The “melody” of volcanic eruptions

Hidden in the chaos of the first moments of a volcanic eruption is an ephemeral ring of smoke and ash that can provide scientists with important clues as to how dangerous an eruption can be. A new study “captures” these volcanic toroidal vortices and their sound on high-speed video, suggesting a new way to monitor eruptions using sound alone.

Volcanic vortex rings are ringed vortices that form around the top of an eruptive cloud during the first few seconds of the event, like eddies that form behind an oar pushed into the water. The size, velocity and characteristics of these vortices reflect factors that are useful in assessing how dangerous the eruption is, such as its intensity or the depth of the eruptive vent from which it began. However – INGV explains in a note – the rings are only appreciable for short periods and can be difficult to measure in small eruptions. Larger eruptions may instead form more visible eddies, but scientists aren’t always able to get close enough to make the necessary observations. Also, if the eruption occurs at night or with the volcanic building enveloped in ash or clouds, the rings may not be visible.

A new study conducted by a team of volcanologists and physicists specializing in fluid dynamics, entitled “Volcanic Vortex Rings: Axial Dynamics, Acoustic Features, and their Link to Vent Diameter and Supersonic Jet Flow”, has made it possible to combine high-speed video with recordings audio focused on the first seconds of the Stromboli eruptions to ‘listen’ to the volcanic vortex rings.

The study, unique in its audiovisual approach and the first to isolate the sound of the rings, was published in the journal ‘AGU Geophysical Research Letters’, which publishes high-impact, short-format studies with immediate implications spanning all the sciences of Earth and Space. “There is a distinctive sound produced by the volcanic vortex, which is a low, steady sound. The beauty of vortex rings is that they are stable, which means that they emit a constant sound that propagates over time ”, explains Jacopo Taddeucci, volcanologist at the National Institute of Geophysics and Volcanology (INGV) and first author of the study.

With this new knowledge of how a volcano’s ‘melody’ is related to its eruption, volcanologists can expand our knowledge of different volcanic processes and sounds to improve monitoring in cases where visual or other techniques are not usable. “Even if we do not see the eruption because, for example, the sky is cloudy or we do not have a fixed camera on the volcano, with this new technique we are able to know what is happening by analyzing only the sound,” continues Taddeucci. “My hope is that, by knowing better the sound of eruptions, we will soon be able to monitor changes in an ongoing eruption only through noise. This is one of the future objectives of our research ».

To detect the sound of volcanic vortexes, Taddeucci and his collaborators installed high-speed cameras and professional microphones several hundred meters from the eruptive vents of Stromboli. They also used a drone to film the volcano while it was emitting eruptive jets, namely columns of hot gas, ash and smoke, which on the Aeolian volcano reach 100-300 meters in height. This allowed the team of researchers to measure parameters such as the size of the eruptive vent.

Although the scientists could not see the rings they were trying to capture with the naked eye, they appeared by analyzing the videos at high speed. After measuring the size and speed of the rings and precisely synchronizing the video with the audio, the researchers were able to isolate the steady low rumble emanating from the eddies.

Since the vortex sounds were distinctive and constant, the team was able to correlate their frequency with other characteristics. In particular, a clear link was highlighted between the movement of the vortex, its sound and the size of the eruptive vent. Other changes in a volcano’s “melody”, including its eruptive jet, may be related to the internal structure of the mouth.

“Vortex rings are quite sensitive to the parameters with which they are generated,” explains Juan José Pena Fernández, fluid mechanics researcher and co-author of the study. “If we record the acoustics generated by a vortex ring we can know quite precisely the conditions that created it. If, therefore, there were a change in the behavior of the volcano we might be able to detect it ». The combination of audio with high-speed images “makes perfect sense,” says Amanda Clarke, a volcanologist at the University of Arizona who was not involved in the study. “I thought he was very creative and intelligent. It’s not easy to do these kinds of field measurements at the same time, ”continues Clark. “In the case of explosive eruptions, the volcanic eddies can really ‘speak’ about the conditions of the source, therefore of the volcano itself. However, it is not easy to extrapolate this information from measurements that are not easily observable ».

Source: RSS Innovazione by

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