A volcano in Alaska "screams" and then goes silent before it erupts, scientists have learned, potentially offering new clues about how volcanoes work around the world.

The haunting cries came from Redoubt Volcano, located about 100 miles southwest of Anchorage, during a two-week eruption in March 2009. Redoubt released a 9-mile-high ash cloud amid more than 20 explosive outbursts, but first it caused a flurry of small earthquakes ranging from magnitude 0.5 to 1.5. In the final minute of its pre-eruption rumbling, Redoubt triggered 30 tremors every second, blurring into a single "scream."

Listen to a recording of the scream below, condensed from 10 minutes into 10 seconds:

Such earthquake swarms aren't unusual before volcanic eruptions, and the blending of rapid-fire rumbles is known to scientists as a "harmonic tremor." The noise can resemble sounds made by various musical instruments, according to a press release from the University of Washington, but it normally occurs at frequencies too low for humans to hear. Redoubt, however, produced bizarrely high-frequency tremors before six of its 2009 eruptions, piquing scientists' curiosity.

"The frequency of this tremor is unusually high for a volcano, and it's not easily explained by many of the accepted theories," says Alicia Hotovec-Ellis, a University of Washington Ph.D. student and co-author of a new study on volcanic screams in Nature Geoscience.

Some volcano noises come from the echo of magma as it opens tiny cracks in the Earth's crust, but Hotovec-Ellis thinks Redoubt's screams occur when magma is pressurized and bottlenecked through a narrow opening inside the mountain. The viscous material briefly sticks to surfaces in the volcano until mounting pressure forces it higher, where it again sticks, slips and repeats. These lurches produce small earthquakes, which grow smaller and faster as the pressure builds, eventually blending into a harmonic tremor.

"Because there's less time between each earthquake, there's not enough time to build up enough pressure for a bigger one," Hotovec-Ellis explains. "After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes."

While volcanic screams are interesting on their own, Hotovec-Ellis is particularly intrigued by the pause just before the eruption. "We think the pause is when even the earthquakes can't keep up anymore," she says, "and the two sides of the fault slide smoothly against each other." Along with co-authors Stephanie Prejean of the U.S. Geological Survey and Ksenia Dmitrieva and Eric Dunham of Stanford University, Hotovec-Ellis developed a new "frictional-faulting" model to study Redoubt's tremor mechanisms.

"We find that the fault stressing rates rise to values 10 orders of magnitude higher than in typical tectonic settings," the researchers write in Nature Geoscience. "At that point, inertial effects stabilize fault sliding and the earthquakes cease. Our model ... implies that the onset of volcanic explosions is preceded by active deformation and extreme stressing within a localized region of the volcano conduit, at a depth of several kilometers."

Redoubt's rising tremor frequency started around 1 hertz, then rose to 30 hertz. Humans can hear frequencies as low as 20 hertz, and a person lying on the ground directly above the magma conduit might be able to hear the peak of harmonic tremors. Since the noises foreshadow a volcanic eruption, though, scientists splurge on seismic sensors.

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