VOLCANOES SCREAM TO ANNOUNCE ERUPTION

Some volcanoes scream at ever higher pitches until they blow their tops.  But the frequencies are too low for humans to hear without specific instrumentation.

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."

 

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.

 

The theory: 

 

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

 

But Alicia Hotovec-Ellis, a University of Washington doctoral student in Earth and space sciences, 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 earthquake, ranging in magnitude 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, After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes.

 

The pause in the harmonic tremor frequency increase just before the volcanic explosion is when even the earthquakes can't keep up anymore and the two sides of the fault slide smoothly against each other.

 

Alicia Hotovec-Ellis is the lead author of a forthcoming paper in the Journal of Volcanology and Geothermal Research that describes the research, with co-authors John Vidale of the UW and Stephanie Prejean and Joan Gomberg of the U.S. Geological Survey.

Read more -  http://www.sciencedaily.com/releases/2013/07/130714160521.htm

More on volcanoes on this blog - http://ottersandsciencenews.blogspot.ca/search/label/Volcanoes

Link to this post - http://ottersandsciencenews.blogspot.ca/2013/07/volcanoes-scream-to-announce-eruption.html