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Abstract Basaltic lava flows can be highly destructive. Forecasting the future path and/or behavior of an active lava flow is challenging because topography is often poorly constrained and lava has a complex rheology and emplacement history. Preserved lavas are an important source of information which, combined with observations of active flows, underpins conceptual models of lava flow emplacement. However, the value of preserved lavas is limited because pre-eruptive topography and, thus, syn-eruptive lava flow geometry are usually not known. Here, we use tree-mold data to constrain pre-eruptive topography and syn-eruptive lava flow geometry of the July 1974 flow of Kīlauea (USA). Tree molds, which are formed after advancing lava encloses standing trees, preserve the lava inundation height and the final preserved thickness of lava. We used data from 282 tree molds to reconstruct the temporal and spatial evolution of the ~ 2.1 km-long July 1974 flow. The tree mold dataset yields a detailed dynamic picture of staged emplacement, separated by intervals of ponding. In some ponded areas, flow depth during emplacement (~ 5 m) was twice the preserved thickness of the final lava (2–3 m). Drainage of the ponds led to episodic surges in flow advancement, decoupled from fluctuations in vent discharge rate. We infer that the final breakout occurred after the cessation of fountaining. Such complex emplacement histories may be common for pāhoehoe lavas at Kīlauea and elsewhere in situations where the terrain is of variable slope, and/or where lava is temporarily perched and stored. 

Abstract The 2018 eruption on the lower East Rift Zone, Hawaii, involved the opening of 24 fissures before the eruption focussed on a single point source, fissure 8 (F8). This study characterises the preserved medial F8 tephra deposit using an isopach map, maximum clast size data, and total grain size distribution analysis, shedding light on the tephra transport and dispersal mechanisms beyond the F8 cone occurring during the fountaining. The medial sheet-like deposit covers approximately 0.22 km², best fit by a Power-Law thinning rate. The TephraFits model estimated the corresponding volume of the continuous medial tephra blanket to be ~ 2$$\times$$ $\times$10⁴m³, just 0.02% of the total volume erupted from fissure 8. Samples from the preserved medial deposit have grain size modes of − 3.5 to − 4 Φ, compatible with Voronoi tessellation calculations. Maximum clast size did not show a ‘typical’ fining relationship with distance from the vent; instead, it shows no clear pattern. One factor was that the extremely low clast density, a function of a secondary vesiculation event, enabled the pyroclasts to be re-entrained, often repeatedly, by large eddies downwind of the vent. This should be considered in future studies of prolonged fountaining episodes as the clasts involved in the medial fall are rarely well preserved in the geologic record due to their fragile nature but their presence adds complexity to the inferred eruption dynamics. 

We use new geochemical, petrological, and rheological data to constrain the formation and emplacement of the highly compositionally unusual(andesitic basalt) Kīlauea 2018 Fissure 17 (F17) eruptive products. Despite the restricted spatial and temporal distribution, F17 samples are texturally and geochemically diverse. The western samples are enriched in SiO2 by up to 10 wt%, relative to their eastern equivalents; additionally, the western samples contain microcrystalline enclaves, absent from the homogenous eastern samples. The compositions erupted along F17 suggest interaction between the basaltic 2018 juvenile magma and a crystal mush at depth, likely a left-over from the nearby 1955 eruption. Magma mingling caused heating and local melting of remnant mush, leading to melt hybridization and volatile exsolution. Rapid water exsolution likely caused overpressurization of the reservoir underneath the western side of F17, leading to Strombolian explosions of viscous magma, in contrast to sustained Hawaiian fountaining on the eastern side. Remelting of remnant crystal mush and melt hybridization in open-conduit systems may hence be an effective mechanism in inducing volatile saturation. 

Magma mingling and mixing are common processes at basaltic volcanoes and play a fundamental role in magma petrogenesis and eruption dynamics. Mingling occurs most commonly when hot primitive magma is introduced into cooler magma. Here, we investigate a scenario whereby cool, partially degassed lava is drained back into a conduit, where it mingles with hotter, less degassed magma. The 1959 eruption of Kīlauea Iki, Hawaiʻi involved 16 high fountaining episodes. During each episode, fountains fed a lava lake in a pit crater, which then partially drained back into the conduit during and after each episode. We infer highly crystalline tachylite inclusions and streaks in the erupted crystal-poor scoria to be the result of the recycling of this drain-back lava. The crystal phases present are dendrites of plagioclase, augite and magnetite/ilmenite, at sizes of up to 10 μm. Host sideromelane glass contains 7–8 wt% MgO and the tachylite glass (up to 0.5% by area) contains 2.5–6 wt% MgO. The vesicle population in the tachylite is depleted in the smallest size classes (< 0.5 mm) and has overall lower vesicle number densities and a higher degree of vesicle coalescence than the sideromelane component. The tachylite exhibits increasingly complex ‘stretching and folding’ mingling textures through the episodes, with discrete blocky tachylite inclusions in episodes 1 and 3 giving way to complex, folded, thin filaments of tachylite in pyroclasts erupted in episodes 15 and 16. We calculate that a lava lake crust 8–35 cm thick may have formed in the repose times between episodes, and then foundered and been entrained into the conduit during drain-back. The recycled fragments of crust would have been reheated in the conduit, inducing glass devitrification and crystallisation of pyroxene, magnetite and plagioclase dendrites and eventually undergoing ductile flow as the temperature of the fragments approached the host magma temperature. We use simple models of magma mingling to establish that stretching and folding of recycled, ductile lava could involve thinning of the clasts by up to a factor of 10 during the timescale of the eruption, consistent with observations of streaks and filaments of tachylite erupted during episodes 15 and 16, which may have undergone multiple cycles of eruption, drain-back and reheating. 

The 2018 LERZ eruption of Kilauea featured a wide range of eruptive styles. In particular, Fissure 17 (F17) displayed activity ranging from Hawaiian fountaining in the eastern part of the fissure to Strombolian explosions in the western part. Lava erupted from F17-West was highly viscous and contained magmatic enclaves. Magmatic enclaves have previously been observed in many other volcanic systems (e.g. Vulcano Island, IT and Sete Cidades Volcano, PT), where they have been attributed to injection of mafic magma into an evolved magma chamber, resulting in viscous fingering, quenching, and break-off into fragments. The F17 enclaves differ from previous studies in that the chemical compositions of the enclave and host magmas are very similar, and that the enclaves have a limited spatial distribution and lack signs of viscous behavior and quenching, pointing to a different formation mechanism than inferred for other volcanic systems. In order to test a different formation hypothesis, we conducted fractal analysis of the size distribution of 84 individual enclaves from F17-West lavas. Our results, including a fractal dimension of fragmentation Df of 2.59, indicate that the F17 enclaves likely formed by brittle fragmentation. Since the enclave and host magmas were at temperatures far above the glass transition during the magma hybridization, high strain rates have to be invoked to explain the brittle fragmentation. This may have caused the enclave magma to transition into solid-state behavior, allowing it to break off into fragments that were subsequently picked up by the host magma and carried to the free surface. The enclaves from F17-West therefore offer a unique insight into the diversity of processes that characterizes the shallow parts of volcanic systems, as well as the importance of strain rates in modulating the rheological behavior of magmas.