The lava fountains, emitted from the summit craters of the volcano and particularly spectacular, can be preceded by peculiar infrasonic signals whose processing allows you to "see" the progressive ascent of the magma inside the conduit about 24 hours before the start of the fountain itself .
These are the results of the study conducted by an international team of researchers from the Etna Observatory of the National Institute of Geophysics and Volcanology (INGV-OE), the University of Catania (UniCT), the University of Canterbury (New Zealand) and Boise State University (USA). The research 'Infrasonic gliding reflects a rising magma column at Mount Etna' has just been published in Scientific Report.
From 2000 to today the lava fountains are among the most frequent eruptive manifestations of Etna. The lava jets, continuous and of variable height between 200 and over 1000 m, can last from half an hour to about two hours.
Infrasound is a low-frequency acoustic signal, inaudible to humans, whose recordings using special microphones are increasingly used by volcanological observatories for monitoring. In fact, volcanoes behave like enormous musical instruments and generate acoustic signals mainly in the infrasonic band both during eruptive activity and during simple degassing. In particular, the volcanic conduits resonate like organ pipes whose notes (and therefore frequencies of the acoustic signal) mostly depend on the length of the "pipe".
The team of researchers discovered that the lava fountain of February 20, 2021 was preceded by an intense acoustic signal in the infrasonic band emitted by the South-East crater over 24 hours before the start of the eruption, characterized by a progressive increase in frequency.
Due to the deep explosions, the vent of the Southeast crater reverberated like an organ pipe. The rising of the magma inside and the progressive filling of the conduit in the hours preceding the eruption, determined a gradual reduction in the length of the resonant portion of the "organ pipe" and, therefore, an increase in the frequency of the acoustic signal emitted. The modeling of the infrasonic signal, integrated with the results of a topographic survey using drones, has made it possible to accurately reconstruct the dimensions of the resonant portion of the conduit, thus making it possible to evaluate the progressive ascent of the magma inside it, from a depth of about 170 m to about 80 m during the 24 hours preceding the lava fountain.
From the point of view of volcanic hazards, although the lava flows accompanied by the fountains do not have a great impact on ground infrastructures, also given the altitude at which they occur, these paroxysms produce ash clouds even over 10 km high which cause enormous inconvenience both to air traffic and to the Etna towns due to the ash they deposit.
Careful monitoring, using various instruments such as seismometers, tilt meters and cameras, is therefore essential. With these tools we try to identify precursory signals of eruptions that are linked to the ascent of magma along the volcano's feeding system.
Researchers from the INGV Osservatorio Etneo (Mariangela Sciotto, Massimo Cantarero and Emanuela De Beni), the University of Catania (Andrea Cannata), the University of Canterbury (Leighton Watson) and Boise State University (Jeffrey B. Johnson) showed how the identification and study of acoustic signals in the infrasonic band can provide valuable information for monitoring volcanoes.
Full citation of the publication: Sciotto, M., Watson, LM, Cannata, A., Cantarero, M., De Beni, E., Johnson, JB (2022). Infrasonic gliding reflects a rising magma column at Mount Etna (Italy). scientific reports,
Video: YouTube INGV volcanoes
Video: The video shows: above, the infrasonic signal recorded from 14:00 on February 19 to 02:00 on February 20, 2021 by the ECPN microphone with its time-frequency-amplitude graph (yellow: high amplitudes; blue: low amplitudes) ; below, the images acquired by the EMOT thermal monitoring camera, accelerated by a factor of 500 and synchronized with the vertical red line present in the upper frame of the video. In the box below, the reconstruction of the conduit of the South-East crater (SEC) is also shown with the magma rising during the hours preceding the start of the eruption. The sound that accompanies the video is the recording of the infrasonic signal from the ECPN microphone, accelerated by a factor of 500 to be made audible and synchronized with the images from the thermal camera.
Photo: Lava fountain at Mount Etna (©️Francesco Ciancitto - INGV)