Your ears can’t hear it, but volcanoes are screaming.
Infrasound—sound waves below 20 Hertz, the lower limit of human hearing—rumbles through the atmosphere constantly. Elephants use it to communicate across miles. Whales send infrasonic calls across ocean basins. And volcanoes? They’re broadcasting their intentions in frequencies we evolved to completely ignore.
When Mountains Hum Songs Nobody Asked to Hear
Mount Etna does this thing where it hums. Not metaphorically—literally hums, producing infrasound at around 0.5 Hz. Scientists first documented this persistent signal in 2003, and here’s the thing: the hum intensifies before eruptions. It’s like the mountain is revving its engine, except the engine is several cubic kilometers of magma trying to find an exit strategy.
The mechanism is almost embarrassingly simple.
Magma moving through conduits creates pressure waves. Gas bubbles expanding in magma generate acoustic signals. Lava lakes—those hypnotic pools of molten rock—slosh around and produce rhythmic infrasound. When Mount Erebus in Antarctica churns its permanent lava lake, it broadcasts infrasound pulses that researchers can detect from research stations miles away. Each bubble bursting at the surface sends a pressure wave rippling outward, a subsonic drumbeat of planetary indigestion.
The Volcano That Broke All the Sound Meters in 1883
Krakatoa’s 1883 eruption produced what might be the loudest sound in recorded history. The explosion generated infrasound waves that circled Earth four times—four complete circumnavigations detected by barometers worldwide. People reported hearing the eruption from over 3,000 miles away. But wait—maybe the really wild part is what they couldn’t hear: the infrasound pulse was so massive it temporarily altered atmospheric pressure patterns globally.
Modern volcanologists now deploy infrasound arrays specifically to catch these signals. The technology is delightfully lo-fi compared to seismometers: essentially sensitive microphones designed to pick up pressure changes in air rather than vibrations in rock.
Why Your Body Knows About Infrasound Even If You Don’t
Turns out, complete inaudibility is negotiable. Stand close enough to an active volcano and you might feel infrasound even if you can’t hear it—nausea, disorientation, chest vibrations, a creeping sense of unease. It’s why some volcanic areas feel wrong before they look dangerous. Your inner ear contains organs that detect infrasonic pressure waves, evolutionary holdovers that probably helped our ancestors detect approaching storms or distant predators.
Yasur volcano in Vanuatu erupts almost constantly, ejecting lava bombs every few minutes, each explosion accompanied by infrasound signatures that scientists have used to classify eruption types. Big booming infrasound? Strombolian eruption. Sustained roar? Continuous ash emission. It’s volcanic semaphore, if semaphore could liquify rock.
The Surveillance System That Listens to Threats You Can’t
The International Monitoring System—originally built to detect clandestine nuclear tests—accidentally became the world’s best volcanic infrasound network. Sixty infrasound stations scattered globally pick up major eruptions automatically. When Iceland’s Eyjafjallajökull disrupted European airspace in 2010, infrasound arrays tracked the eruption plume in real-time, providing crucial data about eruption intensity when visual observation was imposible due to weather.
Indonesia deploys portable infrasound sensors around its 127 active volcanoes because traditional seismic monitoring sometimes misses critical precursors. Merapi, which killed 353 people during its 2010 eruption, now broadcasts its infrasonic signatures to monitoring stations that analyze the acoustic data for early warning signs.
What Happens When Scientists Actually Start Listening to Rocks
Infrasound reveals volcanic processes that seismology alone misses entirely. Gas emissions produce distinct infrasonic signatures separate from magma movement. Lava fountains create different frequencies than lava flows. Researchers at Hawaii’s Kilauea have cataloged infrasound patterns corresponding to specific eruptive behaviors, building an acoustic library of volcanic moods.
The 2018 Kilauea eruption generated months of continuous infrasound as lava drained from the summit, creating pressure changes that resonated through the atmosphere. Scientists tracked the lava’s underground journey toward residential areas by following the infrasonic breadcrumbs—acoustic shadows marking where magma redirected its path beneath the surface.
So volcanoes aren’t just dangerous geological features. They’re broadcasters, constantly transmitting information about their internal state in frequencies we finally learned to monitor. We just needed to start listening to the songs mountains sing when they’re preparing to reshape the landscape.
