Infrasonic Resonance of Volcanic Craters

At Villarrica, infrasound is generated by the roiling of the lava lake and heavily modulated by the crater geometry. The character of the infrasound changed in the lead-up to the eruption. The pitch of the infrasound increased and the duration decreased. Using numerical modeling, we were able to attribute this change due to the lava level rising in the crater.

Villarrica volcano can be thought of as analogous to a trombone. Similar to a person blowing into a trombone, explosions from gas bubbles rising and then bursting at the surface of the lava lake create sound waves. Just as the shape of a trombone can change the pitch of the notes it produces, the geometry of the crater that holds the lava lake modulates its sounds. When the lava lake is deep down in the volcano’s crater, the sound registers at a lower pitch or frequency just like when a trombone is extended. When the lava lake rises up in the crater the pitch or frequency of the sound increases, just like when the trombone is retracted.

Harmonic infrasound signals of exceptionally low frequency and long duration were observed at Cotopaxi Volcano (Ecuador) after a sequence of eruptions in 2015 changed the shape of the crater. The infrasound was remarkably stable over a period of six months providing an opportunity to understand the unique “voiceprint” of Cotopaxi. We were able to use infrasound observations to constrain the geometry of the immense crater at the summit.

• Johnson, J. B., Ruiz, M. C., Ortiz, H. D., Watson, L. M., Viracucha, G., Ramon, P., and Almeida, M. (2018) Extraordinary infrasound tornillos produced by Volcan Cotopaxi's deep crater, Geophysical Research Letters, 45, doi: 10.1029/2018GL077766.