The viscosity of multiphase lava: New insights from integrating laboratory and field measurements

Laboratory measurements on remelted rocks are the standard technique for characterizing the rheology of lava at eruptive temperatures. This approach enables precise measurements but struggles to recreate natural emplacement conditions. Particularly, lavas erupt as multiphase suspensions (melt + crystals + bubbles), but current laboratory methods cannot retain bubbles, and experiments are limited to two-phase (melt+crystals) suspensions. The oxygen fugacity of lavas is known to influence the kinetics of crystallization, and despite its importance, few studies have considered this factor to date. The only available technique to measure the three-phase viscosity of natural lava is through in-situ measurements while it is flowing. Here, we present the first study that integrates viscosity data from laboratory-derived single (melt) and two-phase (melt+crystals) measurements with data from three-phase (melt + crystals + bubbles) field measurements, using the 2023 Litli-Hrútur eruption in Iceland as a case study. We present a rheological characterization of the remelted lava during crystallization at thermomechanical equilibrium and disequilibrium at log fO₂ = −8.7, and deformation rates of 3.3 s^-1. These bubble-free laboratory experiments were designed to overlap with field temperatures (1150–1165 °C), enabling direct comparison with the in-situ three-phase field viscosities. We find laboratory and field data overlap in viscosity space (10^2.5 to 10^4.5 Pa s) and at cooling rates of 0.5–1 °C/min. Isothermal experiments successfully recreate crystal phases and volumes comparable to natural samples (∼20–50 vol.% crystals). This comparison allows the first field validation for laboratory measurements of the effective viscosity of natural lava.