After spending a few weeks pondering the physics of clouds and sound waves I'm just in the right frame of mind for this article, Volcano Sparks New Type of Lightning. It basically says scientists got some detectors out onto Alaska's Redoubt Volcano when it first started erupting last year. They have detected tiny sparks of lightning lasting just a millisecond or two inside the ash plume at the beginning of the eruption. The equipment also mapped large bolts of lightning similar to a thunderstorm and medium sized two mile bolts from volcanic vents. The article conservatively avoids explaining where the electrical potential comes from to make this lightning. It's one of those "we're looking into it" stories.
But the comments after are full of interesting ideas to ponder. Metals in the ash, lack of lightning in some volcanoes because of higher concentration of water vapor, piezoelectric effects that generate voltage (this only works for crystals, so they have to try hard to sell me on this).
The whole time I was reading this story I had this REM song stuck in my head. This volcano needs a theme song. I recommend Windout. Find out. No doubt. You're young and red.