It’s an explosion that starts within the earth, a release of pressurized gases and bits of rock; either as sharp shards or molten fragments or both. A volcanic eruption is one of the most powerful demonstrations of the dynamism of the planet that we usually think of as solid and unyielding.
It’s also loud. Really, really loud. Underwater eruptions can sound like gunshots or bombs reverberating through the water. Looking for a single, ephemeral sound within all that noise of tons of lava and gas and ash and rock all getting slammed out of the Earth’s crust is like listening for a whisper in a thunderstorm.
Or like, you know, listening for thunder in the middle of a volcanic eruption. That’s exactly what some researchers managed to record during eruptions of Alaska’s Bogoslof volcano last year.
They noticed that cracks and pops in the recordings lined up with the timing of volcanic lightning in the same area. Volcanic lightning occurs when eruptions that send a lot of ash into the atmosphere. During their speed run into the air, the ash particles rub against each other, creating an electric charge a lot like when you rub a balloon against your hair. As the particles spread out, that electric charge discharges into lightning….and apparently, thunder.
Researchers had heard volcanic thunder before, so they knew it existed, but it hadn’t been recorded before. These first recordings were just announced in a paper published in Geophysical Research Letters by Matt Haney, a seismologist with the Alaska Volcano Observatory and colleagues.
You can listen to one of their sped-up recordings here:
The deep rumble is the eruption itself, which stops around 10 seconds in, and the cracks are thunder. The researchers had set up the microphones to monitor volcanoes in the area, and seem to have caught the thunder on tape by happenstance. The Bogoslof volcano is not only remote, but is mostly underwater, and so volcanologists couldn’t set up instruments directly on the volcano. Instead, they relied on instruments set up on nearby volcanoes, including microphone arrays, to monitor Bogoslof for eruptions and ash plumes.
Being able to isolate thunder within a volcanic eruption could help researchers better understand the nature of an ash plume, which, in addition to just being an interesting insight into an eruption can also help inform air traffic controllers. Ash from volcanic eruptions can be hazardous to planes, so avoiding the plume is crucial.
“Understanding where lightning is occurring in the plume tells us about how much ash has been erupted, and that’s something that’s notoriously difficult to measure,” geophysicsist Jeff Johnson, who was not connected to the study told the American Geophysical Union. “So if you’re locating thunder over a long area, you could potentially say something about how extensive the plume is.”
There’s still so much to learn about volcanic thunder, including how it relates to volcanic lighting, and what kinds of other characteristics it might have. Dense arrays of microphones closer to the eruption might help tease out some of those tantalizing details.
Using acoustics to listen in on volcanoes is pretty common. Installing sensors directly onto a volcano is rare and expensive, but using more remote options like microphones or seismometers can help researchers keep track of volcanoes, sometimes in really interesting ways.
Recordings of earthquake activity before the 2009 eruption of Redoubt (also in Alaska) showed that activity near the volcano created a high-pitched “scream’ just before the eruption. The noise from an erupting underwater volcano can travel thousands of miles through the ocean. Other researchers are using hydrophones to record exactly when lava spills onto the seafloor.
The sounds of volcanoes are incredible, and while some, like some of the noises at Redoubt are hard for us to hear on the surface, others, like that volcanic thunder would have been noticeable for people observing the eruption. So the next time you find yourself in a safe and secure location watching part of the earth explode in front of you, replete with lightning-laced plumes of ash, listen up. Maybe you’ll get to hear not only the erupting earth, but also a rumble from the sky.