An operation similar to that of a beating heart, with a deeper magma reservoir that constantly feeds a more superficial one, where the gases pressurize giving rise to the burst of lava fountains: it is the result of the model developed for Etna by a team by researchers from the National Institute of Geophysics and Volcanology (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 that involved the Voragine crater of the Sicilian volcano in December 2015.
Scientists analyzed the deformations of the volcano to trace the magmatic sources of the sequences of violent eruptions, to understand their dynamics and define the power system of Etna capable of producing such a rapid accumulation and violent release of magma.
«Our analysis of the ground deformation data, obtained using high frequency tilt and GNSS (Global Navigation Satellite System) data and DInSAR (Differential Interferometric Synthetic Aperture Radar) satellite images, covered a period of 12 days comprising the entire sequence eruptive in December 2015 », explains Alessandro Bonforte, INGV researcher and first author of the article. «These measurements allowed us to define the complex interactions between the different storage areas where the magma erupted with the paroxysms was temporarily stored».
The study made it possible to define the dynamics and transfer rates of magma from a deep magma chamber to a more superficial one. There the magma, rich in gas, temporarily settles, building up pressure.
“The source of deep pressurization supplies gas-rich magma to a more ‘shallow’ reservoir located at a depth of about 1.5 / 2 km,” explains Bonforte. “When the gas pressure exceeds that of the rock’s containment, 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 magmatic material accumulated in the most superficial reservoir, but also part of what is stationed in the rest of the volcano’s feeding system, with an eruptive rate of over 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) again begins to recharge the superficial one, as well as the blood flow in the heart that it is pumped from the atrium to the ventricle and then from the ventricle to the outside of the heart.
«The model proposed by us, therefore, suggests a mechanism similar to that of a beating heart, in which a tank of medium depth, at about 6 km, loads a more superficial tank, at about 2 km; this tank is at a depth that allows the gas to separate from the rest of the melt, thus increasing the pressure, a bit like when you see bubbles forming in a bottle of a carbonated drink. Everything is silent until the pressure exerted by the gas present inside the magma is not too high, essentially the valve opens and paroxysm occurs, which drains the magma from the most superficial source 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 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 other volcanoes in the world », Bonforte concluded.