A foolhardy person, with their ear to the ground moments before a volcanic eruption, might hear the volcano "scream", according to new analysis of the 2009 Mount Redoubt eruption in Alaska.
Recordings of "the screams" -- as the researchers from USGS Alaska Volcano Observatory have dubbed the sounds -- can be heard here and here.
In the sped up recordings, you can clearly hear how the pre-eruption tremors occur more and more frequently, eventually merging into a "continuous high frequency tremor", as described in a paper published in Nature Geoscience on 14 July.
Just seconds before the volcanic eruption, the harmonic tremor or "screams" went completely silent
The phenomenon of mini pre-eruption earthquakes combining to form a harmonic tremor, vibrating at up to 30HZ, is further described in another paper published in the Journal of Volcanology and Geothermal Research.
A person with their ear to the ground on Mount Redoubt in 2009 "would have heard a very soft, low bass hum that rose slowly in pitch just at the limit of perception," says Alicia Hotovec-Ellis, a University of Washington doctoral student in Earth and space sciences and lead author of the study published in the Journal of Volcanology and Geothermal Research.
"This would be followed by about 30 seconds of silence and [then] the deafening boom of the explosion," she told Wired.co.uk in an email.
As magma forces its way upwards before a volcanic eruption, the disruption caused by the fluid causes tremors that are telltale signs of an impending eruption.
In the case of Mount Redoubt, which exploded in a series of eruptions over months in 2009, a series of small repeating earthquakes, around magnitude 0.5-1.5, occurred at a tremendous pace of up to 30 events a second.
Just seconds before the volcanic eruption, the harmonic tremor or "screams" went completely silent.
It is thought that the repeating earthquakes are caused by blockages of magma, sticking and then slipping with the surrounding rock in the mountain.
As the magma forces its way up, it becomes stuck. Pressure then builds behind the magma, forcing it forward again. Each shudder pushes the magma forward only 1mm, but creates an earthquake that rumbles through the mountain.
These shudders become more and more frequent until, around 30 seconds before the explosion, this stick and slip situation becomes unstable. The screams go silent as the magma is let loose, sliding smoothly against the rock in a state the researchers call "aseismic sliding".
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