Our Beautiful Planet: Volcanic lightning

When the awesome power of a volcano combines with the dramatic might of lightning, the display is spectacular. So what exactly is volcanic lightning and how is it created?

The stunning phenomenon has been little studied, so scientists can't say with certainty what causes it. For a start, it's difficult to study. According to German photographer Martin Rietze, who has photographed the phenomenon, lightning activity is at its highest at the start of an eruption, meaning if you're not already at the right place at the right time, it will probably be too late by the time you get there.

Also, with big thunderbolts present only during very big eruptions, the smaller eruptions may only generate smaller storms, which tend to be hard to see through the ash clouds.

In thunderstorms, ice crystals collide with each other, producing enough electric charge to create lightning. Ash clouds are less predictable, however, and harder to study than thunderstorms.

But two studies have brought us closer to understanding how a combination of ash and ice can trigger volcanic lightning — each offering up a different reason as to what causes it.

One study suggests that ash particles are responsible for lightning that strikes near the ground. Researchers found that at Sakurajima volcano in Japan, particles were rubbing together in the dense ash clouds, creating static electricity, according to a study in the journal Geophysical Research Letters. For that study, the scientists recorded video of volcanic lightning at Sakurajima — one of the world's most active volcanoes.

By comparing the video to electromagnetic and infrasound data, the researchers found that the charge buildup from the static electricity was generating the lightning strikes.

The other study looked at lightning that happens high above the earth's surface near the stratosphere, where ice crystals unleash bursts of energy. In the study, also published in Geophysical Research Letters, researchers looked at the location of lightning strikes during an April 2015 eruption of Calbuco volcano in Chile. There, the lightning bolts were emerging around 100 kilometers from the eruption, and 20 kilometers above earth's surface.

The scientists think ice formed in the top of the ash cloud — which was also carrying water vapor — and produced lightning like a thundercloud does. Both studies bring scientists closer to solving what still, for now, remains the mystery of volcanic lighting.