It must be the greatest light show on Earth. The most vivid demonstration of power, beauty and mystery the natural world has to offer. And it must be terrifying to witness at close quarters. In fact, the greatest mystery about the phenomenon of volcanic lightning is how, with every instinct urging them to run like the clappers in the opposite direction, anybody hangs around long enough to see it.
There are now more than 150 recorded cases of vicious electrical storms breaking out directly above craters of erupting volcanos, dating back several centuries. 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. The electrical activity does not pose the same hazard as a volcano's boiling lava, choking dust clouds and drowning mud slides - though there are reports of people and animals being struck as they fled - but it sets a spectacular seal on mother nature's most awesome display of destruction.
Awesome, but not really understood. Exactly what causes volcanic lightning is still hidden in the clouds spewed from the crater. Most volcanologists seem happy with the vague notion that ash particles thrown into the air rub against each other and generate enough static charge to trigger sparks. It's the boiling lava, choking dust clouds and drowning mud slides that really concern them - particularly if they are close to the action.
There is more to the lightning than shock and awe. A better understanding of processes that cause it deep within eruption debris could help predict how the giant clouds will behave. Airlines have long feared the way volcanos can suddenly fill the sky with hazardous vertical smoke columns several miles high that rise at speeds up to 400 metres per second.
Now, an intriguing new idea that could explain volcanic lightning has emerged. Earle Williams of MIT and Stephen McNutt at the University of Alaska, say it might simply be caused by a build up of ice. Because thunder and lightning in conventional storms are down to ice and water, the two claim that large volcanic eruptions are nothing more than dirty thunderstorms.
McNutt is a seasoned volcano watcher, but Williams is trained in meteorology with an interest in thunderstorms. And like one of his favoured bolts from the blue, inspiration struck. "There's just so much water in magma, that's the main issue," Williams says. When magma explodes during eruptions, this water escapes.
The pair will tell the American Geophysical Union meeting in San Francisco next week that conventional thinking on what causes volcanic lightning is plain wrong. If their calculations are correct and the clouds bellowing from an eruption contain as much water as they think, then solid particles like ash will become coated with ice as the plume rises and water condenses.
"In that case, all bets are off on dry particle interactions. It has to be water," Williams insists. If they're right then it has important implications for managing the threat posed by volcanic eruptions. The water could trigger devastating mud slides, called lahars. "Conventional wisdom for a lahar is that it's caused by the melting of ice and snow already on the slopes of a volcano, but if there's this much water in an eruption then you could have a contribution from condensed water falling out," he says. Lahars, in other words, could still pour down from volcanos where there is no ice and snow.
Magma is a mixture of solid and molten rock containing dissolved gases. Up to 6% by weight is water, which means that a cubic metre of magma holds up to 100kg of water. In liquid form that's 100 litres, which, turned to vapour at 30C, will saturate 4,000 cubic metres of air. At -50C, the kind of temperatures experienced at the top of the tallest volcanic eruption clouds, it's more like a hundred million cubic metres. "In a meteorological context those numbers are huge," Williams says. "There hasn't been a calculation that takes that water out of the Earth and puts it into the atmosphere."
There has now. Called a relaxation calculation, Williams and McNutt used it to work out the water content and likely temperature of eruption clouds, based on explosion data used to set thresholds in nuclear test bans. At the heart of the idea is something called a relaxation volume, which indicates how far around an explosion material is likely to be spread. A Chinese firecracker has a relaxation radius of a few centimetres, while a volcanic explosion similar in scope to the great 1883 Krakatoa eruption in Indonesia would hurl material more than 4km.
When Williams and McNutt ran the simulation they found that water from the magma dominated the picture; it saturates the air for miles around with moisture and is far more important than any water vapour sucked in from the surrounding atmosphere. More surprisingly, their calculations suggest that in tropical regions, with an air temperature of 30C, the average cloud temperature was barely hotter at just 30.2 C. At those temperatures, the huge amounts of water vapour in the cloud will rapidly condense to liquid and solid ice. Just like inside a conventional thundercloud.
"Near the ground the temperature of the cloud will be hundreds of degrees C, but as the plume rises, expands and mixes with the environment it cools. By the time it reaches the upper troposphere the contrast with the environment is very minor," Williams says. "We know the essence of thunderstorm electrification is water substances, ice and liquid water. And it appears from our calculations that there will be as much, if not more, water in a volcanic eruption than in a conventional thunderstorm."
Reports on the ground seem to back up the idea. Observations of the 1806 Vesuvius eruption reported that "two places were deluged with a thick black rain, consisting of a species of mud filled with sulphureous particles". Descriptions of the Mount St Helens blast said that "dark grey mud fell from the second high level cloud," and that "mud balls the size of a half-dollar fell like rain for several minutes".
Other reports mention ice. When the Surtsey volcano in Iceland blew its top in 1966, observers noted a "fallout of icy pyroclasts onto local ships described as hail showers with a grain of ash within each hailstone".
Not that the presence of water is enough on its own to bring lightning. First, the particles must bang into each other enough times to promote a build-up of static charge (think a balloon rubbed on a jumper). Second, the positive and negative particles must separate; lightning is just a spark between these two, differently charged, regions. In a thundercloud smaller, positively charged hail generally gathers 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. Williams and McNutt think the same thing happens above a volcano.
Again, reports from eruptions seem to agree. Analysis of the Mount St Helens plume found "negatively charged particles at lower altitudes and positively charged particles higher up". When the Sakurajima volcano erupted in 1996, scientists recorded that "positive charges dominate at the top of the plume and negative charges dominate at the base".
Williams says minute ash particles will serve as cloud condensation nuclei and ice nuclei and that the larger particles will undergo a process called riming, in which they aggregate and become coated with ice. "This tends to shut down the conventional view that volcanic lightning is down to dry ash particles rubbing together," he claims.
The idea has not yet been published in a scientific journal, but other volcanologists got a preview when the pair presented it at a conference in Chile this year. What do they make of it?
"It's always interesting when someone from a completely different background comes in with different ideas," says Mike James of the volcanology and geohazards research group at Lancaster University. "It certainly hasn't been explained thoroughly and we're quite happy that thunderstorm mechanisms are part of it, but we believe the primary charging mechanisms to actually produce lightning in the first place are somewhat different."
He prefers the idea that magma fractures during the explosion create the bulk of the charge. Lightning sometimes occurs too quickly, he says, to be explained by the formation, charging and the separation of ice particles. "I don't think any volcanologist has ever been killed by lightning in the call of duty," he says. "There are worse things to worry about."
Williams accepts that his idea is not perfect - some volcanos produce no lightning, while there are reports of bolts following very small eruptions that produce clouds too small for the charge to separate. "People have come to me and reported eruptions with shallow clouds and lightning and in that case there has to be another explanation," he admits. "But it's tough to get good observations."
Model aeroplanes could be the answer. Meteorologists already fly toys fitted with probes into conventional thunderstorms to record data. Williams says the same drones could be lined up for more hazardous flights - into eruption clouds to check ice levels. "You could do it cheaply but here's the problem, the eruptions are very sudden and you don't know when they're going to take place. You can have an idea but you would have to be camped out with your vehicle waiting, waiting, waiting to send it in at the right time."
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