(Correction appended, July 24, 2012, 2:56 PM)
On March 23, 2009, after several months of dithering, Mount Redoubt in Alaska began to blow its top. It cleared its throat with five cannon-ball like explosions laden with ash and rock, and then, as it ramped up to the next phase of eruption, it began to scream. Really. It let loose with a cry in a gradually rising pitch and then abruptly stopped before the next of its explosions—and the next and the next after that; all told, six of the next seven times it went pyrotechnic it gave its own form of vocal warning first, reaching pitches as high as 30Hz. About ten days later, as the volcano constructed a dome of lava over itself, it screamed again.
These sorts of rising infrasonic screams, picked up by sensitive instruments arrayed around mountains and known to volcanologists as gliding harmonic tremors, are not uncommon when volcanoes are active. If you had been standing on the flank Mt. Redoubt, however, you probably wouldn’t have been aware that any screaming at all was going on. You might have been distracted by the rocks falling from the sky, of course, but the primary reason is that even the highest-pitched 30Hz frequency is only a little bit above the lower limit of human hearing, which starts at 20 Hz and tops out at about 20,000 Hz. Most volcano screaming is at the completely sub-audible 1 to 5 Hz. The mystery this time is what made Mount Redoubt so high-pitched—by volcano standards at least—and a new study just published in the Journal of Volcanology and Geothermal Research may have the answer.
A volcanic eruption is a complex, many-splendored event, and one of the things that makes it so dynamic is that even before the actual blast begins, the surrounding ground may be shaken by numerous small earthquakes. These may be caused by magma being forced through the narrow pipe in the volcano’s throat—a less easy passage than it seems since earlier flows of magma may have hardened against the wall. As pressure builds and the old magma breaks free, the tremors are triggered. The same mechanism may also be responsible for the screaming, as the high-pressure passage of the constricted magma produces the vibrations that seismic instruments and microphones record. The quakes and the screaming should happen in the same narrow time window—but they don’t have to sync up exactly. In this case, the researchers believe, they did, with the resonant shaking of the quakes raising the volume of the screaming.
Volcanoes are not alone in producing sounds at dramatic moments. Icebergs are known to “sing” in harmonic tremors as they run aground, when water rushes through innumerable tiny cracks in the ice. Each crack, much like the throat of the volcano, is converted into a kind of organ pipe by the moving fluid, the air throbbing with sound. There’s a kind of poetic symmetry here: what causes natural structures to cry out at Earth’s lowest temperatures produces a similar effect at its very highest.
An earlier version of this story incorrectly used the term ultrasonic, or sounds above the frequency of human hearing. The correct word is infrasonic, or below the frequency.