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.