In order to predict volcanic eruptions, one must understand the physical behavior of magma at the depth where it forms. Magma is the molten rock ejected by volcanoes when they erupt. It rises from the interior of the Earth, slowly permeating the surrounding rocks, until the pressure rises too high and molten rock spews from the volcano (Fig. 1). Magma can be thin or thick, sticky or smooth, gaseous or not. In general, more-viscous magmas produce more-violent explosions. Much depends on where a volcano is located. Campi Flegrei, for example, is a large, active volcano west of Naples, Italy, that erupts catastrophically approximately every 20,000 years. It has also had less dramatic eruptions as recently as 1538. Around 550,000 people live near the Campi Flegrei volcanic field, and the volcano is currently recharging, with magma gradually refilling its reservoirs. A team of researchers studying the area used the unique facilities of the U.S. Department of Energy's Advanced Photon Source (APS) to understand how magmas behave in volcanoes that, like Campi Flegrei, sit over cracks in the Earth where a piece of ocean crust slides under a continent. Their findings, published in the journal Scientific Reports, will allow scientists to better understand Campi Flegrei and similar volcanoes, potentially protecting people around the world from unexpected volcanic eruptions.

Basalt is a common volcanic rock. It’s usually about 50% silicon oxide, 10% iron oxide, and 15% aluminum oxide, with the remaining 25% a variable mix of calcium oxide, magnesium oxide, and other elements. But the properties of the basalt can change depending on that mix of elements. The magma beneath Campi Flegrei is alkaline basalt, with about 9% magnesium oxide and almost twice as much sodium and potassium as other basalts. This makes them less dense and less viscous, and likely to rise more quickly than other basalts. Alkaline basalts have not been well studied. A team of researchers from La Sapienza University of Rome, Ehime University (Japan), the University of Bayreuth (Germany), and HPCAT-XSD at the APS, an Office of Science user facility at Argonne National Laboratory, set out to fill that knowledge gap using the VX-3 Paris-Edinburgh press at the HPCAT-XSD 16-BM-B x-ray beamline at the APS.

Beamline 16-BM-B takes advantage of the APS’s very high photon flux and bending magnet white beam x-ray spectrum to do the difficult job of illuminating liquids at high pressures and temperatures representative of natural magmas. The V-X Paris-Edinburgh press on this beamline can maintain the molten rock sample at pressures as high as 7 gigapascals and temperatures of 2,300 K for more than 24 hours, allowing a full run of experimental viscosity data to be collected in one long session on a single sample. The combination of the Paris-Edinburgh press and the APS high photon flux makes HPCAT the only facility in the world capable of experiments to determine the viscosity of synthetic magmas.

In addition to the high temperature and pressures required for the experiment, the semi-liquid state of natural magmas also complicates the investigation of its atomic structure. Classic x-ray diffraction techniques use the patterns created by many, many x-rays passing through a regular repeating crystal to figure out the structure. Liquids, due to their amorphous nature, generally do not have a repeating crystalline structure. So, these researchers used multi-angle energy dispersive x-ray diffraction to collect diffraction patterns from 10 different angles, and then analyzed the data with a software package developed at HPCAT-XSD specifically for structural analysis of high-pressure liquids.

The pressure at the depths where magma forms is high enough to squeeze the atoms closer together than they would be at the surface. As the distance between atoms changes, the viscosity of the magma changes as well. The researchers were able to determine both the distance between the primary atoms in the magma (silicon, oxygen, aluminum, calcium, iron, magnesium, and sodium) and their arrangement in space. They captured two specific measurements that can explain the viscosity of the magma: the basic size of the silicon-oxygen tetrahedrons in the melt (the T-O bond length), and the distance between each tetrahedron (T-T bond length). The viscosity of the alkaline basalt magma increases along with the T-O and T-T lengths.

Knowing the viscosity of the magma that forms just beneath Campi Flegrei allowed the researchers to estimate that the magma ascends 2.5 meters to 3 meters per year, leading them to hypothesize that the source of the magma is about 60 kilometers deep. This will help further geochemical and geophysical models of the volcano to be implemented and integrated with the experimental viscosity data, thus improving the tools used to predict dangerous eruptions.   ― Kim Krieger

See: Barbara Bonechi¹*, Vincenzo Stagno^1,2*, Yoshio Kono³, Rostislav Hrubiak⁴, Luca Ziberna^5,6, Giovanni B. Andreozzi¹, Cristina Perinelli¹ , and Mario Gaeta¹, “Experimental measurements of the viscosity and melt structure of alkali basalts at high pressure and temperature,” Sci. Rep. 12,:2599 (2022). DOI: 10.1038/s41598-022-06551-7

Author affiliations: ¹Sapienza Università di Roma, ²Istituto Nazionale di Geofisica e Vulcanologia, ³Ehime University, ⁴Argonne National Laboratory, ⁵Università degli Studi di Trieste, ⁶University of Bayreuth

Correspondence: * barbara.bonechi@uniroma1.it, ** vincenzo.stagno@uniroma1.it

HPCAT-XSD operation is supported by the U.S. Department of Energy (DOE)-National Nuclear Security Administration under Award No. DE-NA0001974, with partial instrumentation funding by NSF. V.S. acknowledges financial support by the DeepCarbon Observatory, the Sloan Foundation, and “Fondi di Ateneo Sapienza 2019”. L.Z. acknowledges the Alexander von Humboldt foundation. This work was financed by Sapienza Ateneo 2020 medi/Sottili and supported by the Joint Usage/ Research Center PRIUS, Ehime University, Japan. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.

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