Why does Lightning occur during eruptions?
I have just supplied this answer to a similar question on this site: The vast amounts of rising hot gases and ash passing through the cooler atmosphere result in the transfer of charge. This excess of electrons within the cloud makes it act like a capacitor, and should the conditions be correct, discharge as lightning. An alternative, older explanation relates to friction between particles resulting in a static electrical charge buildup, and if and when conditions are right the charge passes to earth as a lightning flash. The cause is really speculative and not proven, as is the case of lightning in thunderstorms.
