Volcano Watch: Understanding volcanic blasts using water cannon experiments

The 1980 Mount St. Helens eruption in Washington State illustrated the hazards and impacts of ground-hugging volcanic blasts on natural landscapes and human infrastructure.

The eruption devastated hundreds of square miles and killed 57 people. In the more than 40 years since, several additional laterally directed explosive eruptions have occurred worldwide.

An eruption at Ontake, Japan in 2014 showed the tragic impacts of laterally directed eruptions in near vent environments. But lateral eruptions at volcanoes are not only confined to the main eruption blast.  

Hot gas, ash and mud can flow laterally from a mostly vertical eruption located in confining topography, like a valley, focusing ground-hugging volcanic flows (known as pyroclastic density currents). They may impact the near vent environment even for small eruptions.  

If a valley or other topography exists, these types of flows can move several miles from the eruption vent. In some cases, such events can produce mudflows, called lahars, which can be particularly dangerous even farther, tens of miles from the eruption source. 

Due to the devastating impacts these events can have on nearby areas, the global volcano monitoring community wants to improve the detection and characterization of hazards posed by explosive eruptions using automated sensors like seismometers and microphones for early warning systems.   

A new preliminary field-scale experiment was recently completed by a U.S. and New Zealand research team, where the energy characteristics of a human-made volcanic eruption was measured on a surrounding microphone acoustic recording system.  

The experiment used a tiltable water cannon that was surrounded by pressure sensors like those used for volcano monitoring, both globally and at the Hawaiian Volcano Observatory.

The scientists wanted to determine if there were differences in the sound measured in the direction of the eruption blast, compared to the sounds measured behind the cannon. These differences may give scientists insight into the eruption processes and better understand the hazards associated with real ground-hugging eruptions.  

The figure shows an example explosion from the inclined water cannon experiment. The barrel is comprised of a standard 55-gallon drum with one end open, filled one-third full of water at ambient temperature. A sealed soda-pop bottle filled with liquid nitrogen is dropped into the water. Because the liquid nitrogen is at a temperature of -320 degrees Fahrenheit, it will expand in the warmer surrounding water.  

Shortly after the bottle is immersed, it rapidly bursts, producing a small, controlled explosion. Normally an explosion would expand in all directions, but because the bottle is at the bottom of an open-ended barrel, the energy is focused out of the barrel opening. The preferential direction of energy expansion and the explosion direction is then recorded on the surrounding sensors. 

Each experiment was recorded with video cameras facing in three unique directions to document the blast direction and speed. Vertically directed blasts were found to have similar acoustic recordings on all the surrounding microphones.

For more ground-hugging eruptions, the experiments suggest that the strongest blasts show higher frequency energy in the direction of the blast while lower frequency energy is recorded behind the blast source, which in this case is the cannon.   

While more tests are required, the observations might reflect features of eruption blast dynamics that can be used as part of future eruption detection systems near hazardous eruption vents.  

The observational data may also have implications for hazardous mass flow events including pyroclastic-flows and lahar monitoring. While our Hawaiian volcanoes have fewer explosive eruptions in general, the observation results may be useful to understand the lateral migration of our Hawaiian fissure eruptions. 

If you want to learn more about this experiment, check out this recent publication in Earth, Planets, and Space.

Editor’s note: Volcano Watch is a weekly article and activity update written by scientists and affiliates with the U.S. Geological Survey Hawaiian Volcano Observatory.