An operation similar to a beating heart, with a deeper magma reservoir that constantly feeds a shallower one, where the gas pressurizes, giving rise to the burst of lava fountains: it is the result of the model developed for Etna by a team of researchers of the Istituto Nazionale di Geofisica e Vulcanologia (INGV), just published in the scientific journal ‘Applied Sciences’.
The study, entitled “Combining high- and low-rate geodetic data analysis for unveiling rapid magma transfer feeding a sequence of violent summit paroxysms at Etna in late 2015”, focused on a series of four lava fountains produced by the Voragine crater of the Sicilian volcano in December 2015.
Scientists analyzed the deformation of the volcano to trace the magmatic sources of the sequence of violent eruptions, in order to understand their dynamics and to define the feeding system of Etna able to produce such a rapid accumulation and violent release of magma.
“Our analysis of the ground deformation data, obtained using high-rate tilt and GNSS (Global Navigation Satellite System) data and DInSAR (Differential Interferometric Synthetic Aperture Radar) satellite images, covered a period of 12 days encompassing the entire eruptive sequence in December 2015”, explains Alessandro Bonforte, INGV’s researcher and first author of the article. “These measurements allowed us to define the complex interactions between the different storage volumes where the magma, erupted during the paroxysms, was temporarily stored”.
The study made it possible to define the dynamics and transfer rates of magma from a deep magma storage to a shallower one. There the magma, rich in gas, temporarily settles, building up pressure.
“The deeper storage supplies gas-rich magma to a more ‘superficial’ reservoir located at a depth of about 1.5 / 2 km”, explains Bonforte. “When the gas pressure exceeds a threshold pressure, a violent eruption occurs in the form of paroxysm. This combined mechanism of two levels of ‘storage’ of magma at different depths therefore represents the possible ‘engine’ of such rapid and violent sequences of events”.
These paroxysms drain not only the magma accumulated in the most superficial reservoir, but also part of the magma stationing in the rest of the volcano’s feeding system, with an eruptive rate of more than 300 cubic meters per second. Conversely, when the gas pressure decreases, the paroxysm stops, let’s say that the valve closes, and the deeper reservoir (located about 6 km deep) begins to refill the superficial one again, like the flow in the heart, pumping the blood from the atrium to the ventricle and then from the ventricle to outside the heart.
“The model proposed, therefore, suggests a mechanism similar to that of a beating heart, in which a medium depth storage, at about 6 km, feeds a shallower one, at about 1.5 / 2 km; this shallower reservoir is located at a depth that allows the gas to separate from the rest of the melt, thus increasing the pressure, like when you see bubbles forming in a bottle of a fizzy drink. Everything is quiet until the pressure exerted by the gas inside the magma is too high, essentially the valve opens and paroxysm occurs, draining the magma from the most superficial tank and from the rest of the system, which is continuous. Once the excess pressure has been released, the valve closes and the cycle begins again, with the magma starting again to move from the deep reservoir to the superficial one. This mechanism could represent a valid conceptual model for events of a similar nature on Etna and on other volcanoes in the world”, concludes Bonforte.
(Ufficio Stampa INGV)