Volcanic Lightning

Volcanic Lightning is a weather phenomenon that occurs when lightning is produced in a volcanic plume. Rock and ash collide in the air after the eruption causing electricity to build up, when the charge is large enough it releases in the form of lightning. Volcanic lightning is hypothesized to occur from the site of eruption as a result of charge-separation.  As positively charged material is ejected skyward oppositely charged regions of particulars take shape creating electric discharge. If you would like to read more about Volcanic Lightning  there is a nice detailed article here on http://news.discovery.com/.
Whatever the bizarre cause of this rare event, it is defiantly one of the most beautiful and destructive sights this planet has to offer.