Some volcanoes 'scream' at ever-higher pitches until they blow their tops

Redoubt Volcano on March 31, 2009. View to the east of the summit crater of the volcano, heavily covered with deposits from recent eruptions, many of which were preceded by harmonic tremor. Credit: Game McGimsey

It is not unusual for swarms of small earthquakes to precede a volcanic eruption. They can reach a point of such rapid succession that they create a signal called harmonic tremor that resembles sound made by various types of musical instruments, though at frequencies much lower than humans can hear.

A new analysis of an eruption sequence at Alaska's Redoubt Volcano in March 2009 shows that the harmonic tremor glided to substantially higher frequencies and then stopped abruptly just before six of the eruptions, five of them coming in succession.

"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 in Earth and space sciences.

Documenting the activity gives clues to a volcano's pressurization right before an explosion. That could help refine models and allow scientists to better understand what happens during eruptive cycles in volcanoes like Redoubt, she said.

The source of the earthquakes and harmonic tremor isn't known precisely. Some volcanoes emit sound when magma – a mixture of , suspended solids and – resonates as it pushes up through thin cracks in the Earth's crust.

But Hotovec-Ellis believes in this case 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 sticks 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.

Each of these sudden movements results in a small , ranging in from about 0.5 to 1.5, she said. As the pressure builds, the quakes get smaller and happen in such rapid succession that they blend into a continuous 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 said. "After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes."

She is the lead author of a forthcoming paper in the Journal of Volcanology and Geothermal Research that describes the research. Co-authors are John Vidale of the UW and Stephanie Prejean and Joan Gomberg of the U.S. Geological Survey.

Hotovec-Ellis is a co-author of a second paper, published online July 14 in Nature Geoscience, that introduces a new "frictional faulting" model as a tool to evaluate the tremor mechanism observed at Redoubt in 2009. The lead author of that paper is Ksenia Dmitrieva of Stanford University, and other co-authors are Prejean and Eric Dunham of Stanford.

The pause in the harmonic tremor frequency increase just before the volcanic explosion is the main focus of the Nature Geoscience paper. "We think the pause is when even the earthquakes can't keep up anymore and the two sides of the fault slide smoothly against each other," Hotovec-Ellis said.

She documented the rising tremor frequency, starting at about 1 hertz (or cycle per second) and gliding upward to about 30 hertz. In humans, the audible frequency range starts at about 20 hertz, but a person lying on the ground directly above the magma conduit might be able to hear the harmonic tremor when it reaches its highest point (it is not an activity she would advise, since the tremor is closely followed by an explosion).

Scientists at the USGS Alaska Volcano Observatory have dubbed the highest-frequency harmonic tremor at Redoubt Volcano "the screams" because they reach such high pitch compared with a 1-to-5 hertz starting point. Hotovec-Ellis created two recordings of the seismic activity. A 10-second recording covers about 10 minutes of seismic sound and harmonic tremor, sped up 60 times. A one-minute recording condenses about an hour of activity that includes more than 1,600 small earthquakes that preceded the first explosion with harmonic tremor.

Upward-gliding tremor immediately before a volcanic explosion also has been documented at the Arenal Volcano in Costa Rica and Soufrière Hills volcano on the Caribbean island of Montserrat.

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

Explore further

No Redoubt: Volcanic eruption forecasting improved

More information: Paper: DOI: 10.1038/ngeo1879
Journal information: Nature Geoscience

Citation: Some volcanoes 'scream' at ever-higher pitches until they blow their tops (2013, July 14) retrieved 18 June 2019 from
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Jul 14, 2013
The sound they describe should be very similar to what you get by pouring water from a pitcher into a bottle. The pitch keeps rising, until just before the bottle "erupts" the pitch jumps very high and stops.

Imagine the same in slow motion on a grander scale.

Jul 14, 2013
Nein, I agree all those AGW-ites, and Progressives with all their sock-puppets.... keep screaming at ever-higher pitches when you prove them wrong.

Jul 15, 2013
"Some volcanoes 'scream' at ever-higher pitches until they blow their tops"
Much like certain anti-science types who post here.

We know you AGW Alarmist turds don't scream, you stink.

Jul 15, 2013
Re: ""Some volcanoes 'scream' at ever-higher pitches until they blow their tops"
Much like certain anti-science types who post here."

People have good reason to be making noises. From "Rocks That Crackle and Sparkle and Glow: Strange Pre-Earthquake Phenomena" ...

"Seismic waves are the most dramatic and most intensely studied manifestations of earthquakes. However, we also know of non-seismic phenomena, which precede large earthquakes. Some of them have been reported for centuries, even millennia. The list is long and diverse: bulging of the Earth's surface, changing well water levels, ground-hugging fog, low frequency electromagnetic emission, earthquake lights from ridges and mountain tops, magnetic field anomalies up to 0.5% of the Earth's dipole field, temperature anomalies by several degrees over wide areas as seen in satellite images, changes in the plasma density of the ionosphere, and strange animal behavior ..."

Jul 15, 2013
From "Alaskan Volcano Spits Lightning" at http://www.thunde...ning.htm

"Though lightning was known to occur in the debris clouds above the volcano, the researchers found an earlier phase of volcanic lightning that had never before been observed and occurred right at the volcano's mouth just as it began erupting." ... In other words, the lightning preceded the supposed "charge separation" process from friction that has traditionally been claimed to occur in billowing volcanic clouds.

Jul 15, 2013
What people seem to be confused on is the notion of a "professional scientist" -- and for good reason, since the two terms have conflicting meanings. Scientists are generally thought to be open-minded to new ideas (at least that's the ideal), whereas a professional is an obedient thinker trained to think within the confines of the problem they are assigned to. Critical thinking, by contrast, is the act of questioning inherent assumptions. What seems to have been forgotten in all the rush to quarantine the cranks and pseudoscientists is that critical thinking necessarily involves critique of conventional theories.

Ever since the publication of Disciplined Minds by Jeff Schmidt, we now understand that the PhD programs weed out the grad students who question the scientific framework, and favor the more "gung ho types" who eagerly apply the dominant ideology without questioning it. "Thinking like a scientist" today includes a student's ideological beliefs, as evidenced by people here.

Jul 16, 2013
How considerate of Free_From_Thinking and his buddy Anal Orifice to confirm Neinsense99's point.

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