Just before a volcano spews hot ash and lava in an eruption, it might in some cases emit a high-frequency “scream.”

A study by the University of Washington of the 2009 explosion of Alaska’s Redoubt Volcano recorded such a “harmonic tremor,” which researchers believe could provide clues as to what happens during cycles of eruptions.

Volcanoes Scream Due to Seismic Activity Before Eruption Says Study

(Photo: Chris Waythomas/Alaska Volcano Observatory/ US Geological Survey via Wikimedia)

Sometimes, preceding a volcanic eruption are a series of small earthquakes, which researchers believe might cause the higher frequencies, which cannot be heard by humans, that then stop before the actual explosion.

“The frequency of this tremor is unusually high for a volcano, and it’s not easily explained by many of the accepted theories,” Alicia Hotovec-Ellis, a University of Washington doctoral student, said in a statement of the Redoubt Volcano’s 2009 eruption.

Here’s how the University of Washington describes Hotovc-Ellis’ perspective of how “the screams” might occur:

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 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 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,” Hotovec-Ellis said. “After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes.”

Humans are not likely to hear such a scream in nature because the frequency of the tremor starts at 1 hertz and gets up to about 30 hertz. Humans begin hearing frequencies at 20 hertz, according to the study. But, as the university pointed out, lying above such an area prior to an eruption to hear the sound when it reaches an audible level wouldn’t be advisable.

Listen to such a “scream” as it starts off slowly and the frequency builds:

The above recording represents an hour of condensed volcanic activity from Redoubt Volcano in 2009.

“The ‘drumbeats’ are more than 1,600 small earthquakes that occur at an ever-faster pace until they blend into a continuous harmonic tremor that preceded an explosion,” the description of the audio stated.

The research Hotovec-Ellis was involved in is published in the Journal of Volcanology and Geothermal Research. She also worked on research with Stanford University to develop a tool that would evaluate the tremors. Information about this tool was published in Nature Geoscience.

Featured image via Shutterstock.com. 

(H/T: Science Daily)