There is nothing on Earth quite like volcanic lightning. Witnessing chaotic towers of tumbling ash emerge from an explosive convulsion at the apex of a mountain is already jaw-dropping enough. The emergence of purplish daggers of light shooting out from that column, however, elevates it to something more otherworldly.
Naturally, scientists want to know how volcanic lightning is made. Although the collision of ash particles within the plume are known to play a key role, the intricacies of this phenomenon have proven to be somewhat elusive. Unperturbed, scientists have taken matters into their own hands: They’ve been creating volcanic explosions in laboratories to produce the lightning themselves.
The latest of these experiments is arguably the most realistic of the lot. In a study recently published in the journal Geophysical Research Letters, scientists report how their rig can tweak the properties of a volcanic plume, revealing how changing temperatures or the dampness of the ash can make or break its ability to generate lightning. Dry ash seems to be an especially good way to get your eruption to summon Thor’s thunderous lightshow.
This work is about more than just scratching a scientific itch. Volcanic lightning is emerging as a leading way to spot ash-rich volcanic eruptions from afar. The more we know about how it works, the more precisely we can identify the type of eruption long before experts have set eyes on it—a huge help to vulnerable planes flying nearby or communities downwind.
Sönke Stern, a volcanic lightning researcher at the Ludwig Maximilian University of Munich and the study’s lead author, is aware of this, but he phrases his enthusiasm for the science a little differently. “To be honest, blowing rocks up on a daily basis is something that is pretty fun, for sure,” he told Gizmodo.
Volcanic lightning has a key principle in common with lightning in regular clouds: You need a separation of positive and negative charges. When this segregation becomes too much for physics to bear, a lightning bolt appears, cutting through the insulating air and neutralising the charge difference.
The ingredients in eruption plumes are different from those in classic clouds. Here, ash rules, and “we’re pretty sure that particle collisions create electrification,” Alexa Van Eaton, an experimental volcanologist at the U.S. Geological Survey’s Cascades Volcano Observatory, told Gizmodo.
The bumper car-like action of the ash involves plenty of friction, which generates electric charges. This process, known as triboelectricity, also occurs when you rub a balloon against your head and it magically stays put. Scientists also think that the tearing apart of volcanic debris in the plume helps accumulate electrical charge.
Some of this knowledge has been gleaned from careful observations of volcanic lightning. “Fieldwork is important, as it provides the fully scaled, real-life scenarios that we are trying to explain,” Cassandra Smith, a National Science Foundation Postdoctoral Fellow at the U.S. Geological Survey’s Alaska Volcano Observatory, told Gizmodo.
Inconveniently, “you don’t have control over what the volcano decides to do,” said Smith, and it is hard to study individual influences on volcanic lightning in the real world. It’s simply too difficult to investigate how lightning is made or what environmental conditions can enhance or nix it when it’s concealed by a colossal, superheated ashy maelstrom.
That’s where lab experiments come into play. There aren’t many scientific institutions capable of making realistic-enough volcanic blasts and generating lightning; the Ludwig Maximilian University of Munich is one of just a handful. Many lightning experiments there have been led by Corrado Cimarelli, an experimental volcanologist who is a co-author of the new paper.
For their latest experiments, they used a setup known as a fragmentation bomb: a chamber of pressurised argon gas containing ash that, at a certain high-pressure breaking point, explosively decompresses and propagates up into a tall steel collector tank. This unleashes the ash as an expanding jet, in which volcanic lightning is born. Crucially, the water content of the ash could be varied. Thanks to a furnace, so could the temperature, peaking at 320° Celsius.
The ash itself was the real deal, taken from the remnants of a 13,000-year-old eruption of Germany’s sleeping Laacher See volcano. They got their hands on 660 pounds of the stuff via a company that runs quarries in the volcano’s neighbourhood.
Stern’s team found that, at room temperature, there were fewer but larger lightning discharges. At higher temperatures, the discharges were smaller but more plentiful. Although this effect may be down to temperature’s influence over the turbulence in an ashy plume, it is unclear what’s going on here. “That’s something we can’t fully untangle with this kind of experiment,” said Stern.
The effect of water was far more pronounced. With even just a slight dampening of the ash, the team witnessed a decrease in the overall electrification of the plume by an order of magnitude. At a certain sogginess, there is essentially no lightning present—but why?
It turns out that water vapour expands even more dramatically than the argon gas upon release. That creates a more potent explosion, widening the ash jet and throwing the ash particles all over the place. With fewer collisions between the ash particles overall, you are less likely to produce lightning. Conversely, dry ash detonations made more focused jets, featuring more charge-generating collisions and more lightning.
In the real world, there is some fieldwork evidence that suggests especially damp ash plumes generate less lightning. It isn’t clear why, but it’s worth noting that adding water to hot magma enhances the eruption’s explosiveness. This isn’t quite the same thing as what happens during the experiments, but perhaps this also leads to fewer ash particle collisions.
On the other hand, drier ash is just inherently better at generating electricity. It is a very poor electrical conductor, explained Stern, so as ash particles separate, they are likely to keep hold of their charges, setting up big charge separations that can only vanish via lightning.
The clues these experiments have unearthed are enormously helpful in understanding how volcanic lightning is forged, Smith said. But she emphasised that it remains difficult to isolate individual processes, even here. The change in water content always changes the ash jet style, meaning it is impossible to separate the two factors and say which is more responsible for snuffing out the lightning.
This experiment only looks at the lower, hotter, ash-rich part of the eruption plume. If plumes are buoyant enough, they reach altitudes that are low pressure and decidedly frigid. Here, ice droplets are thought to play a big role in generating lightning. “That’s a completely new realm for volcanic electrification studies that has never been tackled yet,” said Van Eaton, who calls those lofty heights the “wild west” of volcanic lightning.
In other words, things are more complicated than these experiments can show. “We’d love a simple story, but it’s not always like that when it comes to natural eruptions,” said Van Eaton.
Still, these experiments, even with their limitations, make meaningful contributions to a possibly lifesaving endeavour.
Van Eaton recently co-authored a study demonstrating how the World Wide Lightning Location Network can be used to track the development of volcanic lightning and eruptions in remote locations. Eruptions don’t always produce ash, but they can’t seem to make lightning without it, she explained.
Aside from satellites, whose view can be obscured by cloud cover, no other monitoring method that spots volcanic eruptions from afar—except lightning detection—can tell you anything about the ash content on an eruption. Ash can down planes, smother settlements, pollute waterways, and harm human health, so knowing it’s present before we can physically see it is a huge help. And the more we understand what style of lightning matches with what type of ash plume, the better we can mitigate their dangers.
Then again, perhaps volcanologists are just in it for the spectacle. It is difficult not to be awed, said Van Eaton, when lightning discharges shoot out of an already gorgeous and mind-blowing plume. Who wouldn’t want to research that?