As Alaska's Redoubt volcano grew closer and closer to erupting in 2009, the earth seemed to start screaming. Scientists were intrigued -- this was unusual, even for the myriad noises typical of a volcanic eruption.
Four years later, two studies attempt to explain the noises. A team from Stanford University studied sonified earthquake data from the days leading up to the 2009 eruption. Their research is published in the July issue of Nature Geoscience. The Stanford team, led by Eric Dunham, determined that seismic unrest, most likely caused by the movement of magma within deep cracks and conduits, sparks a series of earthquakes and tremors.
"Before many explosions in that eruption, these small earthquakes occurred in such rapid succession--up to 30 events per second--that distinct seismic wave arrivals blurred into continuous, high-frequency tremor," the study authors wrote.
"We think the sound is caused by two rocks (or rock and almost solidified magma) rubbing against each other," researcher Alicia Hotovec-Ellis told CBSNews.com in an email. "The model is kind of similar to screeching tires, a creaky door, or chalk on chalkboard."
A similar series of earthquakes accompanied the 2005 Mount St. Helens eruption in Washington state, but at a much slower pace than the quakes that preceded the Redoubt eruption, according to Hotovec-Ellis's research. The Mount St. Helens quakes occurred at a rate of five per second. With Redoubt, it was 30 per second. She published her findings last month in the Journal of Volcanology and Geothermal Research.
As part of her study, Hotovec-Ellis sped up the sound of the seismic activity by 60 times to make it audible. The result is a 30-second clip that sounds like a mix between fireworks and a revving car engine, followed by the massive explosion of Redoubt.
Understanding the screeching will not necessarily aid experts in predicting an impending eruption.
"By the time the screams get going, the volcano was already quite active. If we saw something similar on another volcano it might give a few minutes or hours of warning before the next explosion," she explained, but added that the research is more useful in understanding the mechanisms of pressurization right before a volcano erupts.
For anyone hoping to hear the sound, it's best to stick with Hotovec-Ellis's recordings.
"If you were on the volcano, you might be able to hear a soft, bass hum just at the limit of your perception," she explained. But, "This would be followed by about 30 seconds of silence and the roar of the explosion."