Tavurvur erupting.

Volcanic sonic boom – what, why and how?

Recently (2016) an eruption at Tavurvur in Papau New Guinea has been going viral due to its amazing ability to produce a volcanic sonic boom. But, what is a volcanic sonic boom, why and how does it occur? In this article I hope to answer some of these questions with reference to the recently published (16/01/2016) research of Medici & Waite 2016, who investigate the formation of volcanic ‘sonic booms’ during eruptions.

Tavurvur erupting.
Tavurvur erupting. Credit: T.Taylor & R.Bartz wikimediacommons

What is a sonic boom?

A sonic boom occurs when shock-waves (pressure waves) travel through the air faster than the speed of sound [1]. A sonic boom which sounds like an explosion, is commonly known from its link to military aircraft when they go supersonic.

The boom is created when sound waves (pressure waves) combine and coalesce, and effectively become a single shock wave wall [1]. When the sonic boom applies to an aircraft the aircraft is in motion. But, when a sonic boom is activated by a volcano the volcano itself acts as a point source of energy.  The shock wave produced creates a bubble that travels outwards from the volcano.

In the Youtube video below you can actually see this shock wave expanding outwards from the volcano. And the sound that the sonic boom makes when it reaches the cameraman on the boat.

What causes a volcanic sonic boom?

When an eruption begins pressure is suddenly released from a volcano, along with gases and various solids and liquids. These components can be ejected from the volcano at supersonic speeds [2], and can lead to the creation of a ‘sonic boom’.

The video of a sonic boom experienced during the opening stage of the eruption of Tavurvur, above, was recorded at a relatively short distance from the volcano. Therefore the shock wave would have lost relatively little energy before it reached the cameraman stood on the boat, allowing the tremendous explosive sound that is heard as the sonic boom strikes.

At greater and greater distances away from the volcanic source the sonic boom will decrease in energy eventually becoming a sound wave and then a small pressure wave.

The eruption of Tavurvur has produced several sonic booms during its activity so far. This shows that a sonic boom can be created during an eruption and not just at the beginning. These booms may be related to changes in the eruption dynamics but at the moment more research is needed to properly understand their cause. Some problems with this is that it is difficult to study the dynamics of active eruptions up close.  Live experiments can be conducted in labs instead, but the real challenge comes when testing these analogue experiments with real volcanic eruptions as they occur. A science and understanding which is still being developed.

Historical examples

The catastrophic eruption of Krakatau, in Indonesia, in 1883 created the loudest sound in recorded history. This sound could well have been a sonic boom or series of sonic booms that could be heard as cannon fire as far away as Australia. The sonic boom eventually became a sound wave which was detected around the entire globe as a small decrease in pressure.

An eruption at Mount Yasur, Vanuatu, also produced volcanic sonic boom during its eruption in 2011 [2], [3].

References/Links

[1] https://en.wikipedia.org/wiki/Sonic_boom

[2] Medici, E.F. & Waite, G.P. 2016. Experimental laboratory study on the formation of multiple shock waves observed during volcanic eruptions. Geophysical Research Letters, 43(1), 85-92

[3] Taddeucci, et al. 2014. High-speed imaging, acoustic features, and aeroacoustic computations of jet noise from Strombolian (and Vulcanian) explosions. Geophysical Research Letters41, 3096-3102

 

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