Weatherwatch: Glass globes blown by lightning

The Icelandic volcano Eyjafjallajökull erupted in April 2010, emitting a cloud of volcanic ash which grounded air traffic across Europe for six days. Five years later, researchers led by Kimberly Genareau, of the University of Alabama. have written, that, among the volcanic debris, were glass spherules (small spheres) measuring less than a 10th of a millimetre across, which had been created by lightning.

In their paper published in Geology, the team describe how lightning melts ash particles inside the volcanic plume. Molten ash forms a ball like a raindrop before re-solidifying as a glass spherule. The team confirmed this theory by comparing natural spherules with ones created artificially in high-voltage experiments. The quantity of spherules, which made up about 5% of the debris sampled, showed how much electrical activity there was around the volcanoe.

“We hypothesize that the ash would need to be very close, likely within the discharge channel, to be affected,” Genareau said.

Intense lightning can occur inside a volcanic plume when the ash particles become charged by friction as they are driven upwards by the heat. There are also electrical discharges around the mouth of a volcano during an eruption, which are less well understood.

This is the first time that these glass spherules have been studied, and they may provide a novel way of investigating volcanic lightning after the event. The number and size of spherules in debris can show indirectly the amount of lightning in an eruption that occurred many years ago even if nobody witnessed it.