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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 (June 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. Dynamics of one of the largest Quaternary explosive eruptions in the Andes: The ca. 29 ka Campo de la Piedra Pómez VEI ≥ 6 caldera-forming eruption, NW Argentina

   https://doi.org/10.1007/s00445-025-01858-5  

   Báez, Walter; de_Silva, Shanaka; Cirer, Mercedes; Bertea, Esteban; Constantinescu, Robert; Danišík, Martin; Schmitt, Axel K; Ramos, Frank C (September 2025, Bulletin of Volcanology)

   MEdmonds (Ed.)

   Large explosive volcanic eruptions (Volcanic Explosivity Index ≥ 6) have the potential to profoundly impact modern society. Databases that encompass information on age, size, dynamics, and geological context of large eruptions are essential for devising effective long-term strategies for hazard and risk assessment. A critical limitation of these databases is the underreporting of events, particularly of those that occurred in remote regions like the southern edge of the Central Volcanic Zone of the Andes (24–27°S). In this study, we provide the first lithofacies description of the proximal and distal fall deposits linked to a previously overlooked eruption in this region, the Campo de la Piedra Pómez caldera-forming eruption from the Cerro Blanco Volcanic Complex (NW Argentina). Importantly, a zircon double-dating age of 28.9 ± 3.2 ka (2σ) for the Campo de la Piedra Pómez eruption is significantly younger than a previously reported 40Ar/39Ar sanidine age. In addition, a new (U-Th)/He age for the Barranca Blanca Ignimbrite indicates that it is significantly older and unrelated to the Cerro Blanco Volcanic Complex, clarifying a long-standing stratigraphic question. Major and trace element compositions of bulk rock and glass allow for a reliable regional correlation of proximal and distal fall deposits, and a clear geochemical distinction of the Campo de la Piedra Pómez eruption from other potential sources with similar compositions. Using inverse modeling of tephra dispersion, we estimate a minimum fall deposit volume of ~ 25.6 ± 5.6 km3. Adding this to the previous estimate of 17.2 km3 for the ignimbrite, the minimum bulk tephra erupted during the Campo de la Piedra Pómez eruption is now estimated at 42.7 km3 (~ 17.5 km3 Dense Rock Equivalent). The dynamics of the Campo de la Piedra Pómez eruption have been reconstructed from field-based data including facies analysis, componentry, and maximum size distribution of lithic clasts. The eruption started with a sustained Plinian phase characterized by a high discharge rate ( 108 – 109 kg/s) and an eruptive plume of ~ 35–40 km above the vent. Partial plume collapse then generated high-mobility pyroclastic density currents. The onset of caldera collapse triggered the transition to a mixed eruptive style, including the development of pyroclastic fountaining (boiling-over) from fissure-like vents accompanied by substantial pyroclastic density currents, and coeval convective Plinian plume activity from a spatially separated narrow vent. As the eruption progressed, fissure-like vents expanded, entirely halting the convective Plinian plume. Despite the huge volume erupted, the Campo de la Piedra Pómez eruption produced only an incipient collapse structure with limited subsidence. 

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4. 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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5. From the ashes of VEI: Decoding explosivity, mass eruption rate and hazards in real time

   https://doi.org/10.22541/essoar.176967627.71453176/v1  

   Sahagian, Dork (January 2026, AGU)

   Ash-producing volcanic eruptions are a major natural hazard. The explosivity of these eruptions has been described for decades by a semi-quantitative “Volcanic Explosivity Index (VEI)” based on a number of interdependent factors (eruption duration, volume of ejected material, eruptive column height, tropospheric and stratospheric injection, and many qualitative descriptors), most of which could only be assessed after eruption had ceased. The utility of this semi-quantitative index has been questioned since then, yet its common usage persists. We suggest that if VEI is to be used in the future, it may as well be an index that can be determined in real time during an eruption, and one that strictly reflects eruption intensity. We find that the same value for the “traditional” Volcanic Explosivity Index of past eruptions can be estimated in real time during an eruption, based solely on eruption column height. This approach is not intended to replace the traditional VEI scale or supplant more detailed mechanistic approaches to mass eruption rate (MER); rather it can be used to provide a more immediate way of calculating the VEI and linking to MER, in real time while an eruption is taking place. As such, if the easily observed height can be measured, both MER and VEI can be estimated. This may facilitate emergency response by aviation, hazard management, and others to mitigate societal impact of hazardous eruptions. This scheme can be inverted to use volume-based VEI of ancient eruptions to roughly retrodict unobserved column height for key historic and pre-historic eruptions. Further, ash particle morphology reflects explosivity in terms of decompression rate at the vent of unobserved eruptions. With sufficiently rapid decompression, this could result in an observable bi-modal bubble size distribution. The morphology of ash particles reveals a threshold between VEI 3 and 4, above which there is sufficiently rapid decompression to trigger a second nucleation event near the vent. 

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