Is it true that during a volcanic eruption lightning can occur, and why?

Yes, during a volcanic eruption lightning is often observed in the eruption cloud. I don't think that anyone completely understands regular lightning and I think that volcanic lightning is similarly not understood. It must have something to do with all the particles of ash in the cloud moving around and bumping into each other, producing strong static charges (similar to how rubing a comb across the carpet produces static charges).

Scott Rowland