Not far from Anchorage, Alaska, there is a volcano that screams. Then falls silent. Then erupts and throws volcanic ash 10 kilometers into the sky. Then starts all over and does it again, and again.
This strange and repeating seismic behavior was discovered in the 2009 eruptions of Redoubt Volcano by Alicia Hotovec-Ellis of the University of Washington. But now she and her colleagues have a model to explain what might cause all the screaming and then the silence before the volcanic storm.
To get an idea what the screaming sounds like – converted from seismic waves, sped up 60 times into audible sound waves – listen to this file which is a real recording of Redoubt Volcano. What you hear is a drum beat of quakes that get faster and faster until they blur together in a rising sound and then go quiet. It's the blurred rising sound that has been called the scream.
"You are getting 20 to 30 earthquakes a second," said Stanford University seismologist Eric Dunham explaining how the screams are really harmonic seismic waves smearing together and becoming one big seismic sound. "Then you get 30 seconds of silence. Part of what we did was to try and figure out why."