Mt Etna erupts with dramatic 'dirty thunderstorm'
In normal storm clouds friction between rubbing ice particles generate a charge, with positively charged particles at the top and negatively charged particles near the bottom. Once the difference in charge is great enough the lightning bolt is produced, neutralising the cloud.
While the exact reason why this "dirty thunderstorm" occurs is highly debated, one prevailing theory is that charge is built up as a result of the movement of ash and other ejected material within the cloud. Instead of ice in a regular storm cloud, the ash particles collide, generating a charge and producing lightning in the volcanic plume.