Scientists Reveal Secret Behind Redoubt Volcano's Explosive Screams

While some volcanoes simply explode, others scream in a relatively high-pitched wail that reverberates through the earth before an eruption. Now, scientists are learning a little bit more about the phenomenon, examining the notorious Redoubt Volcano in Alaska as they try to track down the source of the screams.

Redoubt Volcano is an active stratovolcano in the Aleutian Range, rising to 9,150 feet above sea level. In March 2009, this volcano experienced a series of five eruptions, each of them belting out a sound that was raised to higher and higher frequencies. Intrigued by this event, scientists decided to investigate exactly why the frequencies occurred.

"The frequency of this tremor is unusually high for a volcano, and it's not easily explained by many of the accepted theories," said Alicia Hotovec, Ellis, a University of Washington doctoral student, in a news release.

It's not unusual for swarms of small earthquakes to precede an eruption. These quakes can reach a point of such rapid succession that they create a signal called a harmonic tremor that resembles sound made by various types of musical instruments, though at frequencies that are much lower than humans can hear. Yet with Redoubt volcano, these frequencies reached a pitch that, potentially, humans could hear--assuming they didn't mind pressing an ear against a location that was soon to erupt.

In order to learn more about this event, the researchers analyzed the eruption sequence at Redoubt Volcano. They documented the rising tremor frequency starting at about 1 hertz and rising to about 30 hertz. Since the audible frequency range in humans starts at about 20 hertz, this "scream" could, in theory, be heard by humans.

So what caused the phenomenon? Scientists still aren't sure exactly. Yet it could be that the earthquakes and harmonic tremor happen as magma is forced through a narrow conduit under great pressure into the heart of the mountain. The thick magma would stick to the rock surface inside the conduit until the pressure is enough to move it higher, where it sticks until the pressure moves it again. These movements result in small earthquakes and as the pressure builds, the quakes get smaller. Eventually, they happen in such rapid succession that they blend into a continuous harmonic tremor.

"Redoubt is unique in that it is much clearer that this is what's going on," said Hotovec-Ellis in a news release. "I think the next step is understanding why the stresses are so high."

The findings will be published in the Journal of Volcanology and Geothermal Research and are also published in Nature Geoscience.