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<jats:title>Abstract </jats:title><jats:p>Paleomagnetic and rock magnetic methods for studying volcanoes and their products have been developed since the second half of the twentieth century. These methods have been used to find tephra in sediment cores, date volcanic eruptions and deposits, determine emplacement temperatures of volcanic deposits, and estimate flow directions of dikes, lava flows, and pyroclastic flow deposits. In the twenty-first century, these techniques have steadily improved and expanded, resulting in more probing and precise studies of volcanoes using paleomagnetism. We believe that continual improvement of existing techniques and the increased awareness and interest in paleomagnetic methods should allow more studies to enhance the understanding of volcanic processes.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Forecasts of eruption are uncertain. The uncertainty is amplified when volcanoes reawaken after several generations in repose, because direct evidence of previous behaviour is rarely available. It fosters scepticism about warnings of volcanic activity and may compromise the success of emergency procedures. The quality of forecasts has improved over the past 50 years, owing mainly to a growing sophistication in statistical analyses of unrest. Physics-based analyses have yet to achieve the same level of maturity. Their application has been delayed by a view that volcanoes are too complex to share patterns of behaviour that can be described in a deterministic manner. This view is being increasingly challenged and an emerging line of inquiry is to understand how forecasts can be further improved by integrating statistical approaches with new constraints on possible outcomes from physics-based criteria. The introduction of deterministic reasoning yields rational explanations of why forecasts are not perfect and, as a result, offers new opportunities for increasing public confidence in warnings of eruption.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Glacial environments can have significant impacts on the surrounding landscape and nearby populations when affected by volcanic activity. As such, glaciovolcanic interactions and related hazards have received substantial attention during the last few decades. In contrast, the study of void spaces created by these interactions—glaciovolcanic cave systems—remains underrepresented. This review outlines the global distribution of glaciovolcanic caves and describes examples of both historical and ongoing research advances, most of which are limited to volcanoes of the Cascade Volcanic Arc and Antarctica. Examples range from a largely static fumarolic ice cave system in the crater of Mount Rainier to glaciovolcanic cave genesis and evolution in the crater of Mount St. Helens, where the advancing glacier ice is interacting with ongoing fumarolic activity and generating new cave systems. This review includes various volcanic subfields and also brings together additional disciplines including speleology, microbiology, and astrobiology. Due to the importance of glaciovolcanic caves in the hydrothermal cycle of volcanic systems, the global fight against antibiotic resistance, and their implications for understanding volcano-ice interactions beyond Earth, research on these systems is expanding. Kamchatka, Alaska, and Iceland have notable potential for further studies, while known research sites still hold open questions, including better understanding of the environmental parameters affecting cave genesis and persistence, the effect of glaciovolcanic cave development on underlying hydrothermal systems, and cataloging the biodiversity of glaciovolcanic cave environments.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Volcanic eruption source parameters may be estimated from acoustic pressure recordings dominant at infrasonic frequencies (&lt; 20 Hz), yet uncertainties may be high due in part to poorly understood propagation dynamics. Linear acoustic propagation of volcano infrasound is commonly assumed, but nonlinear processes such as wave steepening may distort waveforms and obscure the sourcing process in recorded waveforms. Here we use a previously developed frequency-domain nonlinearity indicator to quantify spectral changes due to nonlinear propagation primarily in 80 signals from explosions at Yasur Volcano, Vanuatu. We find evidence for <jats:inline-formula><jats:alternatives><jats:tex-math>$$\le$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≤</mml:mo> </mml:math></jats:alternatives></jats:inline-formula> 10<jats:sup>−3</jats:sup> dB/m spectral energy transfer in the band 3–9 Hz for signals with amplitude on the order of several hundred Pa at 200–400 m range. The clarity of the nonlinear spectral signature increases with waveform amplitude, suggesting stronger nonlinear changes for greater source pressures. We observe similar results in application to synthetics generated through finite-difference wavefield simulations of nonlinear propagation, although limitations of the model complicate direct comparison to the observations. Our results provide quantitative evidence for nonlinear propagation that confirms previous interpretations made on the basis of qualitative observations of asymmetric waveforms. </jats:p>

<jats:title>Abstract </jats:title><jats:p>Mocho-Choshuenco volcano has produced several highly explosive eruptions during its history, which make it one of the most hazardous volcanoes in the southern volcanic zone of Chile, although it is still relatively little studied to date. We present a geochemical study of the products of the sub-Plinian, andesitic, Enco eruption that occurred about 1600 years ago. We determined the major and trace elements compositions, as well as the volatile (H<jats:sub>2</jats:sub>O, CO<jats:sub>2</jats:sub>, Cl, and S) contents of melt inclusions trapped in minerals (olivine, plagioclase, and pyroxene) using electron microprobe, ion microprobe (SIMS), and 3D confocal Raman mapping. Though the whole-rock composition of the Enco magma is andesitic (60.2 ± 1.1 wt.% SiO<jats:sub>2</jats:sub>), the melt inclusions have SiO<jats:sub>2</jats:sub> contents ranging from 50.3 to 67.3 wt.%, following the magmatic series of Mocho-Choshuenco, and the compositions of the most mafic melt inclusions are close to those of the most mafic erupted magmas. Geochemical modeling indicates that mixing occurred between a mafic magma and an andesitic-to-dacitic magma. Glass analysis revealed typical parental arc magma values for H<jats:sub>2</jats:sub>O (2.6–3.8 wt.%), S (116–1936 ppm), and Cl (620–1439 ppm). However, CO<jats:sub>2</jats:sub> contents are very high in some melt inclusions with concentrations above 4000 ppm (measured in the glass), suggesting trapping depths &gt;  ~ 17–22 km. Presence of solid carbonates inside inclusion-hosted bubbles clearly indicates that the CO<jats:sub>2</jats:sub> contents measured in the glass phase were minimum values. We conclude that a CO<jats:sub>2</jats:sub>-rich basaltic magma ascended and mixed with a shallower andesitic magma. The magma cooled and exsolved high amounts of CO<jats:sub>2</jats:sub>, which may have dramatically increased the pressure and triggered the highly explosive Enco eruption.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The effects of pyroclastic density currents (PDCs) can be devastating, so understanding their internal dynamics and evolution is important for hazard assessment. We use damaged trees located around Mount St. Helens (USA) as proxy for the dynamic pressure (<jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub>) of the PDC erupted on 18 May 1980. We recorded the location, distribution, and foliage preservation of damaged trees within the medial and distal parts of the devastated forest. Sub-meter resolution aerial photographs from a month after the eruption allow distinction between standing trees that retained foliage from those that were stripped. Heights of standing trees were estimated from the measured lengths of their shadows. The number of standing trees was counted within defined areas along the propagation paths of PDCs. From the measured tree heights, we estimated tree toppling stresses from <jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub>. Overall, <jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub> of the PDC head within the medial to distal portions of the blowdown zone ranged from 10 to 35 kPa. <jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub> likely waned with distance, as shown by the increased number of standing trees in the outer parts of the devastated area. In addition, we find clusters of standing trees on the lee sides of some hills. We propose that these clusters survived because they were primarily impacted by lower dynamic pressures extant within the PDC body, with foliage retention or stripping as a function of the <jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub> evolution in the PDC body. We estimate that <jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub> of the body was less than the estimated maximum <jats:italic>P</jats:italic><jats:sub><jats:italic>dyn</jats:italic></jats:sub> of the PDC head by 12 ± 4 kPa.</jats:p>