Sid is a freelance science journalist.

The bolts of lightning that sometimes dance through the thick ash plumes suspended above volcanic eruptions leave behind a distinct geophysical signature: tiny glass orbs that form when zapped bits of ash melt and are pulled into tiny spheres by surface tension, before they cool and solidify. Lightning is often seen around volcanic eruptions and their resulting ash plumes. One common type results when individual particles of ash (actually broken rock) violently rub against each other, generating huge amounts of static electricity. Even though each discharge lasts only a few milliseconds, temperatures inside and near the bolt can approach a hellish 30,000°C—more than enough to melt fine bits of ash, researchers say. A team sifting through ash samples collected downwind after eruptions at Alaska’s Mount Redoubt in 2009 and Iceland’s Eyjafjallajökull in 2010 found such spherules, the first attributed to volcanic eruptions. (Similar spherules can be formed in a variety of extreme circumstances, from bomb explosions to extraterrestrial impacts.) The samples included clumps of spherules (image shown; scale bar is 20 micrometers long), as well as individual orbs of glass with multiple cracks, possibly die to the sudden conversion of trace amounts of water to steam inside the blobs of glass as they formed, the researchers reported online last week in Geology. Studying ash samples from other eruptions, both recent and in the distant past, may provide geologists with vital clues about how frequently volcanic lightning occurs, and whether it happens during only the most violent eruptions, they suggest.