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As far as Mt. St Helens, that is just one example of a volcano. I cited it to give you an idea of how large a volanic explosion could be. However, you are missing the point entirely. Let's just suppose that the emissions are small (say only 1 billion tons of CO2 in total ) for Mt. St. Helens as you say (and I certainly do not agree with you, but let's just say it is true). Mt. St. Helens was a relatively small eruption. There have been volcanic explosions 100's of times to 1000's of times larger. Several this century in fact.
And look at these monsters:
Here is a little brochure put out by the DOE and I linked this earlier. http://www.eia.gov/oiaf/1605/ggccebro/chapter1.html
Magically, they think about 6-7 billion tons of CO2 was created. That is a little shy of the 9 billion tons you cited, but ok - let's go with that.
I also liked your selective reading above on the analysis of the eruption of Mt St. Helens. First you denied that 60% per volume of the magma can be converted to CO2/SO2, then I shot that one down. Now, your new one is that a sampling of the eruption (one data point) is all the gas that was emitted. But that is just par for the course with you. Read it more carefully next time. Also, math helps here too. 1 cubic mile of magma converted at that rate emits a tremendous amount of CO2/SO2. Since one cubic mile of magma weight about 45 trillion pounds, it doesn't take much mathematical ability to realize how enormous the OVERALL eruption is (on the scale of 100's of billion of tons of CO2/SO2).
Let's see how much gas fraction can potentially yield from magma shall we?
From http://www.agu.org/meetings/fm05/fm05-sessions/ and I quote
"Closed system ascent degassing calculations show that the volume fraction of gas increases to 8 vol% at 4-5 km and reaches 50 vol% at 1 km, where final solidification begins. The gas fraction can potentially increase to >60 vol% during solidification. Allowing for gas separation during extrusion, these results are consistent with observed dacite vesicle fractions averaging 25 vol% (Pallister et al. this session). Ascent degassing calculations also predict melt water contents similar to values measured on rare glassy dacite fragments last equilibrated at depths of 1.2-1.8 km (Mandeville this session)."
Hmm, seems the experts think that gas fraction can increase to 60% of volume. Isn't that amazing?!?