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he first close-range, low-frequency recordings of volcanic eruptions have revealed a surprising similarity to the noise made by jet engines. The finding may provide clues to what happens prior to volcanic explosions.

Hear an infrasound recording of an eruption at Mount St Helens, speeded up 200x (Credit: American Geophysical Union. File: .mov format)

Robin Matoza of the University of California, San Diego, and colleagues measured infrasonic signals from volcanic eruptions around the globe, getting as near to them as 13 kilometres – relatively close for these sorts of measurements. This was the first time that infrasonic signals from very large eruptions had been measured at relatively close range.

When the team speeded up the frequency of the volcano recordings to within the range of human hearing, the distribution of frequencies closely resembled those of a Boeing 747 jet engine.

"The science of jet noise is very well understood," says Matoza. "If we can understand how this works for volcanoes, we may be able to infer properties of eruption columns."

They already have a few ideas. The roar produced in engines is partly down to turbulent air near the inner walls of the engine. The fact that a similar sound is generated by both vulcanian and plinian eruptions – blasts with tall, dense ash columns – suggests it is created as ash and gas blasts up near the inner surface of the volcano's mouth.
Aircraft warnings

Explosive eruptions are far too violent to measure directly using sensors or planes, says Darcy Ogden at Stanford University, so the finding could be a useful way of discovering the velocities, densities and pressures inside the volcanic plume.

"This information about the internal dynamics of the plume then helps us understand more about how plumes work. The more we understand about how plumes work, the better we are at mitigating volcanic hazards," she says.

The recordings could also be used to warn pilots of eruptions hidden by clouds. "If you get a big jet noise like this, it's a very strong indication that a volcano eruption definitely has occurred," says Matoza.

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