The Daily Mail says:
As clouds of toxic ash and dust tower into the sky, they ionise the air, generating an explosive electrical storm. Colossal forks of lightning spark around the noxious plume as it spews from the volcano's crater, creating an image of raw, terrifying energy - as if the air itself were ablaze.
Now, the worst-case scenario is the collapse of the volcano accompanied by a "pyroclastic flow" - a devastating super-eruption of scorching dense gas and molten rock that would roll down the mountainside at 100mph or faster, incinerating and flattening all in its wake.
Flickr link. (High-res.)