Volcanic Lightning Sparked by "Dirty Thunderstorms," Study Finds

Brian Handwerk
for National Geographic News

February 22, 2007

When it comes to volcanic eruptions, mile-high ash clouds and geysers of
molten lava grab most of the attention.

But lightning often accompanies the blasts, and new research suggests that the electrical bolts may be part of a system dubbed a dirty thunderstorm.

(Related interactive feature: Unleash a virtual volcano.)

The study, to be reported tomorrow in the journal Science, describes some of the first direct observations made of volcanic lightning.

The findings offer a rare glimpse of this poorly understood phenomenon, including evidence of one type of lightning never seen before by scientists.

"It's the first real look at the details of at least one kind of volcano lightning—though of course every volcano might not be the same," said Martin Uman, co-director of the University of Florida Lightning Research program, who was not involved in the study.

Static Charge?

The team behind the new research used radio waves to detect the previously unknown type of lightning as it flashed from the crater of Alaska's Mount Augustine volcano (see a photo of Mount Augustine's 2006 eruption).

"During the eruption there were lots of small lightning [bolts] or big sparks that probably came from the mouth of the crater and entered the [ash] column coming out of the volcano," said study co-author Ronald J. Thomas, an atmospheric physicist at the state engineering university New Mexico Tech.

"So we saw a lot of electrical activity during the eruption and even some small flashes going from the top of the volcano up into the cloud. That hasn't been noticed before."

The new evidence suggests that the eruption produced a large amount of electric charge.

"We're not sure if it comes out of the volcano or if it is created just afterwards. One of the things we have to find out is what's generating this charge," Thomas added.

The most common type of lightning that researchers observed occurred shortly after the volcanic eruption and flashed high in the towering plume of ash and gas.

This lightning may be the product of dirty thunderstorm activity similar to that found in conventional thunderstorms.

The volcanic eruption itself is unable to generate sufficient electric charge to spark lightning for long after the eruption occurs or far from the crater, the researchers said.

Instead the scientists believe that electric charges are generated when rock fragments, ash, and ice particles in the plume collide to produce static charges—in much the same way that ice particles collide to create charge in regular thunderstorms.

"As the plume started going downwind, it seemed to have a life of its own and produced some 300 more or less normal [lightning bolts]," the University of Florida's Uman said.

"The implication is that it has produced more charge than it started with. Otherwise [the plume] couldn't continue to make lightning."

Volcanic eruptions release significant amounts of water, which may help fuel these dirty thunderstorms.

"Most of the volcano literature treats volcanic lightning as a different animal than natural cloud lightning," Uman said.

But, he noted, at least one kind of volcanic lightning appears to be "not all that much different than [conventional lightning in] a thunderstorm."

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