Artículo de publicación ISILakes formed in the Aysén region of southern Chile after the retreat of mountain glaciers, established by ~17,900 calendar years before present (cal years BP) or earlier, contain numerous late-glacial and Holocene tephra layers derived from >70 eruptions of the volcanoes in the region, including Hudson, the southernmost in the Andean Southern Volcanic Zone (SVZ). Sediment cores fromseven of these lakes contain an unusually thick late-glacial age tephra layer, which based on its distribution and bulk trace-element composition was derived from a large explosive eruption of Hudson volcano between 17,300 and 17,440 cal years BP and is termed Ho. In 13 cores from six of these lakes, each located ~100 km generally northeast of Hudson, the Ho tephra layer ranges between 50 and 88 cm in thickness, and contains pumice grains up to 2 cm in maximum diameter. Comparison with three previously documented large explosive Holocene Hudson eruptions (H1 at 7,750 cal years BP, H2 at 3,920 cal years BP, and H3 in 1991 AD) suggests that Ho was larger, with an estimated tephra volume of >20 km3, the largest post-glacial eruption documented for any volcano in the southern Andes and most likely responsible for the formation of the Hudson caldera. In total, Hudson has erupted ≥45 km3 of pyroclastic material in the last ~17,500 years, making it the most productive volcano in the southern Andes in terms of the total volume erupted since the beginning of deglaciation in the region. Chemical stratification is not seen in the waterlain Ho tephra, but these deposits are bi-modal, consisting of a much greater proportion of dark glassy basaltic-trachyandesite dense fragments and pumice, with glasses which range between 55 and 59 wt.% SiO2, along with volumetrically lesssignificant lighter-colored trachydacite pumice, with glass of 66 wt.% SiO2. In contrast, H1 products are trachyandesitic in composition, H2 ones are more felsic than H1, being composed essentially of trachydacite, and although H3 1991 AD again produced tephra of bi-modal compositions, it erupted a much smaller proportion of mafic compared to felsic material than did Ho. Thus, the repetitive large explosive eruptions of Hudson volcano have evolved to progressively less-mafic overall compositions from late-glacial to historic times, and their volumes have decreased. Sr-isotopic composition of bulk samples of the most mafic dense glass and most felsic pumice components of the Ho tephra, as well as samples from other Hudson eruptions, which overall range from 51 to 66 wt.% SiO2, with 525 to 227 ppm Sr, are all similar (0.70444± 0.00007), indicating that crystal-liquid fractionation rather than crustal assimilation was the main process responsible for these chemical variations.This research was supported by Fondecyt (Chile) grant #1121141, the Institute of Ecology and Biodiversity grants ICM P05–002 and PFB-23, and the Department of Geological Sciences, CU-Boulder. We also would like to thank A. Vichick, T. Ni...

We conducted a lithological mapping of the Viedma volcano, one of five volcanoes in the Andean Austral Volcanic Zone (AVZ), using remote sensing techniques. We used data of the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) sensor which is highly effective in geological research, to understand build-up processes and to deduce compositional variation of the Viedma volcano emerging from the South Patagonian ice field. The volcanic edifice was divided into bright parts that mainly compose the eastern flank of the volcano and dark parts at the central crater area based on the observation in visible and near infrared ranges. The SiO 2 concentration was calculated using the bands in the visible and thermal infrared regions. The dark part and the bright part have approximately 51 wt% and 63 wt% average SiO 2 content respectively, indicating that the exposures of the Viedma volcano have a wide variation in SiO 2 concentration. Although, according to other authors, ejecta from the Viedma volcano have 64-66 wt% SiO 2 and other AVZ volcanoes are essentially monolithologic dacite/andesite volcanoes, the edifice of the Viedma volcano appears to be composed mostly of basalts or older rocks/basement with low silica contents. RESuMEN. Variaciones composicionales reveladas mediante análisis de imágenes ASTER del volcán Viedma, Zona Volcánica Andina Austral. Mediante el uso de técnica de sensoría remota se ha desarrollado un mapeo litoló-gico del volcán Viedma, uno de los cinco volcanes de la Zona Volcánica Andina Austral (ZVA). Para este efecto, se ha utilizado el radiómetro 'Advanced Spaceborne Thermal Emission and Reflection' (ASTER) que es muy efectivo en investigación geológica, para entender los procesos que han controlado la estructura y deducir las variaciones composi-cionales del volcán Viedma, que sobresale levemente de la superficie del campo de hielo Patagónico Sur. Sobre la base de la observación en el intervalo del espectro visible e infrarrojo cercano, en el edificio se distinguen partes brillantes que corresponden al flanco oriental del volcán y partes oscuras en el área del cráter central. La concentración de SiO 2 se calculó a través del uso de las bandas en las regiones visibles e infrarrojo termal. Las partes oscura y brillante tienen un promedio aproximado de 51% y 63% de SiO 2 en peso, respectivamente, lo que indica que el volcán Viedma expone una amplia diferenciación de las concentraciones de SiO 2. Aunque, según otros autores, las muestras analizadas de material piroclástico del volcán Viedma tienen contenidos de 64-66% SiO 2 y que otros aparatos de la ZVA son volcanes esencialmente monolitológicos dacíticos/andesíticos, el edificio del volcán Viedma estaría compuesto principalmente de basaltos o las rocas más antiguas de su basamento tienen un bajo contenido de sílice.

ABSTRACT. Sediment cores from lakes and bogs in the Río Cisnes valley contain tephra from explosive eruptions of volcanoes in the southern part of the Andean Southern Volcanic Zone (SSVZ). These tephra, which thicken and coarsen to the west, are attributed to eruptions from Melimoyu, Mentolat, Hudson, and potentially either Macá, Cay or one of the many minor eruptive centers (MEC) located both along the Liquiñe-Ofqui Fault Zone (LOFZ) and surrounding the major volcanoes. Correlation of the tephra between two new cores in the lower Río Cisnes valley, and amongst other cores previously described from the region, and source volcano identification for the tephra, has been done using lithostratigraphic data (tephra layer thickness and grain size), petrography (tephra glass color, vesicle morphology, and type and abundance of phenocryst phases), and by comparison of bulk tephra trace-element characteristics with previously published whole-rock and bulk tephra chemical analysis. Four tephras in these cores are attributed to eruptions of Mentolat, four to eruptions from Melimoyu, one possibly to Hudson, and six cannot be assigned to a specific source volcano. Some of these tephra correspond to pyroclastic tephra fall deposits previously observed in outcrop, including the MEL2 eruption of Melimoyu and the MEN1 eruption of Mentolat. However, others have not been previously observed and represent the products of newly identified small to medium sized eruptions from volcanoes of the SSVZ. These results provide new information concerning the frequency and magnitude of explosive eruption of SSVZ volcanoes and contribute to the evaluation of volcanic hazards in the region.Keywords: Andean volcanism, Tephra, Tephrochronology, Chile. RESUMEN. Tefrocronología holocena del curso inferior del valle de río Cisnes, Chile austral. Los testigos de sedimentos de lagos y pantanos obtenidos en el valle del río Cisnes contienen niveles de tefra originados por erupciones explosivas de volcanes localizados en la parte sur de la Zona Volcánica de los Andes del Sur (SZVS). Estos depósitos de tefra, que aumentan de espesor y tamaño del grano hacia el oeste, son atribuidos a erupciones de los volcanes Melimoyu, Mentolat, Hudson y potencialmente a los volcanes Macá, Cay o alguno de los numerosos centros eruptivos menores (CEM) localizados a lo largo de la Zona de Falla Liquiñe-Ofqui (ZFLO) y que rodean a los volcanes principales. La correlación de los niveles de tefra identificados en dos nuevos testigos, obtenidos en el curso inferior del valle del río Cisnes, con aquellos presentes en testigos descritos con anterioridad en la región y la identificación de los volcanes fuente de las tefras, se realizó con la ayuda de datos litoestratigráficos, (espesor de la capa de tefra y tamaño del grano), petrografía (color de los fragmentos de vidrio, morfología de las vesículas y tipo y abundancia de fenocristales) y por comparación de sus patrones de elementos trazas con análisis químicos de roca total y tefra total publicados con anterioridad. Cuatro ...

Active hydrothermal systems develop during repose periods of volcanoes within the first kilometers of their edifices when ascending hot magmatic fluids (liquid or gas) encounter meteoric water recharge and/or seawater. Free convection and forced circulation occur, leading to surface manifestations such as fumaroles and hot springs.Intensive volcanic hazards are associated with pervasive hydrothermal systems. Phreatic and phreatomagmatic eruptions represent the most common hazards occurring when hot magmatic fluids or magma are injected into a pre-existing hydrothermal system (Heiken & Wohletz, 1987;Stix, 2018). Pore-water is flashed creating an overpressure until the potential rupture of host-rock, leading to a sudden explosive eruption (e.g., Mannen et al., 2019;Yamaoka et al., 2016). Other hydrothermal-related hazards exist without necessarily involving magmatic origin. Indeed, long-term host-rock interactions between heat, water, and magmatic fluids lead to numerous modifications of the physical-chemical properties of host rocks and fluids. Percolation of hot and acidic fluids (<400°C) dissolves host-rock primary minerals and precipitates low-permeability clay minerals (Pirajno, 2008). Such alteration products create a barrier to the flow of fluids (called Abstract Phreatic and phreatomagmatic eruptions represent some of the greatest hazards occurring on volcanoes. They result from complex interactions at a depth between rock, water, and magmatic fluids. Understanding and assessing such processes remain a challenging task, notably because a large-scale characterization of volcanic edifices is often lacking. Here we focused on Miyakejima Island, an inhabited 8-km-wide stratovolcano with regular phreatomagmatic activity. We imaged its plumbing system through a combination of four geophysical techniques: magnetotellurics, seismicity, self-potential, and thermal image. We thus propose the first comprehensive interpretation of the volcanic island in terms of rock properties, temperature, fluid content, and fluid flow. We identify a shallow aquifer lying above a clay cap (<1 km depth) and reveal its relation with magmatic-tectonic features and past eruptive activity. At greater depths (2-4.5 km), we infer a seismogenic resistive region interpreted as a magmatic gas-rich reservoir (≥370°C). From this reservoir, gases rise through a fractured conduit before being released in the fumarolic area at ∼180°C. During their ascent, these hot fluids cross a ∼1.2-km-long liquid-dominated zone causing local steam explosions. Such magmatic-hydrothermal interaction elucidates (i) the origin of the long-period seismic events and (ii) the mixing mechanism between magmatic and hydrothermal fluids, which was previously observed in the geochemical signature of fumaroles. Our results demonstrate that combining multidisciplinary large-scale methods is a relevant approach to better understand volcanic systems, with implications for monitoring strategies. GRESSE ET AL.

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The chronology of over 50 tephra layers preserved in a lake sediment core from Laguna La Trapananda (LLT) in the southern portion of the Andean Southern Volcanic Zone (SSVZ), Chile, is constrained by new radiocarbon age determinations, which span the period from late Pleistocene glacial retreat to the late Holocene. The tephra are correlative with tephra previously described from other lake cores in the region and are attributed to explosive eruptions of the SSVZ volcanoes Mentolat, Hudson, Macá, and potentially Cay. The new age determinations are used to estimate the ages of the >50 tephra in the LLT core, as well as those from the other previously described lake cores in the area, by a Bayesian statistical method. The results constrain the frequency of explosive eruptions of the volcanic centers in the southernmost SSVZ. They indicate that there was essentially no increase in the rate of eruptions from late-glacial to recent times due to deglaciation. They also provide isochrones used to constrain the depositional histories of the small lacustrine systems within which they were deposited and they provide a tephrochronologic tool for other paleoclimatic, paleoecologic, archaeologic and tephrochronologic studies in central Patagonia.

Defining the variations in petrological characteristics of erupted magmas within a high-resolution chronostratigraphy provides a necessary framework for monitoring the long-term activity and eruption potential of an active volcano. Here, we investigate the evolution of the magmatic system of Miyakejima volcano, Japan, between the last two caldera-forming eruptions, at ~ 2.3 ka and AD 2000, based on new stratigraphic constraints, radiocarbon ages, and whole-rock geochemical data. The activity of Miyakejima during this interval can be divided into three magmatic periods based on cyclic whole-rock compositional trends. Period 1 spans the interval between ~ 2.3 ka and the 7th century, from the Hatchodaira eruption with caldera collapse to immediately before the Suoana–Kazahaya eruption. Period 2 spans the time period between the seventh century and the fourteenth century, from the Suoana–Kazahaya to the Sonei–bokujyo eruptions. Period 3 covers the period from the two major flank eruptions that occurred in the sixteenth century to the end of the twentieth century until the last caldera-collapse event in AD 2000. The eruption rate decreased from 0.5 km3 per 1000 years in Period 1 to ~ 0.2 km3 per 1000 years in Period 2 and 3. Recharge of primitive basaltic magmas into shallower crustal systems triggered extensive basaltic fissure eruptions at the beginning of each period. Progressively increasing whole-rock SiO2 contents of the hybrid magmas in subsequent eruptions indicates continuous fractional crystallization in small shallow magma chambers which formed at the start of each magmatic period. Intermittent injections of basaltic magma into shallow magma chambers induced magma mixing that caused eruption of hybrid basaltic andesite in each period. We suggest that some basaltic magmas formed isolated magma reservoirs at shallow depth, in which rapid fractionation was able to occur. Rupturing of these isolated magma storage regions filled with gas-rich evolved magma can lead to violent ejection of andesitic magmas, such as for the Suoana–Kazahaya eruption in the seventh century. Our results suggest two main scenarios of eruption for the basaltic magma system at Miyakejima and similar mafic volcanoes in the northern Izu–Bonin arc; (1) eruption of voluminous basaltic lavas after the recharge of primitive basaltic magmas into the shallow magmatic system, and (2) explosive fissure eruption by rupturing of isolated magma bodies filled with gas-rich evolved magmas. Graphical Abstract

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