The popular image of a volcano often focuses on the dramatic visual display of molten rock, but lava is far from silent. The soundscape of an eruption is a complex chorus of natural noise that changes dramatically with the type of volcanic activity and the lava’s movement. From subtle cracking near a slow-moving flow to thunderous booms from a violent explosion, the sounds produced by an eruption offer a sensory window into the physical processes occurring beneath the Earth’s surface. The auditory experience is highly variable, dictated by the lava’s viscosity, the speed of its flow, and the pressure of escaping gases.
The Immediate Auditory Experience of Flowing Lava
The most accessible sounds come from the slow, effusive flows of low-viscosity, basaltic lava, often described as a constant, grinding symphony. Close to the flow front, a rapid cracking and popping noise is distinctive, which many witnesses liken to continuously shattering glass. This sound is generated when the hot, molten rock causes the surface crust to cool and fracture almost instantly upon exposure to the air, driven by thermal expansion and mechanical stress.
As the lava moves, a softer hissing or sizzling can be heard, resulting from the release of trapped volcanic gases like water vapor and sulfur dioxide. This effect intensifies when the flow encounters water, such as a stream or the ocean, causing sudden, violent steam generation and a louder, more forceful hiss. The movement of highly fluid basaltic flows can also create sounds similar to a turbulent stream or flowing water as the material rushes down a slope. Small, discrete pops occur as tiny gas bubbles within the liquid reach the surface and burst.
Roars and Explosions: Sounds of Volcanic Pressure Release
In contrast to the localized sounds of flowing lava, explosive eruptions generate sounds of immense power, often heard across vast distances. These noises, which include booms, roars, and sounds like a distant jet engine, signal a rapid release of pressure from within the volcanic conduit. The most violent eruptions, such as Strombolian or Plinian events, are characterized by the rapid decompression of volcanic gases previously dissolved in the magma chamber.
When gas-rich magma rises quickly, the gases expand and accelerate to supersonic speeds, creating massive shock waves perceived as thunderous blasts. The intensity of these sounds is directly related to the amount of gas and the viscosity of the magma. More viscous lavas trap gases more effectively, leading to more violent, louder explosions when the pressure is finally released. These intense acoustic events can be powerful enough to cause minor damage to structures miles away from the eruption site.
Detecting the Unheard: Infrasound Monitoring
Beyond the range of human hearing, volcanoes emit acoustic waves below 20 Hertz, known as infrasound. This low-frequency energy is undetectable by the human ear but is a crucial tool for scientists monitoring volcanic activity. Infrasound is generated by large-scale atmospheric movements, the vibration of magma within the conduit, and the sudden expansion of gas pockets.
Specialized instruments called microbarometers are deployed around active volcanoes to record these pressure changes. By analyzing the patterns of infrasound, scientists can track the movement of magma and gases deep within the volcano, effectively “listening” to the eruption’s internal dynamics. This monitoring technique is particularly useful for identifying the start and end of an eruption and for providing an early warning of an impending explosive event, giving researchers information about the volcano’s behavior.