The Daily Telegraph has compiled a 31-image slideshow of amazing photographs of the volcano currently erupting in Iceland. Scientists aren't sure why electrical discharges are visible during some volcanic eruptions, but:
One theory is that that the spewing magma bubbles or particles of volcanic ash are themselves electrically charged, and by their motion create separately charged areas.
Link via Geekologie | Photo: US Environmental Protection Agency
http://antwrp.gsfc.nasa.gov/apod/ap100419.html
Check it out, there are some amazing pictures there!