Volcanic eruptions generate a complex and often overwhelming soundscape, ranging from whispers humans cannot hear to blasts that shatter windows hundreds of miles away. These sounds are acoustic signals that scientists interpret to understand processes occurring deep within the volcano’s plumbing system. These audible and inaudible pressure waves indicate the eruption’s style, intensity, and potential hazards. Listening provides direct insight into the physics of magma and gas movement that visual monitoring alone cannot capture.
The Physical Mechanisms Generating Volcanic Noise
The source of nearly all volcanic sound is the energy released when gas separates from magma. As molten rock, or magma, rises toward the surface, pressure decreases, causing dissolved gases like water vapor and carbon dioxide to rapidly expand and form bubbles. When these bubbles burst and the gas escapes through the volcanic vent, the sudden release of pressure creates powerful sound waves. This degassing process generates a continuous, low-frequency roar.
The speed at which gas escapes through the vent conduit affects the quality of the sound. High-velocity gas jets create a turbulent, loud acoustic signal that, when accelerated into the range of human hearing, closely resembles the frequency distribution of a large commercial airliner.
Acoustic energy also comes from mechanical stresses placed on the surrounding rock structure. The movement of magma and pressurized fluids causes the solid rock to crack, fracture, and move, producing sounds associated with seismic tremors. These events occur deeper within the volcano as magma forces its way toward the surface, physically breaking the crust.
Audible Sounds of Different Eruption Types
The sounds a volcano makes are directly tied to its eruption style, differentiating quiet effusive flows from explosive events. Effusive eruptions, characterized by the calm outpouring of low-viscosity lava, produce less violent, persistent sounds. These include rumbling noises from the sustained release of gas and the distinct sounds of the lava itself.
As the surface of a lava flow cools and hardens into a crust, the underlying molten rock continues to move. This movement causes the fragile surface to crack and fracture, generating a continuous sound described as crackling, hissing, or similar to breaking glass. These quieter sounds, combined with the deep, steady roar of degassing, create a localized soundscape.
Explosive eruptions generate the loudest acoustic signals known in nature, often heard thousands of miles away. These eruptions are caused by the violent, instantaneous expansion of gas-rich magma, creating impulsive, broad-frequency blasts. The sound is often described as a deafening boom, a concussive blast, or sharp artillery fire.
The 1883 eruption of Krakatoa produced an explosion considered the loudest sound ever recorded, heard over 4,800 kilometers away. The sound waves were so intense that they ruptured the eardrums of sailors on nearby ships, and the atmospheric shockwave traveled around the globe multiple times.
Phreatic eruptions occur when superheated water flashes to steam, producing a sudden, sharp explosion rather than a steady roar. This event lacks fresh magma, deriving its explosive power from the rapid expansion of water contacting a deep heat source. The resulting steam-driven burst is a singular crack as the confining pressure is overcome.
Infrasound and Seismic Monitoring
The majority of the acoustic energy released by a volcano is inaudible to human ears. This low-frequency sound, known as infrasound, consists of pressure waves below 20 Hertz. Infrasound is generated by nearly all volcanic activity, particularly by large explosions and the continuous turbulence of gas jets.
Scientists use specialized microphones, often deployed in arrays, to detect and analyze these waves. Because infrasound is not affected by cloud cover and can travel thousands of miles with little energy loss, it is a tool for monitoring remote volcanoes. Analyzing these signals allows researchers to detect, locate, and characterize an eruption, providing real-time data on the volume of gas being released.
Infrasound monitoring is often combined with seismic monitoring, which measures ground vibrations (seismic waves) rather than air pressure waves. Volcanic tremor, a sustained seismic signal, is related to the subterranean movement of magma and fluids. Combining atmospheric pressure data from infrasound with ground motion data from seismometers provides a comprehensive picture of the volcano’s internal and external dynamics.