In A Nutshell
Right before a volcano erupts, the Earth emits a series of shock waves, miniature earthquakes that ripple through the bedrock at increasing frequencies as the pressure below the volcano builds—a rising scream straight from the bowels of the Earth.
The Whole Bushel
In 2009, a stratovolcano in the Aleutian mountain range of Alaska erupted, shooting clouds of ash, sulfur dioxide, and water vapor 20,000 meters (65,000 ft) into the sky. This particular volcano, Mount Redoubt, has a long history of violent eruptions, so the event itself wasn’t extremely noteworthy. It’s probably most famous for a series of eruptions that spanned six months in 1999 and 2000, sending streams of lava coursing over the ground in a 35 kilometer (22 mi) radius.
But seismologists who were studying the volcano at the time noticed something interesting: It seemed to scream before the actual eruption took place.
The phenomenon had never been fully observed before Redoubt, but now the model is being seen in other volcanoes as well, though not with the same level of intensity as the Redoubt scream. The earthquakes started a few days before the eruption itself, and that’s what brought the researchers to the scene; there’s always some sort of seismic activity before an eruption, and it’s often used as an early warning sign in populated areas.
But this time, there was an audible hum coming out of the ground.
When the researchers looked at their charts after the eruption, they found a series of “microquakes” that rippled one after the other in perfect unison, like the beat of a tribal drum. As the pressure built, the microquakes increased in frequency until they were coming at a rate of 30 every second. For contrast, the devastating eruption of Mount St. Helens in 1980 only created earthquakes five times per second, and that’s considered the deadliest eruption in the history of the US.
In the case of Mount Redoubt, the earthquakes were being caused by a massive burst in pressure along the fault lines inside the volcano. The pressure was grinding magma against the sides of the volcano’s chute with enough friction to send out surges of shock waves. As they came faster and faster, the howling scream got louder. Deep inside the Earth this would have sounded like a series of oscillating shrieks, although by the time the sound reached the surface it was barely a hum.
And then something else happened: For several seconds before the actual eruption, the Earth went completely silent. The researchers believe this happened because the pressure reached the tipping point; the magma burst past everything blocking its path and, for that briefest of moments, glided through with complete freedom. No more friction, no more scream. But it was just a breath of calm before the storm—at this point the magma was racing toward the top of the volcano, where it rained hellfire onto the Alaskan countryside for the next two weeks.
The first link below has a recording of the planet’s scream. It’s . . . unnerving.
Show Me The Proof
Wired: Listen to a Volcano Scream Just Before It Erupts
Journal of Volcanology and Geothermal Research: Strongly gliding harmonic tremor during the 2009 eruption of Redoubt Volcano