Do volcanoes burn underground oil?

Question

if not, where else does the carbon dioxide from volcanoes come from?

Answer 1

Only if they're located near an oil deposit, which is very unlikely.

Mostly it comes from heating up rocks made of "carbonates" like limestone (calcium carbonate), which releases the CO2.

It's a very small amount, relative to other sources.  See:

http://environment.newscientist.com/chan...

Answer 2

No, but not because they can't.

Because they happen elsewhere. Not by accident - oil fields and volcanoes are created in pretty opposite conditions. Oil fields are millenia of undisturbed growth of prehistoric forests followed by millions of years of compression under solid, undamaged layer of rock. Volcanoes are a result of violent tectonic activity, tectonic plates pressing, breaking, bending.

It's the same question as 'what would win a fight, a lion or a shark'. In the nature the two species never meet because they live in entirely different conditions. Same about oil fields and volcanoes.

As for carbon dioxide, there's quite enough of carbon in normal soil and most of standard rock. You don't need pure fossil fuel.