Credit: C. GUTIERREZ/UPI PHOTO/NEWSCOM

The Chaitén volcano in southern Chile erupts after more than 9,000 years of dormancy, spewing ash some 30 kilometres into the sky. Its plume stretches hundreds of kilometres across neighbouring Argentina to the Atlantic Ocean. The eruption has blanketed the surrounding province with a layer of soot, forcing the evacuation of thousands of people and threatening to bury the town of Chaitén, ten kilometres away. The spewed gases are high in silica and low in sulphur particles, so the eruption is unlikely to have a large effect on the climate.

For decades, scientists observing eruptions have seen lightning of the sort pictured here, but its origins are still not well understood. A study of an Alaskan volcano found that the ash particles spewing from the mouth of its crater were highly charged (R. J. Thomas et al. Science 315, 1097; 2007). Experts think that similar particles are behind these 'dirty thunderstorms'.