The speed with which magma forms and rises from the depths of the Earth to the surface also depends on its viscosity; therefore, it is of fundamental importance to know this physical property of magmas to define scenarios and estimates of volcanic hazard and related risk.

In an experimental study, an international team of researchers led by Barbara Bonechi and Vincenzo Stagno of the Department of Earth Sciences of Sapienza University of Rome, and carried out with the National Institute of Geophysics and Volcanology (INGV), Ehime University in Japan, Argonne National Laboratory in the USA (ANL) and the University of Trieste (UniTS), developed estimates of the rate of magma ascent of the Phlegraean Fields system from the mantle source, which the authors position at a depth of about 60 km, towards a hypothetical crustal feeding system of the caldera located at 25 km.

In the study "Experimental measurements of the viscosity and melt structure of alkali basalts at high pressure and temperature" just published in Scientific Reports, the scientists calculated that the average magma ascent velocity in the deepest region of the volcanic system is about 2.5-3 metres per year, considering a time interval of about 20 thousand years between the major historical eruptions in the Phlegraean Fields region ("Ignimbrite Campana" of 39000 years ago and "Tufo Giallo Napoletano" of 15000 years ago). They, therefore, hypothesised that the mantle source could be at a depth of about 60 km.

"We used a synthetic magma made by melting lava from the Solchiaro volcanic deposit, from the homonymous volcano formed on Procida some 17-19000 years ago," say Barbara Bonechi and Vincenzo Stagno. "The experiments, which we carried out at the prestigious Argonne National Lab synchrotron in Chicago, were aimed at determining the viscosity of magma at high pressures and temperatures."

Volcanic eruptions are surface phenomena that represent the final stage in the life of magma, from its formation in the Earth's interior to its migration towards the surface. The volcanic hazard of an area is, among other things, related to the explosiveness of an eruption, which depends on various factors, including magma chemical composition as it rises, the amount of gas released from it and any interaction with surface waters. An essential role in the eruptive style of a volcano is therefore also given by the density and viscosity of the magma, knowledge of which contributes to improving assessments of processes linked to magmatic dynamics and reducing uncertainties in the modelling of these processes.

"It is important for geochemical and seismological models to consider the depth of magma formation and its potential rate of ascent," say the researchers. Using the falling sphere technique, whose fall can be observed in real time through X-ray imaging produced by an accelerated particle beam at Argonne National Lab, we found that the viscosity of rising basaltic magma feeding the Phlegraean Fields system varies between 0.5 and 3 Pascal seconds.

Luca Ziberna, Professor of Petrology and Petrography at the University of Trieste, focused on the oxidation-reduction conditions of lava, a fundamental chemical parameter for understanding magmatic processes. At the Department of Mathematics and Geosciences of the University of Trieste, some theoretical and experimental models were studied to help understand the effect of chemical composition, with particular reference to the oxidation state of iron, on magma ascent processes.

"The experimental data presented in this paper – concludes Bonechi and Stagno - are of absolute importance in the study of the Phlegraean Fields and constitute additional information to supplement the geochemical data and geophysical models available to date."

A contribution that could be useful in the future to improve prediction and prevention of civil protection but that currently has no direct implication on measures that affect the population's safety.

References:

Experimental measurements of the viscosity and melt structure of alkali basalts at high pressure and temperature - Barbara Bonechi, Vincenzo Stagno, Yoshio Kono, Rostislav Hrubiak, Luca Ziberna, Giovanni B. Andreozzi, Cristina Perinelli & Mario Gaeta - Scientific Reports 2022 https://doi.org/10.1038/s41598-022-06551-7

Further Information

Vincenzo Stagno
Department of Earth Sciences
vincenzo.stagno@uniroma1.it