20 February 2014
Volcanic lightning is one of the few things that could make an eruption even more spectacular. Doubtless you’ve seen the recent photos of this phenomenon from the eruption of Mount Kelud in Indonesia – lighting up a huge ash plume at night – or from any number of eruptions in the past few years, from Puyehue-Cordón Caulle in Chile in 2011 to Sakurijima in Japan in 2013 to Eyjafjallajökull in Iceland in 2010. It makes for some incredible photography, to say the least.
Volcanic lightning forms as a result of electric charges building up in the ash-laden plume of an eruption. Ash particles are bits of pulverized volcanic rock less than 2 mm(or 1/12 of an inch) in diameter, and although they may look like smoke from a distance, the fact that they’re composed of rock means that they can, under the right conditions, conduct electricity. A significant portion of the gas escaping a volcano is composed of water vapor – we all know that wet things conduct electricity better – and some researchers think that ice crystals forming from water vapor in the column freezing at higher altitudes can also increase the electrical conductivity of the plume.
Though it is a dramatic spectacle, volcanic lightning also poses dangers that most people might not associate with an eruption. Like lightning in an ordinary thunderstorm, it can strike objects, people and animals, and start fires; if people are concentrating on the more obvious hazards of a volcanic eruption, it could be easy to overlook the hazards created by lightning in the plume. But it’s very difficult to observe volcanic lightning up close without…well, being in the path of an eruption. And the field is relatively new – we still don’t know a whole lot about the exact process by which it’s generated. Which is why it was quite exciting to see the article by Cimarelli et al. in January’s edition of Geology – because they managed to observe it in the lab!
Dr. Cimarelli emailed me about the article some weeks ago, but I also had the chance to hear one of his co-authors, Don Dingwell, speak about it at a talk at the Carnegie Institute of Washington last month. Accompanying a discussion of the rock fragmentation experiments which the lab conducts, Dr. Dingwell mentioned finding volcanic lightning in the lab was a bit of an accident – a byproduct of the experiments themselves. If I remember correctly, while recording (visually or otherwise) the experiments in progress , the researchers noticed that some of the recording equipment would cut out or malfunction in some way. When they took a super-slow-motion camera and recorded the experiments, they found something like this. (Click to go see the videos on the lab website!)
The fragmentation experiments were set up so samples of volcanic rock could be pulverized in a “pressure bomb”, and the ash-sized material would then be allowed to escape the pressurized chamber via a series of diaphragms into a collection tank. These videos show experiments where just volcanic ash was used to try and recreate the effect, and are slowed down to a rate of 50,000 frames per second. They show tiny lightning bolts forming in the “plume” created by the ash blasting into the collection area – just like you might see in a real eruption!
As it turns out, there are ideal conditions for creating volcanic lighting. Cimarelli et al. found that electrical discharges only happened with specific size ranges of the ash, at certain points in the experimental process: finer ash produced a larger number of discharges concentrated in the turbulent shell around the larger particles in the core of the plume, and when the fines-rich turbulent portion went away, discharges stopped. When they repeated the decompression experiments with glass beads instead of volcanic ash, they were also able to suggest several mechanisms for building charges. In plumes with two different bead sizes, larger beads tended to become positively charged while smaller ones tended to negative charges, and provide a gradient for charges to ‘want’ to cross; however, when the beads were all the same size in a turbulent plume, the authors suggest that there was a mix of charges, but that like charges tended to cluster and thus form the necessary gradients. The key similarity was that the plumes had to be fines-rich and turbulent; once the plume was mainly composed of larger particles, the lightning stopped.
Knowing the conditions under which volcanic lightning is generated is a big step toward treating it like any other volcanic hazard to be mitigated. It is an interesting intersection of atmospheric phenomena and ground-based hazards, and other research groups have taken advantage of technology originally meant to monitor lightning strikes in regular stormclouds to attempt to monitor volcanic lightning. The USGS has used special instruments developed at New Mexico Tech to map lightning during the 2006 eruption of Augustine Volcano, and have suggested the method as a useful way to track the progression of a certain size fraction of the eruption plume (i.e., the fines-rich ashy part). This could be crucial for plume migration forecasts, which are used to protect air traffic as well as track fallout of volcanic tephra. (Also, getting volcanic ash in a jet engine is bad enough without having to worry about being struck by lightning…)
As I mentioned before, volcanic lightning hasn’t been studied as extensively as some other hazards (a quick GeoRef search only pulled up 350 or so articles on the topic), but it will be exciting to see how that changes as studies like this one add to our knowledge.
Thomas, R.J., McNutt, P.R., Rison, W., Aulich, G., Edens, H.E., Tytgat, G., Clark, E., 2010, Lightning and Electrical Activity during the 2006 Eruption of Augustine Volcano in Power, J.A., Coombs, M.L., and Freymueller, J.T., eds., The 2006 Eruption of Augustine Volcano, Alaska, USGS Professional Paper 1769, p. 579-608