A volcanic eruption’s acoustic signature is far more complex than a single, earth-shaking blast. This powerful geological event releases immense energy into the atmosphere, creating a complex soundscape. The sounds generated are a direct result of gas expansion, magma movement, and the violent ejection of material. This acoustic power ranges from frequencies the human ear can easily detect to those that can travel around the globe undetected.
The Audible Sounds of Eruption
When a volcano erupts, the sounds within the human hearing range (roughly 20 hertz to 20 kilohertz) are often the most dramatic. The most prominent audible sound is the explosive “boom,” which is a direct acoustic shock wave created when a large volume of gas and rock is suddenly expelled from the vent. This sound can be heard hundreds or even thousands of kilometers away, depending on the eruption’s magnitude and atmospheric conditions.
A continuous “roaring” or “jet engine” sound often accompanies the initial explosion and may persist throughout the eruption. This persistent noise is generated by the high-velocity venting of volcanic gases, similar to the sustained sound of air rushing out of a high-pressure valve. The continuous escape of gas and steam creates a sustained acoustic tremor.
Deep, persistent “rumbling” sounds are also common, caused by the movement of magma and gas within the shallow subsurface conduits. This low-pitched noise is acoustic energy coupled with seismic tremor, reflecting the volcano’s internal plumbing system under pressure. The eruption also creates secondary, audible noises from the ejected material. These include the sound of hot ash, rocks, and ballistic debris falling back to earth, or the continuous grinding noise of a pyroclastic flow rushing down a slope.
The Science of Infrasound
The most energetic component of a volcano’s acoustic output is often not audible to people, residing in the low-frequency range known as infrasound. This energy exists below the lower threshold of human hearing, specifically at frequencies under 20 hertz. Volcanic infrasound is primarily generated by rapid changes in volume, such as the sudden expansion of gas during an explosive event or the sustained, turbulent flow of gas from a vent.
Infrasound is remarkably efficient at propagation, traveling vast distances with minimal energy loss because its long wavelengths are not easily scattered by obstacles. This low-frequency energy can propagate through the atmosphere for thousands of kilometers, sometimes circling the entire planet. The propagation velocity is approximately the speed of sound at sea level (about 340 meters per second), but this speed is influenced by the temperature and wind structure of the atmosphere.
Scientists use specialized instruments called microbarometers, deployed in arrays, to detect and analyze these inaudible waves. Monitoring infrasound is a valuable tool for volcanologists because it is not affected by cloud cover or darkness, offering clear evidence of an open vent and eruption dynamics regardless of weather. Analyzing the amplitude, frequency content, and duration of the signal helps researchers characterize the eruption style, estimate the volume of material ejected, and track the event’s intensity remotely.
How Eruption Style Dictates Sound
The specific acoustic signature of an eruption changes dramatically depending on the style of volcanic activity. Effusive eruptions, such as the Hawaiian or Icelandic types, are the quietest. These events involve the steady outpouring of low-viscosity lava, with sounds dominated by the hissing of escaping steam and gas, and the crackling or sloshing of the lava flows.
In contrast, highly explosive events like Plinian or Vulcanian eruptions produce the loudest and most powerful acoustic signals. The rapid decompression of gas-rich, highly viscous magma generates massive pressure waves. These manifest as sharp, loud booms and powerful, short-duration infrasound pulses. The sustained, massive eruption column also produces continuous infrasonic tremor, similar to the low-frequency noise of a large jet engine.
Phreatomagmatic and phreatic eruptions, driven by the interaction of magma with external water (like groundwater or a lake), have a unique and violent acoustic profile. The rapid conversion of water to steam generates a sudden and enormous volume expansion, leading to highly explosive bursts of sound. This process can result in events more explosive than a Plinian eruption, characterized by violent, sharp acoustic pulses as the water flashes to steam.