Exploding lava bubble at Etna

Exploding lava bubble at Mount Etna
Photo copyrighted by Tom Pfeiffer
from Volcano Discovery

Audible booms, sometimes very audible, occur at a number of volcanoes around the world. In a previous post, I discussed a variation on this ("flashing arcs") for the "James Bond" volcano, Shinmoedake,  and the Matua, Siberia, eruption. Also, more on shock waves here in the context of China's new fighter jet!

Here, the photographer captured the source of one such boom, a lava bubble (what is the scale??) bursting from the lava lake at Mount Etna.

In text accompanying the photo at the left, the photographer reported that "a loud detonation and a strong shock wave that felt like a punch into the stomach when observed from 300 meters distance, accompanies these bursts. Even at 10 km distance, windows rattle."

Audible booms or atmospheric shock waves are produced when something pushes suddenly on the atmosphere. This is most easily illustrated with a one-dimensional experiment called a "shock tube."  These were developed to explore the effects of bombs on the atmosphere, but have been widely used to investigate basic science problems such as chemical kinetics, and basic properties of gases as a function of pressure and temperature.

Schematic of a shock tube experiment.
Source is here; use is permitted for any purpose.

A diaphragm, which is usually a piece of aluminum, is placed in the tube.  One region of the shock tube is taken up to a high pressure compared to the other region, and then the diaphragm is burst, either mechanically or sometimes with an electric wire. As shown in the upper part of this diagram, a wave, the shock wave, propagates into the low-pressure region. It "carries the message" that the diaphragm has burst into the low pressure region. At the same time, another wave, the expansion fan, propagates into the high-pressure region.  It "carries the message" that the diaphragm has burst into the high pressure section of the tube. The interface between the high and low pressure regions is called the "contact surface", and it moves downstream after the diaphragm is burst. Both waves reflect off the ends of the tube, and the flow becomes very complicated after that. 

The bursting bubble at Mount Etna is a spherical version of a shock tube.  In this case, the "diaphragm" was a coating of lava.  The motion of the "contact surface" is beautifully visible as the red hot lava fragments! Congratulations to the photographer!