The fusion of flash with ash! Say the words aloud, together, and it sounds impossible – the kind of thing a six-year-old might think up. And yet, volcanic lightning is very real. But how does it happen?
Few phenomena can compete with the raw beauty and devastating power of a raging thunderstorm, save for a particularly violent volcanic eruption. But when these two forces of nature collide, the resulting spectacle can be so sublime as to defy reason.
The photograph above offers some important insights into the formation and study of volcanic lightning. It was taken late last month by German photographer Martin Rietze, on a visit to Japan's Sakurajima volcano. Only very big eruptions, he tells us via email, can generate major thunderbolts like the ones seen above.
Smaller eruptions tend to be accompanied by more diminutive storms, which can be difficult to spot through thick clouds of ash. What's more, lightning activity is highest during the beginning stages of an eruption, making it all the more challenging to capture on film. Photographing a big volcanic event at any stage is hard enough as it is; if you're not nearby when it happens, says Rietze, "you will always arrive too late."
It turns out the same things that make volcanic lightning hard to photograph also make it difficult to study. The first organized attempt at scientific observation was made during Iceland's Surtsey eruption in 1963 (pictured here). The investigation was later recounted in a May 1965 issue of Science:
"Measurements of atmospheric electricity and visual and photographic observations lead us to believe that the electrical activity is caused by the ejection from the volcano into the atmosphere of material carrying a large positive charge."
Translation? Volcanic lightning, the researchers hypothesize, is the result of charge-separation. As positively charged ejecta makes its way skyward, regions of opposite but separated electrical charges take shape. A lightning bolt is nature's way of balancing the charge distribution. The same thing is thought to happen in regular-old thunderstorms. But this much is obvious, right? So what makes volcanic lightning different?
Close to 50 years have transpired since Surtsey exploded in November 1963. Since then, only a few studies have managed to make meaningful observations of volcanic eruptions. One of the most significant was published in 2007, after researchers used radio waves to detect a previously unknown type of lightning zapping from the crater of Alaska's Mount Augustine volcano in 2006.
"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 in a 2007 interview with National Geographic. "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 observations suggest that the eruption produced a large amount of electric charge, corroborating the 1963 hypothesis – but the newly identified lightning posed an interesting puzzle: where, exactly, do these charges come from? "We're not sure if it comes out of the volcano or if it is created just afterwards," Thomas explains. "One of the things we have to find out is what's generating this charge."
Since 2007, a small handful of studies have led to the conclusion that there exist at least two types of volcanic lightning – one that occurs at the mouth of an erupting volcano, and a second that dances around in the heights of a towering plume (an example of the latter occurred in 2011 above Chile's Puyehue-Cordón Caulle volcanic complex, as pictured here. (Photograph by Carlos Gutierrez/Reuters.) Findings published in a 2012 article in the geophysics journal Eos reveal that the largest volcanic storms can rival the intensity of massive supercell thunderstorms common to the American midwest. Still, the source of the charge responsible for this humbling phenomenon remains hotly debated.
One hypothesis, floated by Thomas' team in 2007, suggests that magma, rock and volcanic ash, jettisoned during an eruption, are themselves electrically charged by some previous, unknown process, generating flashes of electricity near the volcano's opening. Another holds that highly energized air and gas, upon colliding with cooler particles in the atmosphere, generate branched lightning high above the volcano's peak. Other hypotheses, still, implicate rising water and ice-coated ash particles.
"What is mostly agreed upon," writes geologist Brentwood Higman at Geology.com, "is that the process starts when particles separate, either after a collision or when a larger particle breaks in two. Then some difference in the aerodynamics of these particles causes the positively charged particles to be systematically separated from the negatively charged particles." You can see the diagram here.
The exciting thing about this process is that these differences in aerodynamics, combined with various potential sources of charge (magma, volcanic ash, etc) suggest that there may actually be types of volcanic lightning we've yet to observe. As Martin Uman, co-director of the University of Florida Lightning Research program, told NatGeo back in 2007: "every volcano might not be the same."