Volcanic lightning is something that seems utterly magical when you think about it for the briefest of moments: Sparks, leaping about an eruption column full of fire and fury, illuminating its glassy contours and swirling plumes.
Although its formation mechanisms are still somewhat debated, at least we’ve been able to capture it on camera. Volcanic thunder, on the other hand, has escaped our technological traps – until now. A stunning new paper published in Geophysical Research Letters has revealed that the sound of such hellish thunder has been recorded for the very first time in human history.
It seems initially curious that such a phenomenon hadn’t been recorded until now, but remember that volcanic eruptions are extremely loud. Those that tend to feature volcanic lightning are explosive, producing tall, sustained eruption columns of superheated gas, ash, and lava flecks – so the sound of thunder is generally drowned out by the violence already in play.
In order to put this dramatic quest to an end, the team looked closer at Bogoslof volcano, one of around 50 volcanic islets in Alaska’s Aleutian Island chain. A relatively young volcano currently emerging from the sea, its sporadic eruptions mean that, in recent history, it’s both risen and fallen beneath the waves. Some have termed it a “Jack-in-the-Box” volcano, rather marvelously.
Bogoslof's eruptions often trigger the emergence of sustained eruption columns, perfect for volcanic lightning. Even if the eruption isn't seen at first, thunderstorms are rare in the Aleutian Islands, so when a global network of detectors pick up a lightning strike in the region, it’s almost certainly a sign of an eruption.
The Alaska Volcano Observatory (AVO) – which led the latest study – point out in their paper that even when the eruptions themselves died down, the columns remained, still triggering lightning bolts.
In both March and June of 2017, during two eruptive sessions, the team listened in on infrasound and sonic recording equipment some 60 kilometers (37 miles) from the volcano itself. Finally, during the quieter, dying moments of those eruptions, they managed to isolate clear sounds of volcanic thunder. In some instances, the ashy thunderclaps were so loud that they’d likely be heard over the pandemonium of the eruption itself.
On these sped-up recordings, volcanic thunder can be heard as clicks and pops, while the eruption itself makes lower-pitched grumbling noises.
“I think volcanic thunder has been noticed before using infrasound, but it wasn't published,” co-author Alexa Van Eaton, a physical volcanologist at the Cascades Volcano Observatory, told IFLScience.
Fortunately, at Bogoslof from December 2016 to August 2017, the team observed at least “60 eruptive events with lots of lightning, so there were ample opportunities to detect thunder.”
In all cases, it’s pretty clear where the thunder is coming from.
In crude terms, the particles of ash present in the column bumping up against each other is thought to generate electrical charges through friction, something known as triboelectricity. The same applies to ice particles higher up in the column. Ultimately, this creates charge imbalances across a region in the cloud, and such imbalances are annihilated by lightning strikes.
It’s thought that the aggressive ripping apart of volcaniclastic particles also plays a role in charge buildup, but either way, the results are breathtaking: The bolts such actions create are multi-directional, leaping down to, up from, and laterally over the vent, through the ash. As with regular lightning, this rapid heating of the air causes it to suddenly expand, and thunder is generated.
So why does this new study matter? Well, thunder is a key part of so-called “dirty thunderstorms”, and it’s inextricably linked to the already enigmatic emergence of volcanic lightning.
The more we comprehend one, the more we’re enlightened about the other. Understand the storm, and you’ll better understand how volcanic eruption columns work. Perhaps, now we know what to look for, we can use volcanic thunder to “hear” just how monstrous an eruption column is without ever laying eyes on it.
Recordings: Matt Haney/AVO/USGS