Dirty Thunderstorm, Volcanic Lightning Weather Phenomenon

Causes
A study in the journal Science indicated that electrical charges are generated when rock fragments, ash, and ice particles in a volcanic plume collide and produce static charges, just as ice particles collide in regular thunderstorms.
"As the plume started going downwind, it seemed to have a life of its own and produced some 300 more or less normal (lightning bolts) ... The implication is that it has produced more charge than it started with. Otherwise (the plume) couldn't continue to make lightning." —Martin Uman, co-director of the University of Florida Lightning Research program