Lightning dazzles us and volcanic plumes can be mesmerizing. So a volcano in mid-eruption flanked by zigzags of lightning has got to be one of the coolest spectacles in nature. And it is. People have been watching this display unfold for thousands of years. When Pliny the Younger saw the eruption of Mount Vesuvius in 79 CE, he noticed a "blaze of lightning" brighten up the sky as volcanic ash spewed forth.
That these two things were happening at once may not have been a coincidence. Today it's a known fact that billowing plumes of volcanic ash are able to generate lightning. Now for the first time ever, you can listen to the thunderclaps. Earlier this month, the world learned that a team led by USGS geologist Matt Haney managed to isolate and record the sound of volcano-produced thunder. Such a thing had never been done before — and the achievement could pave the way for life-saving insights.
The Physics of (Normal) Lightning
Regardless of its trajectory, every bolt of lightning is produced by charge separation. A storm cloud is like a huge, floating battery. The base is negatively charged while the upper portion has a positive charge. During thunderstorms, the ground itself also becomes positively charged. All this means is there's a whole lot of polarization going on.
Opposite charges naturally attract and try to balance each other out. Lightning is a rapid electrical discharge that can appear between a positively-charged area and a negatively-charged one. By sending electrons towards one of these poles, lightning temporarily neutralizes the charge of the space in between them.
How do storm clouds get electrified in the first place? It's thought that air currents push cool water droplets and small ice particles upward at an accelerated rate. As these bodies travel higher and higher, they collide with heavier particles called graupel (or "soft hail"), which hang out in the cloud's lower half. The collisions theoretically give those climbing particles a positive charge while the graupel gets negatively charged. Keep that in mind because it'll help us understand how volcanic lightning can form.
Ice, Ash and Eruptions
The manner in which a volcano erupts depends on many things. One important factor is the temperature of the magma lying beneath the surface. If this material is hot — say, in the ballpark of 1,200 degrees Celsius (2,192 degrees Fahrenheit) — and it's runny, you'll get an effusive eruption. In such outpourings, lava gently flows down the sides of the volcano. But if the magma is cooler and more viscous, that means the gasses inside the volcano will have a harder time escaping. Then you'll get a lot of internal pressure culminating in a so-called explosive eruption, with lava and ash plumes shooting skyward.
"Any volcano that produces explosive eruptions and ash plumes could generate lightning," Matthew Haney, Ph.D., a geophysicist with the USGS and Alaska Volcano Observatory in Anchorage, says in an email. "Volcanoes that ooze out lava in an effusive eruption, instead of an explosive one, wouldn't be likely to produce lightning."
The lightning itself is created in one of two ways; both involve ash plumes. Sometimes when there's a cloud of volcanic ash hovering over the ground, the individual ash particles rub together. That produces static electricity, with some particles becoming positively charged and others going negative. The result is a perfect environment for lightning.
"The other way is for ash to become coated in ice at high altitudes in the volcanic plume and for the ice-coated ash particles to collide with each other," Haney says. "This second way is similar to how regular lightning is produced high up in a thundercloud."
Thunder itself occurs after the heat from a lightning bolt rapidly warms up some of the surrounding air particles while pushing others away. Following the strike, the air cools down and contracts at a high speed. The activity emits a cracking noise which can be 10 times louder than the sound of a pneumatic jackhammer. And yet in a volcanic eruption, it's easy for the boom of thunder to get drowned out by long range roars and cracks, which are even more deafening.
That's why the new recordings are so groundbreaking. In December 2016, Haney and five other geologists set up microphones on one of Alaska's Aleutian Islands. The landmass in question was located near Bogoslof volcano, a 6,000-foot (1,828-meter) behemoth anchored on the ocean floor with a summit that's barely above sea level.
Over an eight-month period, Bogoslof erupted more than 60 times over. Haney's team was there to record it all. He said they hit pay dirt in March and June 2017 "by analyzing eruptions at Bogoslof that abruptly quieted down." Once the deafening eruptions faded, their instruments were able to pick up the booms of volcano-generated thunder.
"We showed the thunder signals came from a different direction than the volcanic vent," Haney says. Throughout the study, lightning sensors were used to pinpoint the exact location of bolts within Bogoslof's ash plumes. Haney says his team "showed that the pattern of the thunder in time matched the pattern of lightning." In other words, there was a definite correlation between the two.
The scientists' results were published in Geological Research Letters on March 13, 2018. Now that somebody has finally figured out a way to record the sound of volcanic thunder, future researchers will no doubt try to listen for it. By monitoring these noises, we may be able to do a better job of calculating how big or widespread a given ash plume is. That could help us keep airplanes out of harm's way — and organize post-eruption evacuations.