The impacts of lulls and peaks in effusion rate on lava flow propagation

Abstract

Variable effusion rates have been observed during the eruption and emplacement of lava flows which can complicate lava flow predictability. Conventional wisdom suggests that effusion rates decrease exponentially with time, however, this broad trend may also be subject to short-timescale fluctuations. Flow obstructions, changes in source diameter, channel or pond overflow, and changes within the magma reservoir to name a few factors can increase or decrease local flow rates repeatedly during an active eruption and impact the behavior of the flow. 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. In this work, we address the effects of decreasing and increasing extrusion rates (Q) on flow propagation and four emplacement modes common to lava flows: resurfacing, marginal breakouts, inflation, and lava tubes. We conducted 30 experiments by injecting dyed PEG wax into a chilled bath (∼ 0 °C) on a flat slope. We divided the experiments into two pulsatory extrusion rate patterns, or conditions: stepwise decrease followed by increase in extrusion rate (lull) and stepwise increase then decrease in extrusion rate (peak). We tested a range of flow conditions spanning from flows for which strong crust was favored (low wax temperature; low extrusion rates) and those for which weak crust was favored (high wax temperature; high extrusion rates). We found that a lull in extrusion rates when a strong crust was present promoted flow expansion and thickening via limited resurfacing, localized marginal breakouts, inflation, possible tube formation, with lower rates of flow expansion after the lull. In contrast, a lull and weak crust promoted flow expansion via widespread marginal breakouts, with flow advance rebounding after the lull, and inhibited flow thickening via inflation. A peak in extrusion rates with a strong crust favored flow expansion via widespread marginal breakouts, with flow-advance deceleration after the peak, and possible thickening via inflation. Conversely, a peak in extrusion rate with weak crust promoted flow expansion via widespread marginal breakouts, with flow-advance deceleration after the peak, and inhibited flow thickening via resurfacing and inflation. Our results have implications for pahoehoe flow emplacement and have been used to assess the most appropriate parameters to be used in a probabilistic flow propagation model, MrLavaLoba. Plain Language SummaryVariable effusion rates have been observed during the eruption of lava flows which can complicate lava flow forecasts. In general, lava flow effusion rates decrease with time exponentially although there may be fluctuations in flow rate on short timescales. Flow rates can wax or wane for a variety of reasons, such as flow obstructions, changes in the shape of the erupting source, or the release of more magma from the subsurface. Experiments using materials that behave as lava, such as wax, can be used to investigate how changes in effusion rate and subsequent flow rate impact flow emplacement. In this work, we address the effects of temporary lulls and peaks in extrusion rates on flow propagation. We focus on four emplacement modes common to lava flows: resurfacing, marginal breakouts, inflation, and lava tubes. We conducted 30 experiments by injecting dyed wax into a chilled bath (∼ 0 °C) on a flat slope. We divided our experiments into two extrusion rate patterns, or conditions: a lull (temporary decrease in extrusion rate) and a peak (temporary increase in extrusion rate). We tested a range of flow conditions – from flows with strong crust to those with weak crust. We found that a lull in extrusion rates when a strong crust was present promoted flow thickening via limited resurfacing and inflation and flow expansion via localized marginal breakouts, possible tube formation, and lower rates of flow expansion after the lull. In contrast, a lull and weak crust promoted flow expansion via widespread marginal breakouts, and flow advance rebounds after the lull. A peak in extrusion rates with a strong crust favored flow expansion via widespread marginal breakouts and flow thickening via resurfacing and inflation, and flow-advance deceleration after the peak. Conversely, a peak in extrusion rate with weak crust favored flow expansion via widespread marginal breakouts, and resulted in flow-advance deceleration after the peak. These experiments can help us understand how pāhoehoe flows grow and thicken and the results can be used with numerical models to improve flow forecasts.