May 8, 2012 From low rumblings to concussive blasts, volcanoes emit a broad spectrum of sonic energy. In the case of basaltic eruptions, most of that acoustical energy in the infrasound range, at frequencies below the range of human hearing. A new study reveals that this low-frequency sound can give scientists an enhanced understanding of the behavior of volcanoes and a tool to monitor the lifecycles of their eruptions.
According to geophysicist Aurélien Dupont of the Pusan National University in South Korea, as gasses percolate through underground magma they produce specific low-frequency sounds. "By recording and analyzing the sounds emitted by basaltic volcanoes," Dupont said, "volcanologists can use infrasound to measure the flow and behavior of magma and better survey volcanic activity." He is presenting his research at the Acoustics 2012 meeting in Hong Kong, May 13-18, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.
By tracking the main sources of infrasound during the eruptions, the researcher discovered that the gas pockets trapped in the magma decreased in size throughout the course of the eruption until they disappeared at the eruption's end. "The gas volume fraction constantly decreases in the volcanic conduit during a basaltic eruption," Dupont said.
Basaltic magma has low viscosity and relatively low gas content so it flows easily, producing volcanoes with gentle slopes and impressive displays like lava fountains and rivers of lava.
As the magma travels from the underground magma reservoir to the surface, the gas that is in suspension begins to expand. This expansion produces distinctive infrasound profiles. Once the magma reaches the surface and is under only atmospheric pressure, the gas is able to bubble away and escape.
Dupont made his observations at the Piton de la Fournaise volcano on Reunion Island, which is in the Indian Ocean. This volcano erupts often and is a perfect field laboratory to test and adapt new survey techniques, like infrasonic monitoring.
For this research, Dupont used condenser microphones and microbarometers to detect the sound produced from the gas flow. He then correlated that sound profile with observations of volcanic gasses escaping from vents in and around the crater. "If no volcanic gas escapes anymore from the vents, detections stop and the eruption is over," he noted. "Infrasound can accurately characterize the beginning and the end of an eruption." The series of volcanic noises, which were recorded between 1992 and 2008, brings new constraints to the functioning of the eruptions.
"The quantitative analysis of the noise produced by the gas flow allows us not only to understand a natural system as complex as a volcano but allows us also to better monitor it," concludes Dupont.
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