerals?).
Again, all this is getting pretty deep into igneous rock formation theory (igneous petrology), so I will go no further (expecially since I'm already working the fringes of my personal knowledge).
Critiques from the other geologists welcome, although igneous petrology considerations probably should go to a new topic.
Moose
To all - See terminology you don't understand? Let me remind you of the existence of the forums
Geology Glossary.
This message has been edited by minnemooseus, 03-19-2006 02:56 PM