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!

Volcanoes and atom bombs

Photo of Matua volcano, Siberia, taken by NASA astronauts

Detonation of a thermobaric (fuel-air) bomb by the Russians.
Believed to be the largest of its kind ever detonated.

The New York times recently had a photo documentary of development of the atomic bomb. The photo below is of a "fuel-air bomb", often called a "vacuum bomb". This type of explosive uses oxygen from the surrounding air to increase the duration, and destructiveness, of the shock waves/fireball. They were first developed by the Germans during WWII, and are in use by modern military and in guerilla warfare (1993 World Trade Center bombing, 2002 Bali bombings).

Some volcanoes have eruptions that are "explosive" enough to produce shock waves.  These were first recognized by Perret during explosive eruptions of Vesuvius in 1906, and were later observed and analyzed quantitatively at Ngauruhoe, New Zealand (Nairn, I.A., Nature 259 (5540, pp. 190-192, 1976). On June 12, 2010, NASA astronauts were able to capture the photo at the left of an eruption of Matua volcano, Siberia.  You can't see the shock wave directly, but can see the hole that it punched through the cloud deck.  The rising ash plume has also pushed up a layer of moist air forming a pileus "cap cloud".  A pyroclastic flow is visible at the base of the column, descending toward 5 o'clock on the flank of the volcano.  Volcanologists use some of the same basic concepts about shock waves that were developed to analyze shocks from atomic bombs, and although we talk about the energy released in eruptions in terms of "kilotons" or "megatons". For example, I analyzed the energetics of the lateral blast at Mount St. Helens in 1980, and concluded that about 24 megatons of energy was released during the blast (Kieffer, S.W., Nature, 291, 568-570, 1981).