The Boston Globe's The Big Picture has taken on the eruption of the Alaskan volcano Mt. Redoubt, and pretty much every picture is incredible.
That picture shows lightning erupting around the ash cloud; friction from the ash particles generates a huge static charge in the cloud, and the uplift (maybe, it's not clear) provides charge separation. When the voltage gets high enough, bang! That picture is from the Alaska Volcano Observatory (money well spent, I should think most people would agree). There are lots more amazing shots at The Big Picture. Go see.