(By J. Haxel, 2010)
Loud explosions, bubbles bursting, rumbling, hissing, and roaring like a jet engine describe some of the sounds heard from volcanoes around the world. Stories of sounds from historical eruptions tell of loud booms and explosions from volcanoes heard thousands of kilometers away.
One way to classify the types of sound recorded at both seafloor and land volcanoes is by when they occur. Prior to an eruption while the volcano is building up pressure, earthquakes and tremor from resonating dikes and conduits are common sounds captured by acoustic recording devices. The second group of sounds occurs during the active eruption, consisting mainly of explosions, bubble bursts and gas jets escaping from volcanic vents.
Sarychev Volcano - Kuril Islands, NE of Japan June 12, 2009 taken from the International Space Station. Note the pyroclastic flow towards the upper left from the eruption column. Image credit: NASA
The links on the right hand side of the page will take you to pages describing the sounds and volcanic processes that occur before and during land and seafloor volcanic eruptions.
In August of 1883 the volcano on the island of Krakatoa erupted violently with an enormous succession of blasts killing tens of thousands of people in the surrounding villages. Large pyroclastic flows swept down the flanks of the volcano, even crossing nearby ocean channels to devastate nearby islands. The biggest of these volcanic explosions was heard nearly 4800 km across the Indian Ocean basin on Rodriguez Island off of Africa’s eastern coast. The sound from this blast took roughly 4 hours to travel from the erupting Krakatoa volcano across the Indian Ocean and be heard by inhabitants of Rodriguez Island. Similarly, subsequent explosions and loud booms were heard in Perth, on the west coast of Australia and in Southeast Asia.
27th May 1883: Clouds pouring from the volcano on Krakatoa (aka Krakatau or Rakata) in southwestern Indonesia during the early stages of the eruption which eventually destroyed most of the island. Royal Society Report on Krakatoa Eruption - pub. 1888 Lithograph - Parker & Coward (Photo by Hulton Archive/Getty Images)
Locally, the sound waves created by the blasts were much more damaging. Broken windows and shaking homes resulting from the concussion sound waves of the explosion were reported up to 160km from the volcano around Krakatoa. People within this 160km vicinity of the eruption would have experienced intense ear pain and permanent hearing loss from exposure to these concussion waves. Estimates of exposure levels indicate it would have been like standing on a rocket launching pad with no ear protection.
The concussion or shock waves produced by the blasts at Krakatoa were remarkably high energy and could be heard by pressure sensors around the world. In fact, many stations recording barometric pressure revealed that the atmospheric shock waves created by the explosions travelled 7 times around the Earth before they were dissipated to immeasurable levels.
Scientists record a variety of interesting sounds using very sensitive microphones (on land) and hydrophones (in the ocean) allowing them to study volcanic processes before and during an eruption. Many of the sounds made by volcanoes are below the frequency limit of human hearing (<20Hz). These signals are often referred to as infrasound and can travel further through air and water to reach distant receivers than higher frequency sounds of equal energy.
A hydrophone on the seafloor at the NW Rota volcano used to record acoustic signals from the nearby volcanic vents (image courtesy of NOAA Vents Program)
An infrasound microphone system used to record the volcano-acoustic signals of Mount Saint Helens (image credit Scripps Institution of Oceanography, UC San Diego)
Sounds from volcanic processes (also known as volcano acoustics) that occur before an eruption primarily come from the pressurization of magma bodies in cracks and conduits, bubble explosions, and the resonating superheated hydrothermal systems near the surface of a volcano. As magma ascends, moving toward the surface, gases build pressure within the magma, forcing cracks and dikes to open and expand through the overlying rocks. The high pressures associated with the gas-rich magma within these cracks, pipes and conduits can cause the volume to resonate similar to a pipe organ.
The acoustic signal emitting from resonating bodies of gas rich magma, large bubbles, and or high pressure hydrothermal fluids within the volcanic edifice is often referred to as volcanic tremor. The fundamental frequency of the tremor is related to the volume of the resonating body and can change through time causing an effect known as "gliding". Gliding may occur as a result of a change in the properties of the magma body as more gas is exsolved and/or the crack holding the fluid/gas mixture opens or closes. Volcanic tremor is often used in conjunction with earthquake swarms as a geophysical warning that an eruption is not far off since it is often the direct result magma forcing its way up toward the surface.
This is an example of harmonic tremor from a resonating seafloor volcano. The lower frame shows the raw signal recorded by the hydrophone at 250 times per second (250Hz). The upper figure is called a spectogram and is the frequency representation of the lower frame with brighter colors indicating higher energies. A fundamental frequency around 3Hz with multiple harmonic overtones up to around 60Hz is evident as well as some gliding behavior with the frequency structure changing through time as described in the text above.
Another sound that is often heard and recorded during the periods between and building up to an eruption is caused by earthquakes from cracking and elastic behavior of the overlying rocks as magma pushes its way to the surface. On land, these earthquakes are often difficult to hear and only the larger events are audible due to attenuation properties of the signals in air. In the ocean, the acoustic waterborn phase of earthquakes known as a T-phase can travel great distances and have been well recorded. This earthquake or seismic activity often increases substantially prior to an eruption and is often used as the primary indicator for eruption predictions.
Similar to the above figure, the waveform and spectogram of a T-phase earthquake are shown above. Swarms of these earthquake sounds are a good indication of magma moving beneath the crust and often preclude eruptive activity.
The roar created by an erupting volcano is the result of turbulence and friction created by hot gases accelerating upward through conduits and finally escaping through the volcanic vent at the surface. These hot gases contain magma fragments, ash and other particles that travel violently through the inner walls of the vent conduits. Scientists have measured the low frequency (<20Hz) infrasonic signals created by these gas jets and when sped up to the range of human hearing, these signals sound remarkably like the frequency distribution of sound coming from a Boeing 747 jumbo jet. “The science of jet noise is very well understood. If we can understand how this works for volcanoes, we may be able to infer properties of eruption columns,” says Robin Matoza a scientist studying infrasound of volcanoes from University of California San Diego.
The F22 Raptor Jet Engine (Photo courtesy Pratt & Whitney, A United Technologies Company)
Blasts and Explosions
These impulsive, broad frequency band acoustic signals are the highest amplitude or loudest sounds created by volcanoes. Consequentially, these loud booms and cracks travel the furthest and are capable of doing the most damage from concussive blasts causing hearing injury and breaking glass. Furthermore, the infrasonic energy from these blasts can travel across ocean basins being recorded by pressure recorders thousands of kilometers away.
Listen to the explosive sounds
coming from Arenal volcano in Costa Rica
(sounds and images courtesy of arenal.net)
NW Rota1 Seafloor Volcano
Below is a movie of volcanic explosions from the erupting seafloor volcano NW Rota1 in the Mariannas Arc. The shock waves are visible and are accompanied by explosive blast sounds in the audio link from a hydrophone 20m away. This erutpion is occurring under 500m of seawater adding to the confining pressure at the vent, and still large blocks and huge bubbles are bursting as they are expelled from the vent (courtesy of the NOAA Vents Program and Woods Hole Oceanographic Institution).