RHEOLOGIC INVESTIGATION OF ONE- AND TWO-PHASE SYSTEMS AS INITIAL CONSTRAINTS ON VISCOSITY OF THE AHUʻAILĀʻAU FLOWS OF THE KĪLAUEA 2018 ERUPTION

The rheology of lava flows determines flow propagation and emplacement, and the accurate estimation thereof is important for successful lava flow forecasting in hazard scenarios. The Ahuʻailāʻau flow of the Kīlauea 2018 lower East Rift Zone eruption was one of the best documented flows in the world. Drone overflights and regular ground-based monitoring produced extensive data with which to examine the rheological characteristics of this rapidly emplaced flow. Using drone footage and thermal imaging from the eruption, sampling was carried out along the Ahuʻailāʻau flow field in areas with known emplacement conditions. With these samples, we performed laboratory viscometry experiments to quantify the lava flow rheology and constrain equilibrium crystallization behavior in these lavas.

Super- and sub-liquidus isothermal viscosity measurements of a bulk sample from the channel of the Ahuʻailāʻau flow are performed to determine apparent viscosity in one- and two-phase equilibrium systems. The superliquidus measurements record the apparent viscosity of the bulk melt chemistry as a single-phase system with no gas or crystals. This provides a lower-bound estimation of possible lava viscosities for this eruption. The superliquidus experiments also provide an indication of the liquidus temperature, as crystallization results in a deviation from the one-phase Newtonian viscosity. Using both superliquidus measurements and heat capacity curves to determine the liquidus, two-phase equilibrium systems for this lava were replicated by isothermal subliquidus measurements. At five temperatures below the liquidus, a crystal-free re-melt of the bulk sample was held until viscometric equilibrium was achieved. The samples were quenched from the subliquidus temperatures, maintaining the equilibrium crystalline assemblages which formed during experiment. The resultant crystallinity and crystal populations were then compared to the unmodified natural samples. As the natural samples were evenly distributed both across the flow field and through the time of eruption, the comparison provides estimates of disequilibrium in the flow and helps generate first order viscosity maps across the duration of the eruption. These maps will inform future models for emplacement timescales and lava flow hazards.