A dirty thunderstorm (also volcanic lightning, thunder volcano) is a weather phenomenon that is related to the production of lightning in a volcanic plume.
A study 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.
Volcanic eruptions are sometimes accompanied by flashes of lightning. However, this lightning doesn’t descend from storm clouds in the sky. It is generated within the ash cloud spewing from the volcano, in a process called charge separation.
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.