The effects of unsteady effusion rates on lava flow emplacement: Insights from laboratory analogue experiments

The phenomenology of lava flow emplacement involves complex physical processes related to crystallization, eruption rate, temperature, crust solidification, and a variety of other factors. Changes in effusion rate are a natural part of lava flow emplacement and can complicate lava flow morphology and propagation. Analog experiments are a useful tool for investigating the role of changing effusion rates on flow propagation because they allow reasonably precise control of conditions and detailed documentation of resulting flows. Experimental datasets that investigate the impact of variable effusion rates on flow propagation can be used to enhance fundamental understanding of flow processes and to inform numerical models for hazards forecasts. In this study, we address the effects of decreasing and increasing eruption rates (Q) on four emplacement modes common to lava flows: resurfacing, marginal breakouts, inflation, and lava tubes. Laboratory analogue experiments using polyethylene glycol (PEG) 600 wax were used to derive Ψ, a dimensionless parameter that relates crust formation (t_s) and lateral advection (t_a) timescales of a viscous gravity current. We conducted 120 experiments using a peristaltic pump to inject dyed PEG wax into a chilled bath (∼ 0 °C) in a tank with a roughened base at a slope of 0°. The experiments were divided into two conditions: decreasing Q with time (condition 1) and increasing Q with time (condition 2). We controlled for volume of extruded wax, temperature, instantaneous eruption rate, Ψ, and duration of the decrease or increase in eruption rate. Results indicate that the duration of the pulsatory eruption rate, the experimental condition, initial Ψ, and the extruded volume influence the presence and strength of a crust (or lack thereof) which in turn influences the onset and extent of the four emplacement modes investigated. Prolonged increase in eruption rates favored resurfacing, widespread marginal breakouts and flow advancement, inflation, and some tube formation, while the specific morphology and area covered was controlled by an extensive, coherent crust, which in turn depended on initial Ψ and duration of the initial eruptive stage. Prolonged decreasing eruption rates promoted localized marginal breakouts, inflation, and tube formation. The duration of the pulse during the eruption rate change affected the likelihood and/or significance of the mode of emplacement. Similar observations were made on the early stages of the 2021 Fagradalsfjall eruption in Iceland to demonstrate the utility of the wax experiments in interpreting natural systems. Plain language summary: Predicting where lava will flow remains challenging due to the complex variables affecting its propagation. Lava is a multiphase fluid made of solids, liquids, and gas. In addition, as it cools, it forms a crust that can affect the flow. The eruption of lava is rarely steady over short and long timescales. This variability in eruption rate can also affect lava flow propagation and hence affect its predictability. Such variability in eruption rate may cause the flow to grow in thickness, in length, in width, or all of these simultaneously, and the style of growth affects the area impacted and controls where the flow is most hazardous. While numerical models have been useful in simulating flow advance on slopes, these models often simplify lava flow geometry and propagation mechanics. Laboratory analogue experiments allow for the reproduction of complex physics and morphology that closer approximate processes observed in nature. In this study, 120 experiments using PEG 600 – a water-soluble wax – were used to simulate lava flow emplacement under unsteady vent conditions. Flows were emplaced while increasing or decreasing the eruption rate during an eruption and the duration of the increase or decrease in eruption rate was varied along with other flow conditions. In the experiments, increasing or decreasing eruption rate at the vent, along with other parameters, impacted the formation of a cohesive or brittle crust, which in turn exerted strong control on whether the flows thickened or lengthened, affecting the area impacted and flow morphology. We map out lava flow characteristics in terms of these vent conditions, and compare our findings to a real eruption in Iceland.