Understanding the explosive behaviour of volcanoes and the way magma rises through their conduit is one of the main challenges for volcanologists and could help with forecasting eruptions.
Several probabilistic models have investigated how magma is transported, showing that the viscosity of the fluid – its resistance to deformation and to flow - is a key parameter for determining its behaviour. Now a study1 led by Alex Scarani from Università Roma Tre in Rome and Alessio Zandonà from CNRS in Orléans has added to the analysis, by focusing on the accuracy of methods used to assess the viscosity of magma.
Looking at laboratory samples from eruptions at Mt. Etna in 2011 and at Stromboli in 2007, the team (including researchers from the National Research Council's Institute of Environmental Geology and Geoengineering) found that “these models need to be revisited, because the way measurements have been made for years can lead to macroscopic errors in the determination of the viscosity,” says Zandonà. The reason for those errors, he adds, lies in the presence of crystals that can only be observed at the nanoscale, something that was technically not possible until a few years ago.
The researchers performed experiments based on heating, cooling and decompressing the melts, and used spectroscopy to analyse their chemical composition. The analyses showed that nanocrystals can form both in the laboratory and in natural scenarios.
Temperature and chemical composition are two fundamental parameters that determine how magma is transported, with “important implications for what type of eruption a particular magma can create,” adds Scarani. The higher the temperature, the lower the viscosity. Regarding the chemistry, the sample analyses have shown the presence of titanium and iron oxides that form nanocrystals and saturate the melt when the magma rises, cools, decompresses, and releases gas. This can dramatically affect the tendency of the magma to reach oversaturation, and therefore the type of volcanic activity that is expected for that type of magma.
Mt. Etna and Stromboli volcanoes are similar, having both basaltic magmas, and are usually described as effusive (i.e. with low-intensity explosive activities). However, they can also occasionally behave in an explosive way (with highly energetic activity). In particular, the study revealed differences in the presence of iron and titanium oxides inside the two magmas. In the case of Etna and in a specific thermal range, nanocrystals form, while for Stromboli they are absent. This difference in chemistry and nanocrystal content can explain the differences in explosive behaviour between sustained phenomena lasting more than an hour on Etna, and isolated quick explosions of a few minutes for Stromboli.
Viscosity measurements, the authors suggest, should consider the formation of the nanocrystals, and their possible impact on the magma dynamic.
Additional work needs to be done on the study of decompression and degassing of magma, especially in the presence of water. “This work can serve as a future guide to other studies on viscosity, not only for basaltic samples,” concludes Scarani.