Volcanic Vibrations Give Glimpse into the Heart

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The distinctive howls that emanate from the heart of a volcano could offer us many clues into how and why volcanoes erupt.

This is not common to every volcano, as seen in 2009, where only one out of several volcanic eruptions were preempted with a scream.

March 2009 saw the Redoubt volcano eruption near Anchorage in Alaska. The volcano exploded more than 20 times over a two week period, resulting in ash travelling as far as 15 kilometers into the stratosphere.

Local seismic stations had detected an excitement of activity prior to the explosions. Tiny tremors of magnitude 0.5 to 1.5 were measured. The earthquake frequency rose to 30 events per second in the final minute leading up to an explosion. This means the seismic waves they created fused together into an uninterrupted stream. What followed was a prickly 30 seconds of silence, and then the volcano exploded.

The stream has been nicknamed “the seismic scream” by the seismologists studying Redoubt. They describe it to have built up to a “crescendo of increasing pitch, entering the lowest range of human hearing.”

Eric Dunham, at Stanford University in California says that if you were able to withstand the intense environment within a volcano’s magma chamber, you would perceive the ‘screams’ as a loud and constant, deep rumble. Standing on the volcano’s flanks, you would only just be able to perceive it like a hum. Eric is part of the research team analyzing the vibrational hum.

He is optimistic about what this could reveal about nature’s mysterious volcanoes.

Analysis of the readings given off at the seismic stations reveal the tremors arose from 2 kilometers below the crater. This is near to where the volcanic conduit is fed via the magma chamber. The conduit is a pipe that channels the magma up toward the volcano’s peak, the magma then spurts out the vent.

To understand what caused the volcano to scream, the research team created a mathematic model of the seismic excitement. The results thus far have shown how an increasing amount of pressure within the conduit pipe may have amplified friction between the surfaces in the volcano’s center. This then causes the surfaces to move past each other in an accelerating sequence of shudders just before culminating in an explosion.

Dunham and his researchers are still investigating the exact surfaces that scraped against each other. The current findings on the causes of the scream could lead to varied options, with the most likely at present being explained by the thickness of the magma and degree of change thereof. Helena Buurman, at the university of Alaska Fairbanks says “This suggests to me that the stick-slip mechanism between the magma and the conduit or even the brittle failure of the magma itself is the most likely explanation.”

The 2009 Redoubt eruption is the only recorded example of a volcano’s screams. By establishing the exact cause of the vibrations, our view into what goes on in the heart of a volcano will shift dramatically.

If we get to see how the pressure changes in the heart of volcano just before it erupts, we could begin to understand just what goes on within the mysterious volcano and what causes the differing extremities in eruptions.

 

Jessica Rosslee

 

 

 

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