While people have anecdotally described hearing volcanic thunder in the past, the sound has proven tricky to record. That's because, as Ian Sample reports for the Guardian, it’s difficult to distinguish it from other rumbles and noises that occur during an eruption.
But researchers at the Alaska Volcano Observatory in Anchorage were undeterred by such a challenge. According to a press release, during an eight-month period (from December 2016 to August 2017), they set out to capture the sound of volcanic thunder by monitoring the eruptions of the Bogoslof volcano, which lies on the largest of a cluster of low-lying Aleutian islands of Alaska in the Bering Sea.
The volcano erupted more than 60 times during their research window. During that time, they successfully captured volcanic thunder during two eruptions: on March 8 and June 10 of 2017. They published their findings in the latest Geophysical Research Letters, a biweekly peer-reviewed journal by the American Geophysical Union.
As Sid Perkins reports for Science, this marks the first-ever recording of volcanic thunder.
Volcanic thunder occurs not because of the eruption itself, but rather the resulting ash plumes. As Avery Thompson explains in Popular Mechanics, lighting is triggered when ash plumes react with ice particles in the atmosphere. Thunder, a direct result of lightning, follows, creating the volcanic thunder phenomenon.
"Since the ash plumes stick around even after the eruption is over, researchers get an excellent opportunity to pick up volcanic thunderclaps in an otherwise quiet environment," Thompson explains.
The researchers stationed microphones some 4o miles away from the Bogosloft volcano, which meant the sound reached their mics around three minutes after lighting bolts were seen.
As Perkins writes, the timing and the direction from which the sound arrived suggested to researchers that what they were capturing was thunder. But they had to put clues together to isolate and confirm what their microphone recorded.
Jeff Johnson, a geophysicist at Boise State University who was not involved in the study, explains in the press release that their findings are significant beyond capturing the never-before-recorded sound. “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. So if you’re locating thunder over a long area, you could potentially say something about how extensive the plume is,” he says.
In practical terms, that means the research could offer a new way of detecting volcanic lightning and possibly a way for them to estimate the size of an ash plume, which, as a 2010 National Geographic story explains, poses safety risks for aircrafts.