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1. Determining the umbrella cloud geometry of unwitnessed silicic explosive eruptions: A case study from Mount Mazama (Oregon, United States)

   Wiejaczka, J; Giachetti, T (February 2024, Journal of volcanology and geothermal research)

   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. 

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2. Airfall volume of the 15 January 2022 eruption of Hunga volcano estimated from ocean color changes

   https://doi.org/10.1007/s00445-024-01744-6  

   Kelly, Liam_J; Fauria, Kristen_E; Manga, Michael; Cronin, Shane_J; Latu’ila, Folauhola_Helina; Paredes-Mariño, Joali; Mittal, Tushar; Bennartz, Ralf (May 2024, Bulletin of Volcanology)

   Abstract On 15 January 2022, Hunga volcano erupted, creating an extensive and high-reaching umbrella cloud over the open ocean, hindering traditional isopach mapping and fallout volume estimation. In MODIS satellite imagery, ocean surface water was discolored around Hunga following the eruption, which we attribute to ash fallout from the umbrella cloud. By relating intensity of ocean discoloration to fall deposit thicknesses in the Kingdom of Tonga, we develop a methodology for estimating airfall volume over the open ocean. Ash thickness measurements from 41 locations are used to fit a linear relationship between ash thickness and ocean reflectance. This produces a minimum airfall volume estimate of$${1.8}_{-0.4}^{+0.3}$$ ${1.8}_{-0.4}^{+0.3}$km³. The whole eruption produced > 6.3 km³of uncompacted pyroclastic material on the seafloor and a caldera volume change of 6 km³DRE. Our fall estimates are consistent with the interpretation that most of the seafloor deposits were emplaced by gravity currents rather than fall deposits. Our proposed method does not account for the largest grain sizes, so is thus a minimum estimate. However, this new ocean-discoloration method provides an airfall volume estimate consistent with other independent measures of the plume and is thus effective for rapidly estimating fallout volumes in future volcanic eruptions over oceans. 

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3. A critical magma chamber size for volcanic eruptions

   https://doi.org/10.1130/G47045.1  

   Townsend, Meredith; Huber, Christian (February 2020, Geology)

   Abstract We present a model for a coupled magma chamber–dike system to investigate the conditions required to initiate volcanic eruptions and to determine what controls the size of eruptions. The model combines the mechanics of dike propagation with internal chamber dynamics including crystallization, volatile exsolution, and the elastic response of the magma and surrounding crust to pressure changes within the chamber. We find three regimes for dike growth and eruptions: (1) below a critical magma chamber size, eruptions are suppressed because chamber pressure drops to lithostatic before a dike reaches the surface; (2) at an intermediate chamber size, the erupted volume is less than the dike volume (“dike-limited” eruption regime); and (3) above a certain chamber size, dikes can easily reach the surface and the erupted volume follows a classic scaling law, which depends on the attributes of the magma chamber (“chamber-limited” eruption regime). The critical chamber volume for an eruption ranges from ∼0.01 km3 to 10 km3 depending on the water content in the magma, depth of the chamber, and initial overpressure. This implies that the first eruptions at a volcano likely are preceded by a protracted history of magma chamber growth at depth, and that the crust above the magma chamber may have trapped several intrusions or “failed eruptions.” Model results can be combined with field observations of erupted volume, pressure, and crystal and volatile content to provide tighter constraints on parameters such as the eruptible chamber size. 

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4. Pleistocene to recent evolution of Mocho-Choshuenco volcano during growth and retreat of the Patagonian Ice Sheet

   https://doi.org/10.1130/B37514.1  

   Moreno-Yaeger, Pablo; Singer, Brad S; Edwards, Benjamin R; Jicha, Brian R; Nachlas, William O; Kurz, Mark D; Breunig, Rachel E; Fustos-Toribio, Ivo; Vásquez_Antipán, Daniel; Piergrossi, Ella (June 2024, Geological Society of America Bulletin)

   Mocho-Choshuenco volcano (39.9°S, 72.0°W) produced ∼75 explosive eruptions following retreat of the >1.5-km-thick Patagonian Ice Sheet associated with the local Last Glacial Maximum (LGM, from 35 to 18 ka). Here, we extend this record of volcanic evolution to include pre- and syn-LGM lavas that erupted during the Pleistocene. We establish a long-term chronology of magmatic and volcanic evolution and evaluate the relationship between volcanism and loading/unloading of the Patagonian Ice Sheet via twenty-four 40Ar/39Ar and two 3He age determinations integrated with stratigraphy and whole-rock compositions of lava flows and glass compositions of tephra. Our findings reveal that the edifice is much younger than previously thought and preserves 106 km3 of eruptive products, of which 50% were emplaced immediately following the end of the penultimate glaciation and 20% after the end of the LGM. A period of volcanic inactivity between 37 and 26 ka, when glaciers expanded, was followed by the eruption of incompatible element-rich basaltic andesites. Several of these syn-LGM lavas dated between 26 and 16 ka, which crop out at 1500−1700 m above sea level, show ice contact features that are consistent with emplacement against a 1400- to 1600-m-thick Patagonian Ice Sheet. Small volume dacitic eruptions and two explosive rhyolitic eruptions dominate the volcanic output from 18 to 8 ka, when the Patagonian Ice Sheet began to retreat rapidly. We hypothesize that increased lithostatic loading as the Patagonian Ice Sheet grew prohibited dike propagation, thus stalling the ascent of magma, promoting growth of at least three discrete magma reservoirs, and enhancing minor crustal assimilation to generate incompatible element-rich basaltic andesitic to dacitic magmas that erupted between 26 and 17 ka. From an adjacent reservoir, incompatible element-poor dacites erupted from 17 to 12 ka. These lava flows were followed by the caldera-forming eruption at 11.5 ka of 5.3 km3 of rhyolite from a deeper reservoir atop which a silicic melt lens had formed and expanded. Subsequent eruptions of oxidized dacitic magmas from the Choshuenco cone from 11.5 to 8 ka were followed by andesitic to dacitic eruptions at the more southerly Mocho cone, as well as small flank vent eruptions of basaltic andesite at 2.5 and 0.5 ka. This complex history reflects a multi-reservoir plumbing system beneath Mocho-Choshuenco, which is characterized by depths of magma storage, oxidation states, and trace element compositions that vary over short periods of time (<2 k.y.). 

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5. Stepping back to the source: The expression of eruption column collapse through submarine terraces

   Gilchrist, Johan; Jellinek, AM; Hooft, E; Wanket, S (December 2024, 2024 Fall Meeting, American Geophysical Union)

   Explosive eruption jets rising through relatively shallow water layers form eruption columns that can deliver volcanic ash, gases, and entrained water to the atmosphere and ocean in sequence or simultaneously, depending on eruption source parameters (Gilchrist et al. 2023). Despite the mesospheric eruption column height of the January 15, 2022 eruption of Hunga Tonga-Hunga Ha’apai (HTHH), the majority of erupted material was delivered to the surrounding seafloor via submarine pyroclastic density currents (PDCs). Deposits of HTHH show evidence of axisymmetric terraced deposits, which we show are linked to the mass eruption rate and dynamics of column collapse. We use scaled analog experiments on multiphase sand-water fountains injected into water layers of varying depth to model the collapse dynamics of shallow water eruption columns and to link fountain source conditions to deposit topography. The source strength of multiphase fountains predicts whether they collapse periodically or continuously via sedimentation waves with varying frequency and momentum. In turn, the frequency and momentum of sedimentation waves impacting the tank base determines whether ground-hugging gravity currents flowing out of the sedimentation wave impact zone are initially erosive or depositional. On the basis of experiments, we propose that syn-eruptive shallow submarine caldera deposits that show evidence of terracing and proximal scouring are linked to relatively strong eruption jets in the regime where the jet is in partial collapse or total collapse. In these regimes, the eruption jet collapses periodically as sedimentation waves that erode the deposit in the impact zone and transition into submarine PDCs that deposit the sedimentation wave mixture into regularly spaced terraces thereafter (Fig. 1, black boxes). In contrast, we expect weak eruption jets to occur in the total collapse regime where sedimentation waves descend in rapid succession and effectively supply submarine PDCs continuously which, in turn, build deposits lacking terraces (Fig. 1, blue box). For common values of caldera eruption source parameters, we link submarine PDC deposit morphology to eruption jet strength and plausible mass eruption rates. 

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