An electric eruption

The dramatic volcanic eruption on a remote island in Indonesia's North Sulawesi province on April 17 threw a fiery-red column of lava, incandescent rock and ash as much as 3 km (2 miles) into the sky.

Videos on social media showed bright purple flashes of lightning over the volcano during the eruption.

Data from a ground-based global lightning detection network owned and operated by the Finnish environmental measurement instrument company Vaisala, dubbed GLD360, shows the activity during the eruption. Over 13,000 strikes were recorded between April 17 at 12:10 a.m. and April 18 10:12 a.m., peaking at 42 strikes per second at one stage.

Scientists are still studying the behavior of volcanic lightning. "Each of these eruptions that produce lightning helps us better understand what is happening in the eruptive plume and helps us understand our planet better," said Chris Vagasky, a meteorologist at the University of Wisconsin-Madison.

Despite the striking visuals, the amount of lightning during the Ruang eruption pales in comparison to the massive Hunga Tonga-Hunga Ha'apai eruption in Tonga in 2022.

According to Vagasky, the two eruptions had different characteristics, with the Tonga eruption incorporating much more seawater.

"The Hunga eruptions in January 2022 had around 50 times as much lightning as this one," he said.

Not all of the lightning generated by an eruption actually strikes the ground. Lightning can also travel within an ash plume or between clouds. During the Ruang eruption, about 23% of the lightning struck land or the ocean.

How does volcanic lightning happen?

Indonesia's Centre for Volcanology and Geological Hazard Mitigation (PVMBG) said in an Instagram post that the lightning occurred as particles in the ash clouds collided.

"These collisions could produce static electricity. This is why we could see this volcanic lightning in or around the eruption plume," the centre said.

Although caused by different materials in the atmosphere, volcanic lightning and that from a typical thunderstorm are both a result of particles becoming charged. When there are large positive and negative charge build-ups in the atmosphere, electrons flow between them, causing lightning.

The other way lightning can occur, in eruptions with tall plumes of 7-12 km, is via a mechanism called “ice charging” as the plume reaches heights where water can freeze.  In a typical thunderstorm, it is the different-sized ice particles, such as hail and ice crystals, that collide, causing charge separation. The water for this ice formation comes from the atmosphere.

However, in a volcanic eruption, it is mainly the water content of the magma that supplies the water for ice formation. The water concentrations in eruption plumes can be even greater than in typical thunderstorms.

The occurrence of ice charging-based lightning, along with the presence of ash and other particles, creates what is sometimes known as “dirty thunderstorms”.

Heruningtyas Desi Purnamasari, an official at PVMBG, told Reuters that nobody was reported injured from the lightning, and that the chances of being struck are low.

"The risk of a resident being injured from this lightning is low because they would have to be near the eruption plume,” she said.