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Abstract Determining whether fresh magma has reached the surface during a volcanic eruption can provide important information for forecasts of future activity, especially in the early stages of an eruption. However, identifying fresh, juvenile pyroclasts in tephra fall deposits can be challenging and inconclusive. We studied the products of explosions at Poás Volcano, Costa Rica, in 2016–2019, a period during which the volcano transitioned from a pressurized, hydrothermally sealed state to an open conduit with increased degassing to the atmosphere. The activity consisted of semi-continuous explosions producing < 500-m-high plumes, with the exception of explosions on April 14 and 22, 2017, that produced 4-km-high plumes. We analyzed the grain size distribution, componentry, and particle density of the products of twenty explosions, and collected groundmass glass composition on juvenile particles for three of them. Our work demonstrates varying degrees of magma involvement with the hydrothermal system through time, with juvenile material representing a wide range of abundance (~ 10–70 vol.%) in deposits of individual explosions. Before early April 2017, we infer that small phreatomagmatic explosions were triggered by contact between magmatic fluids and/or magma and the hydrothermal system, based on the presence of abundant hydrothermal fragments and minor juvenile magma. Concurrent with decreasing hydrothermal component in the deposits, explosions in April–May 2017 eroded the walls of the shallow plumbing system, evidenced by an increase in wall-rock lithics in the deposits. These changes coincided with drying of the crater lake, leading to several magmatic explosions in April-September 2017, whose juvenile-rich deposits are consistent with primary fragmentation of fresh magma in the conduit. The eruptive activity changed after this magmatic phase, and in 2019, small explosions mostly recycled the heterogeneous deposits of previous events, producing fine material with high proportions of recycled particles. All explosions from 2016 to 2019 remobilized already-emplaced magma from the 1953–1955 eruptive period, although distinct glass compositions between explosions suggest difference in crystallization or they tapped different portions of this magma body. Our work sheds light on the eruption dynamics and shallow plumbing system of this persistently active volcano and provides a case study for understanding the variable efficiency of phreatomagmatic fragmentation. 

Abstract Most volcanic eruptions on Earth take place below the ocean surface and remain largely unobserved. Reconstruction of past submerged eruptions has thus primarily been based on the study of seafloor deposits. Rarely before the 15 January 2022 eruption of Hunga volcano (Kingdom of Tonga) have we been able to categorically link deep‐sea deposits to a specific volcanic source. This eruption was the largest in the modern satellite era, producing a 58‐km‐tall plume, a 20‐m high tsunami, and a pressure wave that propagated around the world. The eruption induced the fastest submarine density currents ever measured, which destroyed submarine telecommunication cables and traveled at least 85 km to the west to the neighboring Lau Basin. Here we report findings from a series of remotely operated vehicle dives conducted 4 months after the eruption along the Eastern Lau Spreading Center‐Valu Fa Ridge. Hunga‐sourced volcaniclastic deposits 7–150 cm in thickness were found at nine sites, and collected. Study of the internal structure, grain size, componentry, glass chemistry, and microfossil assemblages of the cores show that these deposits are the distal portions of at least two ∼100‐km‐runout submarine density currents. We identify distinct physical characteristics of entrained microfossils that demonstrate the dynamics and pathways of the density currents. Microfossil evidence suggests that even the distal parts of the currents were erosive, remobilizing microfossil‐concentrated sediments across the Lau Basin. Remobilization by volcaniclastic submarine density currents may thus play a greater role in carbon transport into deep sea basins than previously thought. 

The molar abundance ratio of H2 Om /OH of rhyolitic porous pyroclasts, obsidian pyroclasts, and flow obsidians from the 1060CE Glass Mountain eruption at Medicine Lake Volcano (USA) were obtained by Diffuse Reflectance Infrared Fourier Transform spectroscopy. Samples were also analyzed for their total water content, [H2 Ot ], and isotopic composition, 𝜔D, via Temperature Conversion Elemental Analysis (TC/EA). Porous clasts exhibit a H2 Om /OH ratio of 0–4.63 (and a [H2 Ot ] measured by Giachetti et al. (2020) of 0.34–1.2 wt%) that is positively correlated with their porosity and [H2 Ot ], contrary to the low [H2 Ot ] and low H2 Om /OH ratios of both obsidian pyroclasts (0.32–0.72 wt.%, 0.14–0.63) and flow obsidians (0.04–0.07 wt.%, 0.24–0.28). These results confirm the interpretations of Giachetti et al. (2020) that porous clasts were rehydrated for about 1000 years after the eruption via diffusion of overwhelmingly molecular, meteoric water in the matrix-glass, whereas obsidian pyroclasts and flow obsidians are essentially dense and thus rehydrate poorly. Analyses were also conducted on two size fractions of flow obsidian (∱ 63 εm and 63–250 εm) that were ground either (1) immediately before analysis or, (2) about nine years prior to analysis and kept in closed containers ever since. Results show that the ∱ 63 εm ground samples gained 0.06–0.24 wt% of meteoric water in just nine years, with a H2 Om /OH ratio increasing from 0.32–0.54 when measured immediately after crushing, to 3.35–4.64 when measured nine years later. For these smaller particles, even 24 h of heating at 130 ⋛ C under vacuum before analysis is insufficient to remove all the water gained by rehydration. We thus recommend the use of coarser powders (∲ 63 εm) and longer pre-analysis heating time under vacuum (∲2 days) for more reliable [H2 Ot ], H2 Om /OH, and 𝜔D measurements on obsidian samples. Given the thinness of the glass in between vesicles in porous pyroclasts and thus their ability to quickly rehydrate, total water content obtained via analysis of the bulk material (e.g., by TC/EA, Loss on Ignition, Karl Fischer titration) must be interpreted together with 𝜔D and/or H2 Om /OH data to evaluate the extent of rehydration, even for relatively young samples. 

Volcanic Ash Transport and Dispersal Models (VATDMs) make real-time forecasts of tephra fall resulting from explosive eruptions possible. However, these predictions still mainly rely on eruption source parameters, such as erupted mass, total grain-size distribution, and plume height, gathered via thorough studies of past eruptions similar in nature. This dependency of eruption source parameters to analogous eruptions becomes particularly challenging when there are limited instances of similar events. An example is rhyodacitic to rhyolitic eruptions. This type of volcanic eruption has only been witnessed twice, at Chait´ en (2008–2009) and Cord´ on Caulle (2011− 2012), both in Chile. Here, we examine the 7.7 ka Cleetwood eruption of Mount Mazama (Oregon, USA), as a case study. This rhyodacitic eruption started explosively with two initial VEI 4, subplinian phases, and ended effusively with the emplacement of a rhyodacitic flow. We use the results of a detailed study of the proximal and medial tephra deposits as input in a VATDM to investigate the geometry and dimensions of the main plume formed during the Cleetwood eruption. We 1) constrain the erupted mass and calculate a detailed total grain-size distribution, 2) explore the Reanalysis 2 wind database to determine the direction and velocity of the local wind at the time of the eruption, and 3) use the VATDM Tephra2 with a grid-search method to estimate plume height, mass distribution within the plume, and the characteristics of tephra diffusion. We find that a vertical release of the erupted mass along a single line above the vent adequately replicates the measured mass loads but fails to simultaneously fit measured grain-size distributions at the same locations. We thus devise a method that not only accounts for a customized total grain-size distribution, real 1D wind patterns, and variable mass distribution within the plume, but also allows for adjustments to the size and location of an elliptical umbrella cloud. Using this method, we successfully replicate both local mass loads and high-resolution grain-size distributions and show that particles ≥0.125 mm from the lower Cleetwood unit were likely deposited from a 5 ×45 km2 umbrella reaching 16 km a.s.l., elongated in the direction of main wind intensity. This research contributes to enhancing the accuracy of predicting tephra transport from silicic volcanic eruptions. Moreover, it underscores the importance of utilizing grain-size data in combination with mass loads at specific locations to gain insights into the characteristics of the eruption plume, especially for eruptions that have not been directly observed.