The mystery of how volcanic lightning occurs has been solved

Lightning strikes over Volcán de Agua in Guatemala

Mario Dalma Leon/Getty Images

Physicists have solved a long-standing mystery surrounding the process that creates volcanic lightning: when similar particles rub against each other, why do some become positively charged while others become negatively charged?

The exchange of electrical charge when two objects touch, called the triboelectric effect, causes the hair to be attracted to the balloon after friction.

In a volcanic ash cloud, swirling particles of silica exchange electrical charges as they collide. The positively and negatively charged particles separate and lightning occurs when a current flows between them.

But physicists have not been able to explain what breaks the symmetry between two particles of the same material and causes charge to flow in one direction or the other.

“There are a lot of candidates,” he says Galien Grosjeannow at the Autonomous University of Barcelona. “People suspect that moisture is important, or roughness, or crystalline structure.”

Working at Austria’s Institute of Science and Technology in Klosterneuburg, Grosjean wondered if the answer lay in the carbon-containing molecules on the surface of the particles. Such molecules are ubiquitous in nature, and materials scientists strive to minimize these contaminants. However, Grosjean and his colleagues observed what cleaning their samples did to electrification.

Using ultrasound, they levitated a small particle of silicon dioxide, bounced it once against a target plate made of the same material, and then measured its charge. “It can be positively or negatively charged. If it’s positive, we bake or clean it and repeat the experiment – and then it becomes negatively charged,” says Grosjean.

Analysis of the samples showed that the removal of carbon-containing molecules was indeed the controlling factor. “We saw that this effect trumps everything else,” says Grosjean.

Another advantage was that the cleaned sample would become positively charged again after a day or so, which is how quickly it gets a fresh coat of carbon molecules from the air.

Daniel Lacks at Case Western Reserve University in Cleveland, Ohio, the study is impressed. “People know that surfaces have a lot of crap on them. But I’ve never seen that show up in triboelectric charging,” he says.

He fears that the discovery could be bad news for physicists. If the carbon contamination determines the charge direction, it will be very difficult to calculate exactly how the particles become charged. “A prediction can be something that will never happen,” Lacks says.