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Abstract Irazú is one of the largest and most active volcanoes in Costa Rica. We present the tephro-chronostratigraphy of the last 2.6 ka of the Irazú volcano based on detailed field work and C14 radiometric dating, as well as a revision of the geological and historical records. In the stratigraphic record we identified at least 30 tephra units. Eight of them corresponding to the historical period (i.e., after 1700 A.D.), separated by repose periods of different durations. The distribution of the deposits, the volcanic morphologies (craters and pyroclastic cones) and the radiometric ages indicate that most of this recent eruptive activity has occurred from the summit of Irazú along an E–W fissure (~ 4 km long). Toward the west of the summit, near the Sapper hill may be the source of the oldest eruptions at 200 A.D., while the La Laguna cone, located to the east of the summit, could have formed around 1540 A.D., and Main Crater to the west could have formed around sixteenth–seventeenth century. Since then, the historical eruptions (i.e., 1723–1724, 1917–1921, 1924, 1928, 1930, 1933, 1939–1940 and 1963–1965) have been sourced from this crater, but not all of them are registered in the stratigraphy. The eruption frequency of Irazú during this period ranges from 23 to 100 years, with a major event about every 80 years. Irazu’s eruptions have been mainly phreatomagmatic and Strombolian, including some phreatic explosions. We present a detailed tephro-chronostratigraphy that will help to building temporal analysis for hazard assessment and risk management plans to face future eruptions at Irazú.

Irazú is one of the largest and most active volcanoes in Costa Rica. We present the tephro-chronostratigraphy of the last 2.6 ka of the Irazú volcano based on detailed field work and C14 radiometric dating, as well as a revision of the geological and historical records. In the stratigraphic record we identified at least 30 tephra units. Eight of them corresponding to the historical period (i.e., after 1700 A.D.), separated by repose periods of different durations. The distribution of the deposits, the volcanic morphologies (craters and pyroclastic cones) and the radiometric ages indicate that most of this recent eruptive activity has occurred from the summit of Irazú along an E–W fissure (~ 4 km long). Toward the west of the summit, near the Sapper hill may be the source of the oldest eruptions at 200 A.D., while the La Laguna cone, located to the east of the summit, could have formed around 1540 A.D., and Main Crater to the west could have formed around sixteenth–seventeenth century. Since then, the historical eruptions (i.e., 1723–1724, 1917–1921, 1924, 1928, 1930, 1933, 1939–1940 and 1963–1965) have been sourced from this crater, but not all of them are registered in the stratigraphy. The eruption frequency of Irazú during this period ranges from 23 to 100 years, with a major event about every 80 years. Irazu's eruptions have been mainly phreatomagmatic and Strombolian, including some phreatic explosions. We present a detailed tephro-chronostratigraphy that will help to building temporal analysis for hazard assessment and risk management plans to face future eruptions at Irazú. [ABSTRACT FROM AUTHOR]

Long‐lasting, hybrid eruptions can be of complex description and classification, especially when associated with multiple eruptive styles and multiple products. The 2021 Tajogaite eruption of La Palma, Canary Islands, was associated with a magma‐gas decoupled system that resulted in the simultaneous emission of lava flows and tephra plumes from various vents. Even though the tephra blanket (∼2 × 107 m3) represents only 7%–16% of the total erupted volume, it provides fundamental insights into the overall eruptive dynamics. Tephra was mostly dispersed NE‐SW due to a complex regional and local wind patterns and was subdivided into 3 units and 11 layers that well correlate at different distances from the vent and with both tremor data and lava emission rate. While plume height varied at the temporal scale of a few hours, the average mass eruption rate associated with the tephra blanket of the different units remained relatively constant (∼3–4 × 103 kg s−1). In contrast, the emission rate of lava largely increased after the first week and remained higher than the overall emission of tephra throughout the whole eruption (average value of ∼6 × 104 kg s−1). Based on a detailed characterization of the tephra blanket in combination with atmospheric wind, tremor, and lava emission trend, we demonstrate the need of (a) multidisciplinary strategies for the description of hybrid eruptions that account for both the duration of individual phases and the quantification of the mass of multiple products, and of (b) dedicated ash dispersal forecasting strategies that account for the frequent variations of eruptive and atmospheric conditions. Plain Language Summary: Volcanic eruptions are mostly subdivided into effusive and explosive when they are associated with the emission of lava flows or with the fragmentation of magma that results in the generation of widespread deposits (i.e., tephra). The September–December 2021 Tajogaite eruption of La Palma Island (Spain) represents a typical example of long‐lasting, hybrid eruption with alternation, or, more often, contemporaneous emission of lava flows and tephra. The lava field extended toward the western sector of the island, while the tephra blanket was mostly dispersed NE‐SW due to a complex regional and local wind pattern. Even though the tephra blanket represents only 7%–16% of the total erupted volume, it provides fundamental insights into the overall eruptive dynamics. Based on a detailed characterization of the tephra blanket, in combination with wind, seismic tremor, and lava emission trend, we demonstrate the need of dedicated strategies for the description of hybrid, long‐lasting eruptions that account for both eruption duration and for the quantification of multiple products (tephra blanket, tephra cone, lava flows). We also show the need of dedicated strategies of short‐range ash dispersal forecasting that account for the frequent variations of eruptive and atmospheric conditions. Key Points: Tephra blanket of the 2021 Tajogaite eruption is only 7%–16% of the total volume but is key to decipher the variable eruptive dynamicsTephra blanket shows distinct layers and units at different distances from vents that well correlate with tremor and lava emission rateHybrid eruptions are best described based on both event duration and mass associated with multiple products (tephra blanket, cone, lava) [ABSTRACT FROM AUTHOR]

Due to frequent volcanic activity and erosion of dryland soils, magnified by land use after human settlement (ca. 870 CE), peatlands in Iceland receive regular additions of mineral eolian deposits. Their soils may develop not only histic but also andic characteristics. Yet, mineral eolian deposition as an environmental determinant of peatlands in Iceland is still poorly understood, not least with regard to the peatlands carbon (C) stores. This study advances our understanding of the impact of tephra deposition on Histosols by elucidating interactions between C characteristics and andic soil properties. We compare Histosols from three Icelandic peatlands of different degrees of exposure to eolian deposition by evaluating data sets of their C structure derived by 13C NMR spectroscopy, andic soil properties based on selective extractions of Al, Fe, and Si, and decomposition proxies C/N, δ13C, and δ15N. By applying multivariate statistical methods, we are able to present several important patterns. Soil organic matter of Histosols with andic properties is less decomposed than that of Histosols without notable andic properties. Andic soil properties seem to impact their C structure by facilitating the formation of organo‐mineral complexes, which particularly hamper the decomposition of chemically more labile C groups. Tephra layers appear to hamper microbial activity in deeper soil layers by preventing input of fresh organic matter. The interaction of andic and histic soil properties and the protective role of major tephra deposits may enable an unusual potential for long‐term C stabilization in a natural peatland environment. Plain Language Summary: Peatlands belong to the greatest terrestrial carbon stores worldwide. Their organic soils possess exceptional capacities for long‐term carbon storage. Peatlands in Iceland are unusual as they receive comparatively great amounts of windborne mineral material from volcanic eruptions or sparsely vegetated and eroded drylands. These mineral additions of volcanic origin to the otherwise organic substrate of the peatlands shape their soil characteristics and set them apart from similar soils in neighboring countries. On the one hand, they bear characteristics typical for organic soils in Nordic peatlands, on the other hand they possess characteristics of mineral soils of volcanic regions. Little is known about the impact of interactions between these characteristics on the peatlands carbon stores. This study increases our knowledge on carbon storage of peatlands in volcanic regions. We compare soils from three peatlands in northwest Iceland and shed light on interactions between their mineral and organic constituents, with a particular focus on soil organic carbon. We find strong indications that decomposition processes are slower in soils with stronger characteristics of mineral volcanic soils. In undisturbed (anaerobic) peatlands, this seems to impact long‐term carbon storage positively, but in disturbed (aerobic) peatlands, it might pose a threat of accelerated carbon emissions. Key Points: Andic soil properties impact the carbon structure of Histosols of volcanic regions and their carbon storage capacityAndic soil properties and thick tephra deposits appear to enhance long‐term carbon stabilization in undisturbed peatlandsRelatively undecomposed Histosols with andic properties may be a greater source of atmospheric carbon upon disturbance than anticipated [ABSTRACT FROM AUTHOR]

Interactions between tephra deposits from volcanic eruptions and peatland carbon (C) dynamics are poorly understood despite the significant extent of peatlands influenced by tephra worldwide. Tephra falls can affect peat accumulation within a radius of at least 1,000 km. In Iceland, volcanic activity is amongst the highest in the world and it might even increase due to pressure release on subglacial volcanoes. A potent combination of sparse vegetation, low cohesion of Andosols, and frequent strong winds, facilitates the regular input of mineral aeolian material from eroded areas into peatlands in Iceland, in addition to tephra deposits. We present results of a study on the impact of a major tephra deposit, the Hekla 4 tephra, on C dynamics in Icelandic peatlands. We investigated three sites at differing distances from the active volcanic zones and major erosion areas in the interior of the country. A combination of variables was applied, including laboratory C mineralization at 5, 15 and 25°C and C structure obtained by solid‐state 13C NMR spectroscopy. Tephra deposits seem to affect C dynamics. Differences in C mineralization can only be partially explained by C structure. The C mineralization from soils with conspicuously similar C structure differs by a factor of up to 2.7. Temperature sensitivity of decomposition after the tephra deposition increases by a factor of up to 2.2. Changes in inorganic matter and the ratio of soil organic matter to soil organic carbon by a factor of up to 1.3 and 1.4, respectively, were also observed. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the main focus is the application and improvement of four empirical models, which account for the pyroclastic cover deposit thickness (PCDT) spatial distribution with respect to the bedrock surrounding the Somma‐Vesuvius volcano (Campania, southern Italy). Three models, which are already known in the literature, link the depth to bedrock to the morphological features of a slope. An original model called SEPT (slope exponential pyroclastic thickness) is presented in this manuscript and combines the initial total thickness of ash‐fall pyroclastic cover with the slope gradient. All models were applied and validated using field measurements derived from this and preceding studies in the study area. The main finding is that the spatial distribution of the depth to bedrock in mountainous peri‐volcanic areas mainly depends on the initial thickness of air‐fallen material at a given position and slope angle. These findings allowed for the recognition of an ash‐fall pyroclastic depositional environment that is characterized by different processes from those existing in other geomorphological frameworks and in which the soil thickness along the slopes is controlled by the weathering of bedrock and the formation of soil in situ. Finally, in this research, a reliable approach is proposed that is also applicable to other peri‐volcanic areas of the world to assess the spatial distribution of the depth to bedrock, which is a fundamentally important parameter in distributed geomorphologic and hydrologic modeling. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

The characterisation of tephra deposits resulting from almost simultaneous sedimentation and wind remobilisation is complex, and multidisciplinary strategies are required in order to accurately constrain associated processes and eruptive parameters. We present a multifaceted study that aims to characterise the recent eruptive activity and the subsequent aeolian remobilisation of tephra deposits at Sabancaya volcano (Peru), which started erupting in November 2016 with frequent and relatively small explosions (plume heights <6 km above the vent). First, we estimated the bulk volume of tephra deposit produced between November 2016 and August 2018 at 0.04 ± 0.02 km3, and the dense rock equivalent (DRE) volume at 0.02 ± 0.01 km3. This corresponds to a tephra production rate of 1.1 ± 0.5 × 10−3 km3 DRE per month. Second, continuous sampling in a dedicated tephra collector network between April 2018 and November 2019 allowed estimation of the tephra volume at 2.3 ± 1.1 × 10−5 km3 DRE per month, indicating a significant decrease in the mass eruption rate since 2018. Third, by characterising the pulsatory activity through the repose interval between explosions and magma characteristics, Sabancaya's activity was classified as Vulcanian. Finally, aeolian remobilisation phenomena were studied using high-resolution videos, measurements of the airborne concentration of particulate matter with a diameter of ≤10 μm (PM 10) and particle physical characterisation. Subtle morphological differences are identified between remobilised particles and those in primary deposits, and we found that particles moving at ground level and above 1.5 m have grainsizes transportable by saltation and suspension, respectively. • Sabancaya's activity from 2016 to 2021 was characterised by 8 periods of Vulcanian pulsatory behaviour • The DRE volume of tephra deposit produced from Nov. 2016 to Aug. 2018 is 0.02 ± 0.01 km3 • Since 2018, sampling in tephra collectors indicates a decrease of the mass eruption rate • Innovative strategy for sampling syn-eruptive aeolian remobilisation of tephra deposits • Statistically significant differences are observed between the shape of remobilised and primary ash [ABSTRACT FROM AUTHOR]

The Baia-Fondi di Baia eruption is one of the sporadic events that have occurred in the western sector of the Campi Flegrei caldera. It dates back to 9525-9696 bp and opened Epoch 2 of the caldera activity after a 1000-year-long period of quiescence. Although relatively small in terms of erupted volume with respect to most of the events of the past 15 ka, the Baia-Fondi di Baia eruption was characterized by a complex series of events, which have led to different interpretations in the literature. We present a detailed stratigraphic study of 40 outcrops in a sector of about 90 km, coupled with sedimentological (grain size, componentry), physical (density, vesicularity), textural, and compositional analyses of the erupted deposits. Based on these data, we interpret the stratigraphic succession as being related to two distinct eruptive episodes (Baia and Fondi di Baia). These were separated by a short time interval, and each was characterized by different eruptive phases. The Baia eruptive episode started in a shallow-water environment with an explosive vent-opening phase that formed a breccia deposit (Unit I), rapidly followed by alternating fallout activity and dense, pyroclastic density current deposits generation (Unit II). Sedimentological features and pumice textural analyses suggest that deposition of Unit II coincided with the intensity peak of the eruption, with the fallout deposit being characterized by a volume of 0.06 ± 0.008 km (corresponding to a total erupted mass of 4.06 ± 0.5 × 10 kg), a column height of 17 km, and a corresponding mass flow rate of 1.8 × 10 kg s. The associated tephra also shows the highest vesicularity (up to 81 vol.%) the highest vesicle number density (1.01 × 10 cm) and decompression rate (0.69 MPa s). This peak phase waned to turbulent, surge-like activity possibly associated with Vulcanian explosions and characterized by progressively lower intensity, as shown by density/vesicularity and textural properties of the erupted juvenile material (Unit III). This first eruptive episode was followed by a short quiescence, interrupted by the onset of a second eruptive episode (Fondi di Baia) whose vent opening deposited a breccia bed (Unit IV) which at some key outcrops directly overlies the fallout deposit of Unit II. The final phase of the Fondi di Baia episode strongly resembles Unit II, although sedimentological (presence of obsidian clasts which are absent in the Baia deposits) and textural (lower vesicularity, vesicle number density, and decompression rate values) features, together with a more limited dispersal, suggest that this phase of the eruption had a lower intensity. The large range of groundmass glass compositions, associated with variable proportions of highly (phonolitic-trachytic) and mildly (tephriphonolitic-latitic) evolved end-members in the erupted products, also suggests that these eruptive episodes were fed by at least two different magma batches that interacted during the different phases, with an increase of tephriphonolitic-latitic magma occurring during the Fondi di Baia stage. [ABSTRACT FROM AUTHOR]

On 11 February 2010, Soufrière Hills Volcano, Montserrat, underwent a partial dome collapse (~ 50 × 10 6 m 3 ) and a short-lived Vulcanian explosion towards the end. Three main pyroclastic units were identified N and NE of the volcano: dome-collapse pyroclastic density current (PDC) deposits, fountain-collapse PDC deposits formed by the Vulcanian explosion, and tephra-fallout deposits associated with elutriation from the dome-collapse and fountain-collapse PDCs (i.e. co-PDC fallout deposit). The fallout associated with the Vulcanian explosion was mostly dispersed E and SE by high altitude winds. All units N and NE of the volcano contain variable amounts and types of particle aggregates, although the co-PDC fallout deposit is associated with the largest abundance (i.e. up to 24 wt%). The size of aggregates found in the co-PDC fallout deposit increases with distance from the volcano and proximity to the sea, reaching a maximum diameter of 12 mm about 500 m from the coast. The internal grain size of all aggregates have nearly identical distributions (with Mdϕ ≈ 4–5), with particles in the size categories > 3 ϕ (i.e. < 250 μm) being distributed in similar proportions within the aggregates but in different proportions within distinct internal layers. In fact, most aggregates are characterized by a coarse grained central core occupying the main part of the aggregate, coated by a thin layer of finer ash (single-layer aggregates), while others have one or two additional layers accreted over the core (multiple-layer aggregates). Calculated aggregate porosity and settling velocity vary between 0.3 and 0.5 and 11–21 m s − 1 , respectively. The aggregate size shows a clear correlation with both the core size and the size of the largest particles found in the core. The large abundance of aggregates in the co-PDC fallout deposits suggests that the buoyant plumes elutriated above PDCs represent an optimal environment for the formation (particle collision) and development (aggregate layering) of particle aggregates. However, specific conditions are required, including i) a large availability of water (in this case provided by the steam plumes associated with the entrance of PDCs into the ocean), ii) presence of plume regions with different grain-size features (i.e. both median size and sorting) that allows for the development of multiple layers, iii) strong turbulence that permits both particle collision and the transition of the aggregates through different plume regions, iv) presence of hot regions (e.g. PDCs) that promote aggregate preservation (in this case also facilitated by the presence of sea salt). [ABSTRACT FROM AUTHOR]

This research examines the spatial distribution of Haleakalā silverswords ( Argyroxiphium sandwicense DC subsp. macrocephalum ) over basaltic 'a'ā lava flows at Silversword Loop, Haleakalā Crater (Maui, Hawai'i). The overall study goal is to situate an extensive field data set within an historical geomorphic framework to propose a conceptual, biogeomorphic model of landscape development at Silversword Loop. Landscape features were investigated along two belt transects (111–120 m long) crossing lava-flow deposits at ~ 2175 m; the lava flows, > 4.5 km long, originated ~ 2700 YBP at Pu’u O Māui, a large cinder cone. Transects consist of contiguous 3 × 3 m (9 m 2 ) quadrats; plot data include topographic profiles, soil depth to bedrock, size of surface tephra – determined by photosieving – and plant density; 23 soil samples were analyzed within 10 profiles. Weathered lava outcrops, tephra blocks (≥ 10 cm), and cobbles (5–10 cm) cover lava-flow ridges; the gently sloping flanks of lava-flow deposits merge into intervenient concave troughs, filled with volcanic ash and small tephra. Soils along ridges are thin (≤ 17 cm), whereas profiles on troughs are deeper (≤ 110 cm). Soils are sandy volcanic Inceptisols ( Lithic , Typic , and Aridic Lithic Haplustepts ) averaging 21.6% gravel, 96.8% sand, and 3.2% fines (silt + clay); mean organic matter is just 0.92%. These soils provide low water retention (23.2% at field capacity) and rapid infiltration. Deeper trough soils are largely devoid of plants. In contrast, silverswords are virtually restricted to shallow soils on lava-flow crests, where they densely cluster about outcrops and blocks: 90% of plants grow on soils ≤ 40 cm deep, and 71.3% occupy plots with ≤ 40% ash cover; 84.4% of rosettes are rooted on outcrops or near stones; only 4.4% grow on soil; and 11.2% germinated on plant litter. Patterns of lava-flow weathering and soil depth to bedrock control silversword distribution at Silversword Loop. Shallow basalt bedrock and outcrops restrict soil drainage, providing a perched water table that silversword roots can exploit. Surface blocks also provide safe, stable microhabitats, thus rosettes grow preferentially near stones. Geomorphological, pedological, and ecological data are integrated into a biogeomorphic sequence of soil development and landscape evolution at Silversword Loop for the past three millennia, first through the eruptions and ash deposits of Pu'u O Māui, followed (~ 970–940 BP) by pyroclastic activity in Halāli'i and Ka lu'u o Ka 'O'o cinder cones. [ABSTRACT FROM AUTHOR]

Volumes of tephra-fall deposits are difficult to determine due to the commonly poor preservation of proximal and distal areas of the deposit. Typically, these volumes are found by extrapolating thinning trends found from isopach maps drawn for the areas of the deposits that are preserved. However, the construction of isopach contours is dependent on subjective interpretation of field measurements and can be highly variable. Here, we have investigated the spatial correlation relationships of thickness measurements from fall deposits to the vent location to produce a purely statistical method to objectively determine the volume of a fall deposit without the production of isopach maps. Integration of a log linear regression model for thickness measurements with distance from the vent is applied to the field measurements without any prior interpretation, and data and model uncertainty has been accounted for using Bayesian methods. Eruption volumes calculated from our method correspond well to those previously determined by alternative approaches for the deposits of Fogo A, Azores; Askja 1875, Iceland; Santa Maria 1902, Guatemala; and Pinatubo 1991, Philippines. The quantification of uncertainty in field measurements and model error, and the removal of subjectivity incurred in the production of isopach maps suggest that the method presented here can offer benefits in determining the volumes of deposits. [ABSTRACT FROM AUTHOR]

Quantitative hazard assessments of active volcanoes require an accurate knowledge of the past eruptive activity in terms of eruption dynamics and the stratified products of eruption. Teide-Pico Viejo (TPV) is one of the largest volcanic complexes in Europe, but the associated eruptive history has only been constrained based on very general stratigraphic and geochronological data. In particular, recent studies have shown that explosive activity has been significantly more frequently common than previously thought. Our study contributes to characterization of explosive activity of TPV by describing for the first time the subplinian eruption of El Boquerón (5,660 yBP), a satellite dome located on the northern slope of the Pico Viejo stratovolcano. Stratigraphic data suggest complex shifting from effusive phases with lava flows to highly explosive phase that generated a relatively thick and widespread pumice fallout deposit. This explosive phase is classified as a subplinian eruption of VEI 3 that lasted for about 9-15 h and produced a plume with a height of up to 9 km above sea level (i.e. 7 km above the vent; MER of 6.9-8.2 × 10 kg/s). The tephra deposit (minimum bulk volume of 4-6 × 10 m) was dispersed to the NE by up to 10 m/s winds. A similar eruption today would significantly impact the economy of Tenerife (e.g. tourism and aviation), with major consequences mainly for the communities around the Icod Valley, and to a minor extent, the Orotava Valley. This vulnerability shows that a better knowledge of the past explosive history of TPV and an accurate estimate of future potentials to generate violent eruptions is required in order to quantify and mitigate the associated volcanic risk. [ABSTRACT FROM AUTHOR]

Explosive eruptions associated with tephra deposits that are only exposed in proximal areas are difficult to characterize. In fact, the determination of physical parameters such as column height, mass eruption rate, erupted volume, and eruption duration is mainly based on empirical models and is therefore very sensitive to the quality of the field data collected. We have applied and compared different modeling approaches for the characterization of the two main tephra deposits, the Lower Pumice (LP) and Upper Pumice (UP) of Nisyros volcano, Greece, which are exposed only within 5 km of the probable vent. Isopach and isopleth maps were compiled for two possible vent locations (on the north and on the south rim of the caldera), and different models were applied to calculate the column height, the erupted volume, and the mass eruption rate. We found a column height of about 15 km above sea level and a mass eruption rate of about 2 × 10 kg/s for both eruptions regardless of the vent location considered. In contrast, the associated wind velocity for both UP and LP varied between 0 and 20 m/s for the north and south vent, respectively. The derived erupted volume for the south vent (considered as the best vent location) ranges between 2 and 27 × 10 m for the LP and between 1 and 5 × 10 m for the UP based on the application of four different methods (integration of exponential fit based on one isopach line, integration of exponential and power-law fit based on two isopach lines, and an inversion technique combined with an advection-diffusion model). The eruption that produced the UP could be classified as subplinian. Discrepancies associated with different vent locations are smaller than the discrepancies associated with the use of different models for the determination of erupted mass, plume height, and mass eruption rate. Proximal outcrops are predominantly coarse grained with ≥90 wt% of the clasts ranging between −6ϕ and 0ϕ. The associated total grainsize distribution is considered to result from a combination of turbulent fallout from both the plume margins and the umbrella region, and as a result, it is fines-depleted. Given that primary deposit thickness observed on Nisyros for both LP and UP is between 1 and 8 m, if an event of similar scale were to happen again, it would have a significant impact on the entire island with major damage to infrastructure, agriculture, and tourism. Neighboring islands and the continent could also be significantly affected. [ABSTRACT FROM AUTHOR]

Abstract: Villarrica (Chile) is one of the most active volcanoes in South America having erupted about 60 times in the last 460years. Although its historical eruptive activity has been mainly effusive and weakly explosive, it had strong explosive behaviour in postglacial times. Chaimilla (<3.1ka) is one of the best exposed and widely dispersed pyroclastic deposits, related to both fall and flow activity. The deposit is dispersed over an area of 250km2 and consists of 8 units (A–H) which were grouped into four sequences. Stratigraphic data suggest that the eruption had a relatively short duration and evolved from i) an Opening phase, dispersing ash, lapilli clasts, accretionary lapilli, blocks and bombs, to ii) a Pulsatory phase, originating a series of magmatic explosions, to iii) a Collapsing phase, characterised by unstable plumes which emplaced a series of pyroclastic density currents intercalated with thin fallout layers and finally to iv) a Climactic phase forming a more sustained plume which eventually collapsed generating the final pyroclastic density currents. The deposit (fall and flow) has a minimum cumulative volume of 0.6km3, with the main sustained phase being associated with a VEI 4 and the flow units having a minimum estimated total volume of 0.04km3. The erupted material has a homogenous chemical composition but displays a remarkable variability in both textural and physical properties. The density distribution of juvenile products shows a clear bimodality characterised by two main populations: P1 and P2. Population P1 consists of highly vesicular clasts (modal density around 1000kgm−3) with mostly sub-spherical bubbles and moderately crystallised groundmass with large-sized microlites. Clasts from population P2 are poorly vesicular (modal density around 1600kgm−3) with irregular to collapsed bubbles and numerous smaller microlites. The variability of both vesicularity and microlite characteristics suggests the involvement of two magma batches with distinct pre-eruptive degassing and rising histories. Our eruption conceptual model implies the arrival of new magma (represented in the deposit by P1 clasts) into a small, outgassed magma body which was accumulated at shallow level (mainly represented by P2 clasts). A new Chaimilla-type eruption could significantly affect the communities that have recently developed around Villarrica volcano and subsist mainly on tourism and forestry. As a result, a better understanding of the dynamics and evolution of the Chaimilla eruption is necessary for the identification of potential hazard scenarios at Villarrica volcano and, ultimately, for the risk mitigation of this populated area of Southern Chile. [Copyright &y& Elsevier]

Abstract: Explosive volcanic eruptions propel high-velocity turbulent jets into the atmosphere and are one of the most powerful and dangerous turbulent flows on Earth. Such eruptions are particularly difficult to predict due to their unusual dynamics that allow the jet to form a high buoyant Plinian plume or dense pyroclastic flows when the column collapses. A major goal of physical volcanology has been to predict quantitatively the limit between the flow regimes, as a function of exit conditions specified at the vent, which requires a physical model constrained by geologic field data. A first generation of quasi-analytic models, developed over 20–30 years although qualitatively successful, has failed to achieve this goal, because they overestimate by more than one order of magnitude the mass flux at the transition. There has been no consensus on whether this mostly reflects poor knowledge of geologic parameters or inadequacies in the physical description of the dynamics. Direct numerical simulations bring considerable advances in the study of unsteady phenomena that occur within the volcanic column, but their complexity makes the deciphering of the underlying dynamics challenging. Here, we present a basis for a new generation of 1D-models that includes a more sophisticated description of the rate of entrainment of air into the jet as a function of its buoyancy. In this framework, we demonstrate that the inconsistency between previous 1D models and field data is due to the omission of a key ingredient in the modeling of turbulence: reduction of entrainment due to the high density of the volcanic jet. We show that negative buoyancy near the base of the jet reduces the coefficient governing turbulent mixing to much less than the value commonly assumed based on experimental studies of neutrally buoyant jets and positively buoyant plumes. The results of our new model, allowing for variable entrainment as a function of local Richardson number, greatly improve the quantitative prediction of the transition as constrained by field data from a large number of eruptions. [Copyright &y& Elsevier]

Native plant establishment is limited by harsh environmental conditions in areas affected by tephra deposition following volcanic eruptions. Late-successional species might be lacking even decades after the disturbance. We assessed the effectiveness of pine-bark mulch, a by-product of sustainable timber production in the study area, in promoting the establishment and survival of a late-successional species ( Pinus pseudostrobus) and a nitrogen-fixing legume ( Lupinus elegans). We established a factorial experiment in areas covered with tephra during the eruption of the Paricutín volcano in the state of Michoacán, Mexico. After 1 year, P. pseudostrobus survival was significantly higher ( p < 0.001) in plots with pine-bark mulching (46.5%) than in plots without mulching (21.8%). After 2 years, surviving pines with mulching were significantly taller ( p= 0.03) than pines without mulching (45.3 ± 3.8 cm and 31.2 ± 3.7 cm, respectively). Lupinus elegans plants survived longer when grown in plots with pine-bark mulching than without mulching. Mulching reduced tephra temperatures during the dry season (when temperatures can reach up to 58°C 4 cm below the surface of bare tephra). Lupinus elegans plants were affected by herbivory by small rodents, run-off, and frost at the end of the growing season. Our results suggest that mulching can ameliorate harsh environmental conditions on sites covered with tephra while incorporating a by-product of sustainable forestry into restoration practice. [ABSTRACT FROM AUTHOR]

Abstract: Sedimentological and chemical analysis of gravity core samples, along with the interpretation of very high resolution, single channel seismic reflection profiles acquired off the Sarno prodelta system (southeastern Naples Bay), document the sedimentary facies and seismic stratigraphic signature of the tephra deposit erupted by Vesuvius during the plinian eruption of 79 AD. The 79 AD pyroclastic deposits sampled off the Sarno prodelta system revealed significantly different characters with respect to both the subaerial succession documented from the type sections of Pompeii, Herculaneum and Oplontis and the more distal pyroclastic fallout deposits that were recovered from the outer continental shelf off the northern Salerno Bay. This was likely to be expected as, in proximal subaqueous settings, both primary deposition and reworking of tephra are known to be influenced by the hydrodynamics of the water itself (induced by the pyroclastic currents entering the sea) as well as by the early instability of pyroclastic bedforms due to the exceptionally high sediment yield during volcanic eruption. According to the data illustrated and discussed in this study, we suggest that the 79 AD tephra deposit sampled off the Sarno prodelta is likely to be the result of a number of syn-eruptive genetically-related depositional and/or erosional mechanisms associated with (1) anomalous waves (tsunami) and currents generated by the entrance of pyroclastic flows and surges into the sea; (2) subaqueous density flows evolved from the impact of hot pyroclastic currents into the seawater; (3) failure of water-logged, cohesionless pumice deposits, due to instability of pyroclastic bedforms rapidly accumulating on the seafloor. Seismic interpretation revealed that the Upper Holocene sequence off the Sarno prodelta system is affected by extensive creep involving the whole post-79 AD succession. Deformation due to creep is documented by slumping of semi-consolidated strata over a basal surface represented by the lithologic discontinuity between the base of the 79 AD tephra deposits and the underlying hemipelagite. Seismic data also suggest that gravitational instability of this area has been induced, or enhanced, by significant volcanotectonic deformation and local uplift of the seafloor that ostensibly predated the eruptive event. The results of this research may be relevant to studies of other eruption events in close proximity to coastlines in terms of wave- and/or current-generated sedimentary features as a possible explanation of unusual subaqueous facies architecture. [Copyright &y& Elsevier]

Since the late-18th Century, eye-witness accounts have documented a wide-spectrum of eruptive activity sourced from Volcán Calbuco located in northwest Patagonia. Despite these observations there is very little known about the eruptive products themselves that can account for this eruptive diversity. In this study, we examine the tephrostratigraphic record post-dating the interval 1578–1702 cal. yr BP, with emphasis on historical eruptions (i.e. <130 years, including the 2015 eruption) at proximal to medial distances (<16 km from source) to characterize the composition, distribution, volume and style of these units. At least 11 discrete tephra units are recognized which are in accord with documented eruptive activity between ~1760 CE and 2015. Juvenile pyroclasts from these units span a narrow compositional range from basaltic-andesite to andesite (55–60 wt% SiO 2), and contain plagioclase (71–73%), pyroxene (~21%), cristobalite (3–5%) and scarce olivine and Ti-magnetite (1–2%). The largest documented historic eruption occurred in 1893–95, and produced a thick mantle of coarse-grained tephra fallout (0.32–0.50 km3 non-DRE) accompanied by intense ballistic bomb barrage closer to source. The 1893–95 eruption is comparable to the 1961 and 2015 eruptions both in terms of magnitude and explosivity, despite pyroclastic density currents (PDCs) not being documented in 1893–95. Both the 1929 and 1961 eruptions generated lava-flows, tephra fallout and PDCs, from which the 1961 volume totalled ~0.17 km3 non-DRE, affecting valleys northeast, up to a distance of 6 km from the crater. In contrast, the 2015 eruption only produced PDCs, tephra fallout and ballistics (0.26–0.36 km3 non-DRE). Results obtained from dendrochronological analysis of Nothofagus dombeyi trees within the study area reveals growth suppression indicated by structural damage during the 1893, 1929 and 1961 eruptions, probably related to thicker overall accumulations of tephra. Collectively, the componentry, architecture and volume of historic (AD 1893, 1961 and 2015) Calbuco tephra indicate sub-Plinian parental events derived from more mafic products with disequilibrium textures, than smaller eruptions characterized by c. 60% wt. SiO 2 probably triggered by crystal fractionation, providing a centennial time scale eruptive heterogeneity. This data is meaningful in terms of better understanding eruptive diversity at basaltic-andesite centers elsewhere that have high eruption frequencies (e.g. centennial time-scales). • Historic explosive eruptions of Volcán Calbuco reconstructed to the north of the volcano. • Identification, correlation and description of the latest 12 explosive eruptions. • Three sub-Plinian eruptions are mapped and their eruption parameters estimated. • Tephras described in terms of componentry, geochemistry, mineralogy and petrology. [ABSTRACT FROM AUTHOR]