Mystery of Volcanic Lightning Origin Solved

June 17, 2016 | Joanne Kennell

Volcanic lightning
Photo credit: Gordon Dymowski/flickr (CC BY-NC-SA 2.0)

Volcanoes really can produce lightning!

Volcanic lightning may be one of the most beautiful natural atmospheric phenomena on Earth — a combination of red-hot lava, billowing ash clouds, and powerful lightning strikes makes for a spectacular light show.

What causes volcanic lightning has remained a mystery for years, but researchers from Germany think they have figured out the enigma. Hint — it has to do with where it forms and what it materializes in.

Volcanic lightning is similar to normal lightning in that an electrical charge builds up within the cloud. The top of the cloud has a positive charge, while the bottom has a negative one. Air acts as an insulator between the positive and negative charges, but when the opposite charges build up enough, this insulating capacity breaks down and there is a rapid discharge of electricity, aka lightning.

A similar process happens in volcanic lightning; however, there are some major differences. First, volcanic lightning forms closer to the ground, and it doesn’t always move downward like normal lightning does. But the most important difference, as well as the most perplexing to researchers, is that the lightning happens inside clouds of ash, rather than everyday water vapor clouds.

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To solve this puzzle, the research team from Ludwig-Maximilian University observed lightning at Mount Sakurajima, one of the most active volcanoes on Earth, using high-speed video footage and acoustics.

After analyzing the data, the team found that the volcanic lightning most often formed in the lower section of the ash cloud. Why? The magma inside the volcano caused the ash cloud directly above it to become electrified, building the charge — similar to what happens in a normal cloud — producing a lightning strike.

"These data lead us to infer that although volcanic and thundercloud lightning share many common physical characteristics, the conditions presaging the occurrence of electrical discharges at explosive eruptions result from the complex charge distribution within the developing plume," the team concluded in their paper.

But another important revelation was made while analyzing the data. The team noted that the frequency of the lightning strikes increased with the size of the ash cloud, potentially offering a new way to ascertain how large an eruption is, and how much ash will be spewed into the air — without endangering the lives of researchers.

"This is a parameter that can be measured - from a distance of several kilometres away and under conditions of poor visibility," first author Corrado Cimarelli, told Maddie Stone at Gizmodo.

The study was published in the journal Geophysical Research Letters.

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