RHEOLOGICAL TIME SERIES: A CASE STUDY FROM THE 2021 FAGRADALSFJALL ERUPTION (Invited Presentation)

As magma temperature and composition drift and change throughout an eruption, so does its rheology. These changes may span orders of magnitude in magma viscosity and result in orders of magnitude flow velocity changes, as well as transitions in eruptive style.

In this study, we undertook the systematic quantification of the rheological variations that occurred during the 2021 Fagradalsfjall eruption. In order to do so, we measured the viscosity of a suite of 20+ representative samples collected between day 2 and 183 of the eruption in a concentric cylinder rheometer. Temperatures were initially raised to superliquidus conditions, and then lowered to magmatic temperatures as determined through geothermometry. We analyzed our dataset as a time series. An overall viscosity decrease trend emerged. As the eruption progressed, melt viscosity decreased by 25%, from 40 Pa s to 30 Pa s at a constant temperature of 1200 °C. However, this trend was not monotonous. At least 3 positive spikes in viscosity can be identified, at day 80, 120, and 138 of the eruption. Finally, we contextualized the viscosity values and trends measured in the laboratory by correlating them with field observations.