Morphological transitions between lobate resurfacing and distal breakout lava flows in flood basalts: insights from analog experiments

Abstract

Continental flood basalts (CFBs) are dominated by two characteristic lava morphologies. The first type, referred to as ‘compound’ or ‘hummocky pāhoehoe,’ exhibits pillow-like lava flow lobes with cross-sections of ~ 0.5–2 m and thin chilled margins. The second type, referred to as ‘simple’ or ‘sheet lobes’ preserves more massive, inflated flow interiors that are laterally continuous on scales of 100s of meters to kilometers. Previous hypotheses suggest that two factors may contribute to stratigraphic changes in morphology from ‘compound’ to ‘simple’: 1) increased eruption duration or 2) increased extrusion rate. We test the hypothesis that a large increase in extrusion rate would result in flow morphology transitioning from multiple small lobes to inflated sheet lobes due to a shift in flow propagation from intraflow resurfacing-dominated to marginal breakout-dominated. Using polyethylene glycol (PEG) wax extruded into a circular water-filled tank 130 cm in diameter, we produced larger, more complex experiments than previous studies. Our efforts simulated more complex lava fields which change flow morphology with distance from the eruptive vent, characteristic of CFBs. Whereas previous PEG studies linked extrusion rate to near-source surface morphologies, our experiments evaluated how flow propagation mechanisms change with variable extrusion rate and distance from the source. Two flow propagation styles were identified: 1) resurfacing, in which molten material breaks through the surface of a flow and covers the older crust and 2) marginal breakouts, in which molten material extends beyond the crust at the active distal margin of the flow. Flows that propagated via marginal breakouts were found to have lower proportions of resurfaced area and vice versa. We show that significant resurfacing is needed to preserve internal chilled boundaries within a flow and a low-extrusion-rate surface morphology, whereas marginal breakout-dominated flows tend to inflate the pillow-like surface morphology preserving a massive interior at great distances from the vent. Higher and more steady extrusion rates tend to decrease the extent of resurfacing and increase the distance between the source and preserved low-extrusion-rate surface morphologies. We find that an extrusion rate increase equivalent to a jump in the extrusion rate scaling factor, Ψ value, from < 1 to > 5 would be necessary to ensure a switch from resurfacing-dominated lobate morphologies to marginal breakout-dominated propagation style. This amounts to a factor of 125 increase in effusion rate for fissure eruptions and a factor of 625 for point source eruptions, assuming no change in vent geometry. This would be equivalent to an effusion rate of 0.2 m³/s, as documented in 1987–1990 Kīlauea eruptions, increasing to 125 m³/s, which was commonly measured during the 2014 Holuhraun eruption in Iceland and the 2018 eruption at Leilani Estates in Hawai‘i. Thus, we propose that continental flood basalts do not require unusually large effusion rates, but instead were active for a longer and more consistent time period than smaller-volume eruptions.