By Kristina Kassis (Fall 2012 GE 101 student)

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On June 5, 2011, Chile’s Puyehue volcano, located in the Andes Mountains of Southern Chile about 575 miles South of the Capital, Santiago, erupted explosively, demonstrating the dramatic power of nature by sending its plume of ash six miles high all over Chile, over Argentina and out toward the Atlantic Ocean. The Puyehue volcano has not been active since 1960, when it erupted after a massive magnitude 9.5 earthquake.  On June 5, however, tens of thousands of people were evacuated from 22 surrounding rural communities in Chile as a blanket of grey ash rained down like a deadly snow. Perhaps most striking about the Puyehue eruption, however, was the spectacular display of lightning that illuminated the sky above the volcano during the eruption.

Volcanic thunderstorms or “dirty thunderstorms” are rare and not fully understood by scientists, but it is widely believed that friction between particles and gases is largely responsible for the lightning displays that occur in conjunction with volcanic eruptions.[1] A study in the journal Science indicated that electrical charges are generated when rock fragments, ash, and ice particles in a volcanic plume collide and produce static charges, just as ice particles collide in regular thunderstorms. The large amounts of water present in magma may also help to fuel volcanic thunderstorms. Earle Williams of MIT and Stephen McNutt at the University of Alaska hypothesize that “dirty thunderstorms” might simply be caused by a build up of ice. Because thunder and lightning in conventional storms are essentially caused by a buildup of ice and water, Williams and McNutt claim that large volcanic eruptions are similarly a result of the abundance of water present in magma.

“There’s just so much water in magma, that’s the main issue,” Williams says. When magma explodes during eruptions, this water escapes.  But, Williams notes, the presence of water alone is not enough to spark lightning. First, he says, the particles must bang into each other enough times to promote a build-up of static charge, like a balloon rubbed repeatedly back and forth across someone’s head. Second, the positive and negative particles must separate because lightning is just a spark between these two differently charged regions. In a smaller thundercloud, positively charged particles generally gather above larger, negatively charged clumps of faster falling ice; if the cloud grows high enough, the two regions are dragged far enough apart to trigger lightning.[2] This is exactly what Williams and McNutt claim happens above a volcano.

Reports from eruptions seem to agree with Williams and McNutt’s hypothesis. For example, analysis of the Mount St Helen’s plume in 1980 found negatively charged particles at lower altitudes and positively charged particles higher up. Similarly, when the Sakurajima volcano erupted in Japan 1996, scientists recorded that positive charges dominate at the top of the plume and negative charges dominate at the base,[3] again supporting Williams and McNutt’s hypothesis.

Williams and McNutt are not the only scientists who have studied and analyzed “dirty thunderstorms.” For example, researchers with The Scientific American hypothesized that the amount of lightning correlated with the height of the plume: the taller the plume, the more spectacular the lightning show during eruption. This hypothesis is supported by Alaska’s Mt. Redoubt. Redoubt had a series of more than 20 eruptions over 13 days in March of 2009, all of which were accompanied by lighting flashes.[4]

“In general, the higher the plume went, the more lightning we got,” said Ronald Thomas, a physicist and electrical engineer at the New Mexico Institute of Mining and Technology. This information may tell us little about why volcanic lightning occurs, but it is undoubtedly a fascinating correlation and could be beneficial to scientific inquiry in the future.

Though volcanic lightning does not occur with every volcanic eruption, there have been many instances throughout the years. There are now more than 150 recorded cases of electrical storms breaking out directly above craters of erupting volcanoes, dating back several centuries.[5] For example, the 1980 eruption of Mount St Helens in Washington state, one of the most studied eruptions in recent times, produced a lightning bolt every second.[6]  In addition, a famous image of the phenomenon was photographed by Carlos Gutierrez and occurred in Chile above the Chaiten Volcano in 2008.[7] Other instances of volcanic lightning have been reported above Alaska’s Mount St. Augustine volcano when it erupted in 2006 and Mount Redoubt in 1989 and 1990.  Yet another spectacular of example of volcanic lightning occurred with the eruption of the Sakurajima Volcano in 1914, the most powerful eruption in Japan in the 20^th Century. Iceland’s Eyjafjallajokull Volcano on March 24^th, 2010 and Grimsvotn Volcano in November of 2004 both produced spectacular displays of volcanic lightning as well, so it is clear that volcanic lightning, though a rare and misunderstood phenomenon, is not unheard of.

So what does this all mean? It is clear that volcanic lightning is a phenomenon that may never be fully explained by scientists. There are hundreds of possible hypotheses. However, if Williams and McNutt are correct in their hypothesis that water is largely responsible for generating electric lightning, it has important implications:  the water could trigger devastating mudslides known as Lahars, merely adding to the threat posed by volcanic eruptions. All in all, volcanic lightning may be fascinating to watch and learn about, but it proves beyond a reasonable doubt the tremendous force of nature, and the capacity for something beautiful and awe-inspiring to rapidly become deadly. A picture may be worth a thousand words, but a photograph of volcanic lightning says only two: “WATCH OUT!”