Mount Etna in Italy has been in an eruptive phase since July 30 of this year. A good video clip taken from near the site of the eruption can be found at this link. Sadly, I was unable to plant the video clip into this post, \;-(. An excellent blog to keep up to date on global volcanism is Eruptions.
I have to wonder how much combustion of reduced magmatic components is occuring as the magma enters contact with the atmosphere. Certainly the sulfide components must combust at the surface.
Carbonates are prone to thermal decomposition as well, though from disproportionation to metal oxides and CO2. At some depth from the surface, the formation of CO2 must begin to produce at least some amount of PV work on the magma column.