After erupting, one volcano sings a unique ‘song’
The South American volcano Cotopaxi sometimes sings. But people will never hear it deep and distinct voice. It’s frequency is too low for the human ear to hear. But microphones can listen in. And between late 2015 and early 2016, they heard this peak in Ecuador sign out an unusual pattern of sound. Researchers now say that pattern is due in part to the unique shape of the volcano’s crater.
Other volcanoes might have a similarly distinct “voiceprint.” Identifying it could help scientists listen for changes within a crater. Such changes might even warn of an upcoming eruption.
Scientists from Ecuador installed a network of microphones on Cotopaxi’s flanks. They could record the very low frequency infrasound. Two weeks after the volcano’s August 2015 eruption, the network recorded the unusual acoustic pattern. It showed a strong, clear oscillation. The sound curve resembles a screw — or “tornillo,” in Spanish — that tapered off with time.
Scientists described the volcano’s distinctive call in the June 16 Geophysical Research Letters.
Cotopaxi repeated this tornillo pattern 37 times between September 2015 and April 2016. Each time, the signal lasted through a dozen or more oscillations. It resonated much like a musical instrument, before dying away.
Indeed, notes study leader Jeffrey Johnson, the volcano “rang like a bell for more than a minute.” A geophysicist, he works at Boise State University in Idaho.
Too low to hear
Scientists have been listening to infrasound volcano calls for about two decades. These sound waves have frequencies between about 0.01 hertz and 20 Hz. That’s far too low for the human ear to hear. (It is audible to some animals, however, including elephants.) These low acoustic signals are helping scientists study everything from volcanic eruptions to the movement of volcano-induced mudflows, known as lahars.
“If you didn’t have infrasonic sensors, you wouldn’t pick up any of this,” Johnson says. “You would be deaf to this world of unique and beautiful sounds.”
Among recorded volcanos, Cotopaxi’s voice is unusually deep. It’s like a baritone among tenors. The volcano resonates at around 0.2 Hz. That’s a frequency about one-fifth that of many other volcanoes. Johnson’s team attributes this strange voiceprint to air sloshing back and forth within Cotopaxi’s deep, cylinder-shaped crater.
What triggered the sound oscillation after the initial 2015 eruption is difficult to figure out. Scientists looked at video from a nearby webcam. Based on that, Johnson suspects the culprit may be the off-and-on release of gases from the crater’s magma lake.
Other volcanoes may not speak in screw-shaped sound bits. But Johnson suggests that they may have similarly unique voices. Monitors could identify the “voiceprints” of the most worrisome volcanoes. Then they could listen for sudden changes that might provide an early sign of volcanic unrest. For example, a volcano’s voice may change due to the rise or fall of the crater’s magma lake, he says. “Infrasound can tell us what’s happening at the surface [of an active volcano],” he says, “when we can’t walk up to the crater and peer inside.”
Seeing tornillos in volcano infrasound recordings is “a very cool thing,” says Jonathan Lees. He’s a seismologist and volcanologist who was not involved in the new study. He works at the University of North Carolina at Chapel Hill. It takes a very specific internal shape to create a tornillo, he says. That’s why it is not likely to be heard coming from many other volcanoes, he notes.
Johnson’s team listened to another bell-like crater at Congo’s Mount Nyiragongo volcano. “We did detect a low-frequency infrasound tone [coming from it],” Johnson says. However, the sounds didn’t oscillate long enough time to create a tornillo. That is likely due to the width of Nyiragongo’s crater. At 3 kilometers (1.9 miles) across, it is more bowl-shaped than pipe-shaped.
Although Cotopaxi’s call is too low for the human ear to hear, researchers re-created the tornillo pattern. They used oscillations in white noise. It shares the same amplitude pattern as the original tornillo signal, but those oscillations are at twice the speed of those in the volcano’s original call.
acoustic Having to do with sound or hearing.
audible Something that can be heard, usually with ears or other sound-sensing structures.
colleague Someone who works with another; a co-worker or team member.
crater A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.
eruption (in geoscience) The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. Volcanic eruptions on Earth usually send hot lava, hot gases or ash into the air and across surrounding land. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
hertz The frequency with which something (such as a wavelength) occurs, measured in the number of times the cycle repeats during each second of time.
infrasound Sound waves with frequencies below the lower limit of human hearing.
lahar A thick flow of mud, ash, soil and rocks. It can develop when rains mix with the materials being spewed during a volcanic eruption. This viscous mix, can have the consistency of wet, newly mixed concrete. As it slides down a volcano’s slopes, it can destroy nearly everything in its path.
magma The molten rock that resides under Earth’s crust. When it erupts from a volcano, this material is referred to as lava.
monitor To test, sample or watch something, especially on a regular or ongoing basis.
network A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)
online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.
oscillate To swing back and forth with a steady, uninterrupted rhythm.
resonate To reverberate, like a ringing bell, producing a clear tone or frequency of radiating energy.
seismology The science concerned with earthquakes and related phenomena. People who work in this field are known as seismologists.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly. (in biology) The structure that an organism uses to sense attributes of its environment, such as heat, winds, chemicals, moisture, trauma or an attack by predators.
unique Something that is unlike anything else; the only one of its kind.
volcano A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. This plumbing system can become more complex over time. This can result in a change, over time, to the chemical composition of the lava as well. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock.
volcanology The study of volcanoes. Scientists who work in this field are known as volcanologists.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.