June 2008

Mega Basso Profundo – Listening to Volcanoes

Here’s a new take on the “if a tree falls in a forest and no one is around to hear it” riddle: if you can’t hear a volcano, does that mean that it’s not making a sound – in fact, a very loud sound?

Milton Garces emerges from the lava.
Fire field captured with infrared and conventional photography.
Tree ferns and ground cover surround one of ISLA's isolated instrument vaults, which houses microphones and equipment.

Take, for example, Hawaii’s Kilauea Volcano. “It’s one of the loudest things in this whole island,” says Milton Garces, director of the University of Hawaii’s Infrasound Laboratory (ISLA). “This thing is louder than the ocean, this thing is louder than anything else around it. In terms of human perception, you can stand right by it, and you don't hear a thing.”

That’s because the sounds emerging from the volcano fall well below the range of human hearing. Such frequencies are known as infrasounds. Sound is measured in hertz (cycles per second), and humans can hear frequencies in the range of 20 to 20,000 hertz. The frequencies emanating from Kilauea’s Halemaumau crater are in the vicinity of 0.5 hertz – under the radar of our aural perceptions. By using arrays of supersensitive microphones that can detect these low frequencies, Garces’ team records the signal, then speeds up and processes the sound file until it can be heard with our own ears.

A March 2008 explosion exposed a new vent within Kilauea’s Halemaumau crater. As gasses ascend from deep within, this vent acts like a kind of wind instrument and produces a low-frequency resonance, which has been referred to as Pele’s Chant. Pele is the volcano goddess of ancient Hawaiian legend. “We picked up this explosion really loud and clear,” says Garces. “And that is producing a very unique, very loud infrasonic signal that we’re going to be studying in great detail over the next few months.”

Infrasound arrays are used not only to detect volcanic activity, but can also be employed to hear meteors, asteroids or comets that collide explosively with the earth’s atmosphere (these are collectively known as bolides). The arrays can also detect an ongoing chorus of infrasonic activity in our oceans. ISLA’s primary role, however, is listening for the infrasonic activity associated with nuclear weapons testing. Infrasound is one of four technologies used in the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty, and Garces’ lab is an important link in that chain.

Infrasounds produced by things like meteors and nuclear weapons tests are brief and unpredictable, though. Pele’s Chant, in contrast, is providing a persistent source of sound, enabling Garces and his crew to better understand the capabilities of infrasonic monitoring itself. They’re assessing things like range limitations, and––thanks to Kilauea’s location––the effects of wind on acoustic wave propagation. Infrasounds can also help in predicting volcanic eruptions, and these data are made available to government and civil defense organizations.

As you might imagine, volcanic craters are not the best locations for placing microphones, which are very sensitive to wind noise. In addition to the wind, there’s always the possibility that a volcano will bury your instruments in molten rock or ash. This is why Garces places his arrays in remote locations sheltered from wind, at a distance of several kilometers to several hundred kilometers from the volcano. This precaution not only removes the microphones from harm’s way, it’s also a safety measure for human researchers. After all, someone has to maintain all that equipment.

“Every time you go deal with an instrument close to a volcano, you put yourself at substantial risk,” Garces warns. “You have flying fragments of hot rock, you have the shockwaves that can actually produce a lot of internal damage. Glowing avalanches of rocks. Gas that comes out of an open vent can flow down mountains, a little bit like a river, without you even seeing it coming. The only way to perceive it is through a shimmer in the light. By then, it might be too late.”

Interact with Milton Garces and read about his latest research


You can hear some of Garces’ sped up infrasonic recordings in the following stories :
Science Diary: Volcano - Array
Science Diary: Volcano - Silence
Science Diary: Volcano - Pele


Links to other related stories:
Science Diary: Volcano - Climb
Science Diary: Volcano – Eruption
Science Diary: Volcano - Death
Science Diary: Volcano - Analyze