Description from National Geographic:

May 6, 2008—After 9,000 years of silence, Chile’s Chaitén volcano (pictured on May 3) is erupting with lava, ash—and lightning.

Since the volcano awoke on May 2, it has continued erupting intermittently, blanketing the area in ash and forcing more than 4,000 people to flee.

The mingling of lightning and ash seen above may be a “dirty thunderstorm.”

The little-understood storms may be sparked when rock fragments, ash, and ice particles in the plume collide to produce static charges—just as ice particles collide to create charge in regular thunderstorms.

Found via DD32’s blog.