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Volcanic activity associated with the emplacement of Large Igneous Provinces (LIPs) has been linked to most Phanerozoic extinctions/episodes of major environmental change. In recent years, mercury (Hg) enrichments and elevated mercury/total organic carbon (Hg/TOC) ratios have been increasingly utilized as a marker of volcanism in sedimentary records deposited distally from LIPs. The proxy is based on the premise that volcanism is a major natural source of the element to the atmosphere, and that it was especially important prior to anthropogenic emissions. To date, end-Permian and end-Triassic records illustrate the strongest use of Hg as a marker of volcanic activity, supplemented by the use of Hg isotopes and other evidence for LIP eruptions and volatile emissions. Sedimentary records of several other events also document Hg enrichments in at least one region, suggestive of a regional- or global-scale perturbation to the Hg cycle at those times, potentially linked to volcanism. The Cenomanian– Turonian Oceanic Anoxic Event 2 appears to be the exception, with strata documenting peaks in Hg/TOC in a small minority of studied records, suggesting that there was only a minimal perturbation to the Hg cycle at that time. Even for events where a global Hg-cycle perturbation apparently occurred, varying Hg enrichments documented from individual archives of the same event indicate that the complex biogeochemical cycling of mercury can result in a strong influence of local/regional aquatic, biological, or sedimentary processes on the precise signature of any worldwide disturbance. Of additional intrigue is an apparent lack of correlation of Hg and Hg/TOC with other volcanic proxies such as osmium isotopes, suggesting that the two systems record different aspects of LIP volcanism and emplacement. Recent studies are beginning to investigate these above complexities, but further work is needed to fully explore the nuances of Hg in the geological record, and how it can be best employed as a proxy for LIP volcanism.

Understanding the timing of critical changes in volcanic systems, such as the beginning and end of eruptive behaviour, is a key goal of volcanic monitoring. Traditional approaches to forecasting these changes have used models motivated by the underlying physics of eruption onset, which assume that geophysical precursors will consistently display similar patterns prior to transition in volcanic state. We present a machine learning classification approach for detecting significant changes in patterns of volcanic activity, potentially signalling transitions during the onset or end of volcanic activity, which does not require a model of the physical processes underlying critical changes. We apply novelty detection, where models are trained only on data prior to eruption, to the precursory unrest at Augustine Volcano, Alaska in 2005. This approach looks promising for geophysically-monitored volcanic systems which have been in repose for some time, as no eruptive data is required for model training. We compare novelty detection results with multi32 class classification, where models are trained on examples of both non-eruptive and eruptive data. We contextualise the results of these classification models using constraints from petrological, satellite and visual observations from the 2006 eruption of Augustine Volcano. The transition from non-eruptive to eruptive behaviour we identify in mid-November 2005 is in agreement with previous estimates of the initiation of dike intrusion prior to the 2006 eruption. We find that models which include multiple types of data (seismic, deformation and gas emissions) can better distinguish between non-eruptive and eruptive data than models formulated on single data types.

The 6-month-long 2014–2015 Holuhraun eruption was the largest in Iceland for 200 years, emitting huge quantities of sulfur dioxide (SO2) into the troposphere, at times overwhelming European anthropogenic emissions. Weather, terrain and latitude made continuous ground-based or UV satellite sensor measurements challenging. Infrared Atmospheric Sounding Interferometer (IASI) data are used to derive the first time series of daily SO2 mass present in the atmosphere and its vertical distribution over the entire eruption period. A new optimal estimation scheme is used to calculate daily SO2 fluxes and average e-folding time every 12 h. For the 6 months studied, the SO2 flux was observed to be up to 200 kt day−1 and the minimum total SO2 erupted mass was 4.4±0.8 Tg. The average SO2 e-folding time was 2.4±0.6 days. Where comparisons are possible, these results broadly agree with ground-based near-source measurements, independent remote-sensing data and values obtained from model simulations from a previous paper. The results highlight the importance of using high-resolution time series data to accurately estimate volcanic SO2 emissions. The SO2 mass missed due to thermal contrast is estimated to be of the order of 3 % of the total emission when compared to measurements by the Ozone Monitoring Instrument. A statistical correction for cloud based on the AVHRR cloud-CCI data set suggested that the SO2 mass missed due to cloud cover could be significant, up to a factor of 2 for the plume within the first kilometre from the vent. Applying this correction results in a total erupted mass of 6.7±0.4 Tg and little change in average e-folding time. The data set derived can be used for comparisons to other ground- and satellite-based measurements and to petrological estimates of the SO2 flux. It could also be used to initialise climate model simulations, helping to better quantify the environmental and climatic impacts of future Icelandic fissure eruptions and simulations of past large-scale flood lava eruptions.

The Late Triassic Carnian Pluvial Episode (CPE) was a time of biological turnover and environmental perturbations. Within the CPE interval, C-isotope and sedimentary records indicate multiple pulses of depleted carbon into the atmosphere–ocean system linked to discrete enhancements of the hydrological cycle. Data suggest a similar cascade of events to other extinctions, including being potentially driven by emplacement of a large igneous province (LIP). The age of the Wrangellia LIP overlaps that of the CPE, but a direct link between volcanism and the pulsed CPE remains elusive. We present sedimentary Hg concentrations from Western Tethys successions to investigate volcanic activity through the previously established CPE global negative C-isotope excursions (NCIEs). Higher Hg concentrations and Hg/TOC are recorded just before and during NCIEs and siliciclastic inputs. The depositional settings suggest volcanic Hg inputs into the basins over the NCIEs rather than increases of Hg drawdown or riverine transport. Differences in Hg and Hg/TOC signals between the basins might be linked to coeval LIP style or the temporal resolution of the sedimentary successions. Overall, our new data provide support for a link between pulses of Wrangellia LIP volcanism, NCIEs, and humid phases that mark the CPE in the Western Tethys.

Long-lived basaltic volcanic eruptions are a globally important source of environmentally reactive, volatile metal pollutant elements such as selenium, cadmium and lead. The 2018 eruption of Kīlauea, Hawai’i produced exceptionally high discharge of metal pollutants, and was an unprecedented opportunity to track them from vent to deposition. Here we show, through geochemical sampling of the plume that volatile metal pollutants were depleted in the plume up to 100 times faster than refractory species, such as magnesium and iron. We propose that this rapid wet deposition of complexes containing reactive and potentially toxic volatile metal pollutants may disproportionately impact localised areas close to the vent. We infer that the relationship between volatility and solubility is an important control on the atmospheric behaviour of elements. We suggest that assessment of hazards from volcanic emissions should account for heterogeneous plume depletion of metal pollutants. Volatile metal pollutants in basaltic volcanic plumes can be deposited up to 100 times faster than refractory species, and may produce disproportionate impacts at proximal locations, according to extensive sampling of Kīlauea’s 2018 eruption plume.

Monitoring of volcanic emissions (gas, ash and aerosols) is crucial to our understanding of eruption mechanisms, as well as to developing mitigation strategies during volcanic eruptions. Ultraviolet (UV) spectrometers and cameras are now ubiquitous monitoring tools at most volcano observatories for quantifying sulphur dioxide (SO2) emissions. However, because they rely on scattered UV light as a source of radiation, their use is limited to daytime only, and measurement windows are often further restricted by unfavourable weather conditions. On the other end of the spectrum, Open Path Fourier Transform Infrared (OP-FTIR) instruments can be used to measure the concentrations of a series of volcanic gases, and they allow for night-time operation. However, the retrieval methods rely on the presence of a strong source of IR radiation in the background - either natural (lava flow, crater rim, the sun) or artificial – restricting their use to very specific observation geometries and a narrow range of eruptive conditions. Here we present a new approach to derive quantities of SO2, ash and aerosols from measurements of a drifting volcanic plume. Using the atmosphere as a background, we measured self-emitted IR radiation from plumes at Stromboli volcano (Italy) capturing both passive degassing and ash-rich explosive plumes. We use an iterative approach with a forward radiative transfer model (the Reference Forward Model – RFM) to quantify concentrations of sulphur dioxide (SO2), aerosols and ash in the line of sight of the spectrometer. This new method could significantly enhance the scientific return from OP-FTIR instruments at volcano observatories, ultimately expanding their deployment as part of permanent scanning networks (an alternative to DOAS instruments) to provide continuous data on the emissions of gas, ash and aerosols.

Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions.

Funder: EPSRC-CASE studentship, Funder: NERC studentship, Funder: Leverhulme Trust; doi: https://doi.org/10.13039/501100000275, Funder: NERC-CASE studentship, Funder: Rolex Institute, Volcanoes represent one of the largest natural sources of metals to the Earth’s surface. Emissions of these metals can have important impacts on the biosphere as pollutants or nutrients. Here we use ground- and drone-based direct measurements to compare the gas and particulate chemistry of the magmatic and lava–seawater interaction (laze) plumes from the 2018 eruption of Kīlauea, Hawai’i. We find that the magmatic plume contains abundant volatile metals and metalloids whereas the laze plume is further enriched in copper and seawater components, like chlorine, with volatile metals also elevated above seawater concentrations. Speciation modelling of magmatic gas mixtures highlights the importance of the S2− ligand in highly volatile metal/metalloid degassing at the magmatic vent. In contrast, volatile metal enrichments in the laze plume can be explained by affinity for chloride complexation during late-stage degassing of distal lavas, which is potentially facilitated by the HCl gas formed as seawater boils.

The Paleocene–Eocene Thermal Maximum (PETM) was a period of geologically-rapid carbon release and global warming ~56 million years ago. Although modelling, outcrop and proxy records suggest volcanic carbon release occurred, it has not yet been possible to identify the PETM trigger, or if multiple reservoirs of carbon were involved. Here we report elevated levels of mercury relative to organic carbon—a proxy for volcanism—directly preceding and within the early PETM from two North Sea sedimentary cores, signifying pulsed volcanism from the North Atlantic Igneous Province likely provided the trigger and subsequently sustained elevated CO2. However, the PETM onset coincides with a mercury low, suggesting at least one other carbon reservoir released significant greenhouse gases in response to initial warming. Our results support the existence of ‘tipping points’ in the Earth system, which can trigger release of additional carbon reservoirs and drive Earth’s climate into a hotter state., The Paleocene–Eocene boundary coincided with runaway global warming possibly analogous to future climate change, but the sources of greenhouse gasses have remained unresolved. Here, the authors reveal volcanism triggered initial warming, and subsequent carbon was released after crossing a tipping point.

Volcanoes are a large global source of almost every element, including ~20 environmentally reactive trace elements classified as metal pollutants (e.g. selenium, cadmium and lead). Fluxes of metal pollutants from individual eruptions can be comparable to total anthropogenic emissions from large countries such as China.The 2018 Lower East Rift Zone eruption of Kīlauea, Hawaii produced exceptionally high emission rates of major and trace chemical species compared to other basaltic eruptions over 3 months (200 kt/day of SO2; Kern et al. 2019). We tracked the volcanic plume from vent to exposed communities over 0-240 km distance using in-situ sampling and atmospheric dispersion modelling. This is the first time that trace elements in volcanic emissions (~60 species) are mapped over such distances. In 2019, we repeated the field campaign during a no-eruption period and showed that volcanic emissions had caused 3-5 orders of magnitude increase in airborne metal pollutant concentrations across the Island of Hawai’i.We show that the volatility of the elements (the ease with which they are degassed from the magma) controls their particle-phase speciation, which in turn determines how fast they are depleted from the plume after emission. Elements with high magmatic volatilities (e.g. selenium, cadmium and lead) have up to 6 orders of magnitude higher depletion rates compared to non-volatile elements (e.g. magnesium, aluminium and rare earth metals).Previous research and hazard mitigation efforts on volcanic emissions have focussed on sulphur and it has been assumed that other pollutants follow the same dispersion patterns. Our results show that the atmospheric fate of sulphur, and therefore the associated hazard distribution, does not represent an accurate guide to the behaviour and potential impacts of other species in volcanic emissions. Metal pollutants are predominantly volatile in volcanic plumes, and their rapid deposition (self-limited by their volatility) places disproportionate environmental burdens on the populated areas in the immediate vicinity of the active and, in turn, reduces the impacts on far-field communities.Reference: Kern, C., T. Elias, P. Nadeau, A. H. Lerner, C. A. Werner, M. Cappos, L. E. Clor, P. J. Kelly, V. J. Realmuto, N. Theys, S. A. Carn, AGU, 2019; https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/507140.

Satellite remote sensing has been widely used to make measurements of sulphur dioxide (SO2) emissions from volcanoes. The Infrared Atmospheric Sounding Interferometer (IASI) is one such instrument that has been used to examine the emissions from large explosive eruptions. Much less work has been done using IASI to study the emissions from smaller eruptions, non-eruptive degassing or anthropogenic sources, and similarly it is rarely used for examining long term trends in activity. Now, when there are three IASI instruments in orbit and with over ten years of data, is the perfect opportunity to explore these topics. This study applied a ‘fast’ linear retrieval developed for IASI in Oxford, across the globe for a ten-year period. Global annual averages were dominated by the emissions from large eruptions (e.g. Nabro in 2011) but elevated signals could also be identified from smaller volcanic sources and industrial centres, suggesting the technique has promise for detecting lower level emissions. A systematic approach was then taken, rotating the linear retrieval output for each orbit at over 100 volcanoes worldwide, with the wind direction at the volcano’s vent, or in cases where the plume was emitted at a greater height, using the observed plume direction. This isolates the elevated signal downwind of the volcano. The rotated outputs were then averaged over monthly, annual and multi-annual time periods. Analysis of the upwind and downwind values establishes whether there is an elevated signal and its intensity. An inventory was then constructed from these observations which show how these emissions varied over a ten-year period. Trends in SO2 emission were compared against fluxes generated for the Ozone Monitoring Instrument (OMI) and the number of thermal anomalies detected by the MODVOLC algorithm developed for MODIS. It was identified for example, that long term trends are more easily identified at high altitude volcanoes such as Popocatepetl, Sabancaya and Nevado del Ruiz. This is consistent with the idea that the instrument performs better in regions with lower levels of water vapour (e.g. above the boundary layer).

The 2018 eruption of Kīlauea volcano, Hawai'i, resulted in enormous gas emissions from the Lower East Rift Zone (LERZ) of the volcano. This led to important changes to air quality in downwind communities. We analyse and present measurements of atmospheric sulfur dioxide (SO2) and aerosol particulate matter < 2.5 µm (PM2.5) collected by the Hawai'i Department of Health (HDOH) and National Park Service (NPS) operational air quality monitoring networks between 2007 and 2018; and a community-operated network of low-cost PM2.5 sensors on the Island of Hawai'i. During this period, the two largest observed increases in Kīlauea's volcanic emissions were: the summit eruption that began in 2008 (Kīlauea emissions averaged 5 – 6 kt/day SO2 over the course of the eruption) and the LERZ eruption in May-August 2018 when SO2 emission rates likely reached 200 kt/day in June. Here we focus on characterising the airborne pollutants arising from the 2018 LERZ eruption and the spatial distribution and severity of air pollution events across the Island of Hawai'i. The LERZ eruption caused the most frequent and severe exceedances of Environmental Protection Agency 24-hour-mean PM2.5 air quality thresholds in Hawai'i since 2010. In Kona, for example, there were eight exceedances during the 2018 LERZ eruption, where there had been no exceedances in the previous eight years as measured by the HDOH and NPS networks. SO2 air pollution during the LERZ eruption was most severe in communities in the south and west of the island, with maximum 24-hour-mean mass concentrations of 728 µg/m3 recorded in Ocean View (100 km west of the LERZ emission source) in May 2018. Data from the low-cost sensor network correlated well with data from the HDOH PM2.5 instruments (Kona station, R2 = 0.89), demonstrating that these low-cost sensors provide a viable means to rapidly augment reference-grade instrument networks during crises.

Deep carbon emissions from historically inactive volcanoes, hydrothermal, and tectonic structures are among the greatest unknowns in the long-term (∼Myr) carbon cycle. Recent estimates of diffuse CO2 flux from the Eastern Rift of the East African Rift System (EARS) suggest this could equal emissions from the entire mid-ocean ridge system. We report new CO2 surveys from the Main Ethiopian Rift (MER, northernmost EARS), and reassess the rift-related CO2 flux. Since degassing in the MER is concentrated in discrete areas of volcanic and off-edifice activity, characterization of such areas is important for extrapolation to a rift-scale budget. Locations of hot springs and fumaroles along the rift show numerous geothermal areas away from volcanic edifices. With these new data, we estimate total CO2 emissions from the central and northern MER as 0.52–4.36 Mt yr−1. Our extrapolated flux from the Eastern Rift is 3.9–32.7 Mt yr−1 CO2, overlapping with lower end of the range presented in recent estimates. By scaling, we suggest that 6–18 Mt yr−1 CO2 flux can be accounted for by magmatic extension, which implies an important role for volatile-enriched lithosphere, crustal assimilation, and/or additional magmatic intrusion to account for the upper range of flux estimates. Our results also have implications for the nature of volcanism in the MER. Many geothermal areas are found >10 km from the nearest volcanic center, suggesting ongoing hazards associated with regional volcanism.

Distributed volcanic fields of scoria cones, maars, tuff cones and lava flows represent a crucial expression of the interplay between magmatism, tectonics and surface processes in continental rifts. We compare the cone morphology of two fields (Butajira and East Ziway) in the Main Ethiopian Rift (MER), using high-resolution digital elevation models (DEMs) to investigate volumes, surface environment and age of emplacement. Volumes of individual cones are similar in both fields (mean ~4 × 106 m3) and lower than in other distributed volcanic fields located in extensional environments. Volcanism at Butajira has created linear clusters of superimposed cones, suggesting a small number of larger (107–108 m3) fissure eruptions compared to the isolated events at East Ziway. The presence of maars and deep craters within cones at Butajira indicates phreatomagmatic activity, not seen at East Ziway where cones may have been emplaced during a dry climate phase. Shape parameters, including curvature of cone rims and ratios of cone height, width and crater depth, suggest younger eruption ages at Butajira compared to East Ziway and the possibility of ongoing activity in this area. A broader survey of cone fields across the MER using the ASTER global digital elevation model fails to identify systematic variation in height, width and volume from cone morphology, limited by low-resolution. High-resolution topography provides a useful insight into rift environments at the time of emplacement, and further surveys across the Main Ethiopian Rift may advance our understanding of distributed volcanism in extensional settings and the hazards they present.

Santorini Volcano is an outstanding natural laboratory for studying arc volcanism, having had twelve Plinian eruptions over the last 350,000 years, at least four of which caused caldera collapse. Periods between Plinian eruptions are characterized by intra-caldera edifice construction and lower intensity explosive activity. The Plinian eruptions are fed from magma reservoirs at 4–8 km depth that are assembled over several centuries prior to eruption by the arrival of high-flux magma pulses from deeper in the sub-caldera reservoir. Unrest in 2011–2012 involved intrusion of two magma pulses at about 4 km depth, suggesting that the behaviour of the modern-day volcano is similar to the behaviour of the volcano prior to Plinian eruptions. Emerging understanding of Santorini's plumbing system will enable better risk mitigation at this highly hazardous volcano.

As our ability to detect volcanic unrest improves, we are increasingly confronted with the question of whether the unrest has a magmatic origin (magma on the move) or a non-magmatic origin from a change in the hydrothermal system (fluids that are not magma on the move) or tectonic processes. The cause of unrest has critical implications for the potential eruptive hazard (e.g. used in constructing Bayesian Event Trees), but is frequently the subject of debate, even at well-studied systems. Here, we propose a set of multi-disciplinary observations and numerical models that could be used to evaluate conceptual models about the cause of unrest. These include measurements of gas fluxes and compositions and the isotopic signature of some components (e.g. H 2 , He, C, SO 2 , H 2 O , CH 4 and CO 2 ), the spatial and temporal characteristics of ground deformation, thermal output, seismicity, changes in gravity, and whether there is topographic uplift or subsidence spanning hundreds to thousands of years. In several volcanic systems, both magmatic and non-magmatic unrest is occurring at the same time. While none of these observations or models is diagnostic on its own, we illustrate several examples where they have been used together to make a plausible conceptual model of one or more episodes of unrest and whether eruptions did or did not follow the unrest. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

Ash clouds are a geographically far reaching hazard associated with volcanic eruptions. To minimise the risk that these pose to aircraft and to limit disruption to the aviation industry, it is important to closely monitor the emission and atmospheric dispersion of these plumes. The altitude of the plume is an important consideration and is an essential input into many models of ash cloud propagation. CO2 slicing is an established technique for obtaining the top height of meteorological clouds and previous studies have demonstrated that there is potential for this method to be used for volcanic ash. In this study, the CO2 slicing technique has been adapted for volcanic ash and applied to spectra obtained from the Infrared Atmospheric Sounding Interferometer (IASI). Simulated ash spectra are first used to select the most appropriate channels and then demonstrate that the technique has merit for determining the altitude of the ash. These results indicate a strong match between the true heights and CO2 slicing output with a root mean square error (RMSE) of less than 800 m. Following this, the technique was applied to spectra obtained with IASI during the Eyjafjallajökull and Grimsvötn eruptions in 2010 and 2011 respectively, both of which emitted ash clouds into the troposphere, and which have been extensively studied with satellite imagery. The CO2 slicing results were compared against those from an optimal estimation scheme, also developed for IASI, and a satellite borne LiDAR is used for validation. Overall, the CO2 slicing tool performs better than the optimal estimation scheme. The CO2 slicing heights returned a RMSE value of 2.2 km when compared against the LiDAR. This is lower than the RMSE for the optimal estimation scheme (2.8 km). The CO2 slicing technique is a relatively fast tool and the results suggest that this method could be used to get a first approximation of the ash cloud height, potentially for use for hazard mitigation, or as an input for other retrieval techniques or models of ash cloud propagation.

Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time series reveal pulses of accelerating uplift that transition to gradual long-term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2-δ13C of −4.2‰ to −4.5‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic-hydrothermal system can explain the uplift-subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice-wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic systems and will be crucial for interpreting future patterns of unrest.

Sulfur dioxide (SO2) is an important atmospheric constituent that plays a crucial role in many atmospheric processes. Volcanic eruptions are a significant source of atmospheric SO2 and its effects and lifetime depend on the SO2 injection altitude. The Infrared Atmospheric Sounding Interferometer (IASI) on the METOP satellite can be used to study volcanic emission of SO2 using high-spectral resolution measurements from 1000 to 1200 and from 1300 to 1410 cm−1 (the 7.3 and 8.7 µm SO2 bands) returning both SO2 amount and altitude data. The scheme described in Carboni et al. (2012) has been applied to measure volcanic SO2 amount and altitude for 14 explosive eruptions from 2008 to 2012. The work includes a comparison with the following independent measurements: (i) the SO2 column amounts from the 2010 Eyjafjallajökull plumes have been compared with Brewer ground measurements over Europe; (ii) the SO2 plumes heights, for the 2010 Eyjafjallajökull and 2011 Grimsvötn eruptions, have been compared with CALIPSO backscatter profiles. The results of the comparisons show that IASI SO2 measurements are not affected by underlying cloud and are consistent (within the retrieved errors) with the other measurements. The series of analysed eruptions (2008 to 2012) show that the biggest emitter of volcanic SO2 was Nabro, followed by Kasatochi and Grímsvötn. Our observations also show a tendency for volcanic SO2 to reach the level of the tropopause during many of the moderately explosive eruptions observed. For the eruptions observed, this tendency was independent of the maximum amount of SO2 (e.g. 0.2 Tg for Dalafilla compared with 1.6 Tg for Nabro) and of the volcanic explosive index (between 3 and 5).

The timing and processes that govern the end of volcanic eruptions are not yet fully understood, and there currently exists no systematic definition for the end of a volcanic eruption. Currently, end of eruption is established either by generic criteria (typically 90 days after the end of visual signals of eruption) or criteria specific to a given volcano. We explore the application of supervised machine learning classification methods: Support Vector Machine, Logistic Regression, Random Forest and Gaussian Process Classifiers and define a decisiveness index D to evaluate the consistency of the classifications obtained by these models. We apply these methods to seismic time series from two volcanoes chosen because they display contrasting styles of eruption: Telica (Nicaragua) and Nevado del Ruiz (Colombia). We find that, for both volcanic systems, the end-date we obtain by classification of seismic data is 2–4 months later than end-dates defined by the last occurrence of visual eruption (such as ash emission). This finding is in agreement with previous, general definitions of eruption end and is consistent across models. Our classifications have a higher correspondence of eruptive activity with visual activity than with database records of eruption start and end. We analyze the relative importance of the different features of seismic activity used in our models (e.g. peak event amplitude, daily event counts) and find little consistency between the two volcanic systems in terms of the most important features which determine whether activity is eruptive or non-eruptive. These initial results look promising and our approach may offer a robust tool to help determine when an eruption has ended in the absence of visual confirmation.

As well as gases that regulate climate over geological time, volcanoes emit prodigious quantities of metals into the atmosphere, where they have key roles as catalysts, pollutants and nutrients. Here we compare measurements of arc basaltic volcano metal emissions with those from hotspot settings. As well as emitting higher fluxes of metals (similar to those building ore deposits), these arc emissions possess a distinct compositional fingerprint, particularly rich in tungsten, arsenic, thallium, antimony and lead when compared with those from hotspots. We propose that volcanic metal emissions are controlled by magmatic water content and redox: hydrous arc magmas that do not undergo sulfide saturation yield metal-rich, saline aqueous fluid; shallow degassing and resorption of late-stage sulfides feeds volcanic gases in Hawai’i and Iceland. Although global arc magma chemistries vary considerably, our findings suggest that volcanic emissions in arcs have a distinct fingerprint when compared with other settings. A shift in global volcanic metal emissions may have occurred in Earth’s past as more oxidized, water-rich magmas became prevalent, influencing the surface environment. Arc volcanism emits higher metal fluxes to Earth’s atmosphere than hotspot volcanism. The systems’ unique gas compositions are controlled by magmatic water content and redox state, as shown by a compilation of volcanic gas and aerosol metal data.

Satellite remote sensing is a valuable method for detecting and quantifying sulfur dioxide (SO2) emissions at volcanoes. The use of ultraviolet satellite instruments for monitoring purposes has been assessed in numerous studies, but there are advantages to using infrared measurements, including that they can operate at night and during high-latitude winters. This study focuses on the Infrared Atmospheric Sounding Interferometer (IASI). Retrievals developed for this instrument have been shown to be successful when applied to large eruptions, but little has been done to explore their potential for detecting and quantifying emissions from smaller and lower altitude emissions or for the assessment of ongoing activity. Here a “fast” linear retrieval has been applied across the globe to detect volcanic sources of SO2. The results are dominated by emissions from explosive eruptions, but signals are also evident from weak eruptions, passive degassing, and anthropogenic activity. Ecuador and Kamchatka were selected for further study with a more processing intensive iterative retrieval which can quantify the SO2 amount. At Tungurahua in Ecuador, good agreement was seen between IASI, the Ozone Monitoring Instrument (OMI) and ground-based flux data, demonstrating that the retrieval is capable of capturing relative changes in activity. Similarly, good agreement was found between IASI and OMI in Kamchatka. In this high-latitude region, OMI is unable to operate for 3 or 4 months in each year. It is therefore suggested that IASI could be used alongside other instruments for evaluating changes in volcanic activity.

This work was funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc) and a Boise Fund grant from the Department of Zoology, University of Oxford. The Main Ethiopian Rift (MER, ~7–9°N) is the type example of a magma-assisted continental rift. The rift axis is populated with regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of small mafic scoria cones. The recent (latest Pleistocene to Holocene) history of volcanism in the MER is poorly known, and no eruptions have occurred in the living memory of the local population. Assessment of contemporary volcanic hazards and associated risk is primarily based on the study of the most recent eruptive products, typically those emplaced within the last 10–20 ky. We integrate new and published field observations and geochemical data on tephra deposits from the main Late Quaternary volcanic centres in the central MER to assess contemporary volcanic hazards. Most central volcanoes in the MER host large mid-Pleistocene calderas, with typical diameters of 5–15 km, and associated ignimbrites of trachyte and peralkaline rhyolite composition. In contrast, post-caldera activity at most centres comprises eruptions of peralkaline rhyolitic magmas as obsidian flows, domes and pumice cones. The frequency and magnitude of events varies between individual volcanoes. Some volcanoes have predominantly erupted obsidian lava flows in their most recent post-caldera stage (Fentale), whereas other have had up to 3 moderate-scale (VEI 3–4) explosive eruptions per millennium (Aluto). At some volcanoes we find evidence for multiple large explosive eruptions (Corbetti, Bora-Baricha, Boset-Bericha) which have deposited several centimeters to meters of pumice and ash in currently densely populated regions. This new overview has important implications when assessing the present-day volcanic hazard in this rapidly developing region. Postprint

J. Biggs, I.M.Watson, W. Hutchison, T.A. Mather and D.M. Pyle are members of the NERC Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET). This work forms a contribution to the NERC Large Grant RiftVolc (NE/L01372X/1) awarded to J. Biggs. W. Hutchison was funded by NERC studentship NE/J500045/1. I.M. Watson is part of the NASA-ASTER science team and acknowledges NASA and JAXA for the provision of data. Fumaroles are the surface manifestation of hydrothermal circulation and can be influenced by magmatic, hydrothermal, hydrological and tectonic processes. This study investigates the temporal changes in fumarole temperatures and spatial extent on Aluto, a restless volcano in the Main Ethiopian Rift (MER), in order to better understand the controls on fluid circulation and the interaction between the magmatic and hydrothermal systems. Thermal infrared (TIR) satellite images, acquired by the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) over the period of 2004 to 2016, are used to generate time series of the fumarole temperatures and areas. The thermal anomalies identified in the ASTER images coincide with known fumaroles with temperatures > 80 °C and are located on or close to fault structures, which provide a pathway for the rising fluids. Most of the fumaroles, including those along the major zone of hydrothermal upwelling, the Artu Jawe Fault Zone, have pixel-integrated temperature variations of only ~ 2 ± 1.5 °C. The exception are the Bobesa fumaroles located on a hypothesised caldera ring fault which show pixel-integrated temperature changes of up to 9 °C consistent with a delayed response of the hydrothermal system to precipitation. We conclude that fumaroles along major faults are strongly coupled to the magmatic-hydrothermal system and are relatively stable with time, whereas those along shallower structures close to the rift flank are more strongly influenced by seasonal variations in groundwater flow. The use of remote sensing data to monitor the thermal activity of Aluto provides an important contribution towards understanding the behaviour of this actively deforming volcano. This method could be used at other volcanoes around the world for monitoring and geothermal exploration. Postprint

Sulfides are a major potential repository for magmatic metals and sulfur. In relatively reduced magmas, there may be a dynamic interplay between sulfide liquids and magma degassing as magmas ascend/erupt. Sulfide-bubble aggregates may segregate to shallow levels. Exsolved fluids may oxidize sulfides to produce SO 2 gas and metals, which can vent to the atmosphere with chalcophile metal ratios reflecting those in their parent sulfide liquids. Sulfide breakdown and/or sequestration timing and balance define the role of sulfides in both ore formation and the environmental impacts of volcanic eruptions, including during the evolution of large igneous provinces, which are key periods of heightened volcanism during Earth history.

An enigmatic chloride-rich iron (oxyhydr)oxide has been recently identified together with mercury anomalies in End-Cretaceous marine sediments coeval with the Deccan Traps eruptions. The mineral was observed in Bidart (France) and Gubbio (Italy), suggesting a widespread phenomenon. However, the exact nature and origin of this Cl-bearing mineral remained speculative. Here, we characterized the accurate composition and nanostructure of this chloride-rich phase by using micro-Raman spectroscopy, Transmission (TEM) and Scanning (SEM) Electron Microscopy on Focused Ion Beam foils. We also provide new evidence of its occurrence in Zumaia, a reference KPg section from Spain. Results confirm akaganéite (β-FeOOH) as the main phase, with chloride content of 3–5 atomic weight %. Akaganéite particles are constituted by the aggregation of nanorods of akaganéite. Internal structures contain empty spaces, suggesting formation in a low-density (atmospheric) environment. This new mineralogical evidence supports the hypothesis that the observed akaganéite was formed in the Deccan volcanic plume and was transported to the Atlantic and Tethysian realms through the stratosphere. Therefore, akaganéite provides a potential new sedimentary marker to identify the imprint of the Deccan eruptions in the stratigraphic record and is evidence of volcanic halogen degassing and its potential role for the Cretaceous-Tertiary mass extinction.

Cyclic behaviour over a range of timescales is a well-documented feature of many dome-forming volcanoes, but has not previously been identified in high resolution seismic data from Volcan de Colima (Mexico). Using daily seismic count datasets from Volcan de Colima and Soufriere Hills volcano (Montserrat), this study explores parallels in the long-term behaviour of seismicity at two long-lived systems. Datasets are examined using multiple techniques, including Fast-Fourier Transform, Detrended Fluctuation Analysis and Probabilistic Distribution Analysis, and the comparison of results from two systems reveals interesting parallels in sub-surface processes operating at both systems. Patterns of seismicity at both systems reveal complex but broadly similar long-term temporal patterns with cycles on the order of ~ 50- to ~ 200-days. These patterns are consistent with previously published spectral analyses of SO2 flux time-series at Soufriere Hills volcano, and are attributed to variations in the movement of magma in each system. Detrended Fluctuation Analysis determined that both volcanic systems showed a systematic relationship between the number of seismic events and the relative ‘roughness’ of the time-series, and explosions at Volcan de Colima showed a 1.5–2 year cycle; neither observation has a clear explanatory mechanism. At Volcan de Colima, analysis of repose intervals between seismic events shows long-term behaviour that responds to changes in activity at the system. Similar patterns for both volcanic systems suggest a common process or processes driving the observed signal but it is not clear from these results alone what those processes may be. Further attempts to model conduit processes at each volcano must account for the similarities and differences in activity within each system. The identification of some commonalities in the patterns of behaviour during long-lived dome-forming eruptions at andesitic volcanoes provides a motivation for investigating further use of time-series analysis as a monitoring tool.

Volcanic eruptions have been hypothesized as an iron supply mechanism for phytoplankton blooms; however, little direct evidence of stimulatory responses has been obtained in the field. Here we present the results of twenty-one 1–2 day bottle enrichment experiments from cruises in the South Atlantic and Southern Ocean which conclusively demonstrated a photophysiological and biomass stimulation of phytoplankton communities following supply of basaltic or rhyolitic volcanic ash. Furthermore, experiments in the Southern Ocean demonstrated significant phytoplankton community responses to volcanic ash supply in the absence of responses to addition of dissolved iron alone. At these sites, dissolved manganese concentrations were among the lowest ever measured in seawater, and we therefore suggest that the enhanced response to ash may have been a result of the relief of manganese (co)limitation. Our results imply that volcanic ash deposition events could trigger extensive phytoplankton blooms, potentially capable of significant impacts on regional carbon cycling.

The Southern and Austral Volcanic Zones of the Andes comprise 74 volcanic centres with known post-glacial activity. At least 21 of these have had one or more large explosive eruptions in the late Quaternary, dispersing tephra over vast areas. These tephra layers therefore have great potential as tephrochronological marker horizons in palaeoenvironmental studies in southern Chile and Argentina, a region that is particularly useful to study climate dynamics of the southern hemisphere. However, to date tephrochronology has rarely been fully utilised in this region as a correlation and dating tool. Here we review the existing post-glacial tephrostratigraphic record of the Southern and Austral Volcanic Zones, and compile a database of known occurrences of tephra from these volcanoes in ice and lacustrine, marine, peat, and cave sediment records. We address the inconsistencies in and revisions of the tephrostratigraphies presented in prior literature, and discuss the challenges in correlating tephras and the limitations of the tephrostratigraphic record in this area. This study highlights the many gaps that still exist in our knowledge of the eruptive histories of these volcanoes, but also reveals the largely under-utilised potential of tephra as a correlation tool in this region. This is exemplified by the severe lack of adequate geochemical analysis of tephra layers preserved in many lacustrine and peat sediment sections, which are particularly important tephrostratigraphic records in southern Chile and Argentina due to the paucity of surface preservation.

Between January 2011 and April 2012, Santorini volcano (Greece) experienced a period of unrest characterised by the onset of detectable seismicity and caldera-wide uplift. This episode of inflation represented the first sizeable intrusion of magma beneath Santorini in the past 50 years. We employ a new approach using Rn-δ C systematics to identify and quantify the source of diffuse degassing at Santorini during the period of renewed activity. Soil CO flux measurements were made across a network of sites on Nea Kameni between September 2010 and January 2012. Gas samples were collected in April and September 2011 for isotopic analysis of CO (δ C), and radon detectors were deployed during September 2011 to measure (Rn). Our results reveal a change in the pattern of degassing from the summit of the volcano (Nea Kameni) and suggest an increase in diffuse CO emissions between September 2010 and January 2012. High-CO-flux soil gas samples have δ C ∼ 0 ‰. Using this value and other evidence from the literature we conclude that these CO emissions from Santorini were a mixture between CO sourced from magma, and CO released by the thermal or metamorphic breakdown of crustal limestone. We suggest that this mixing of magmatic and crustal carbonate sources may account more broadly for the typical range of δ C values of CO (from ∼ - 4 ‰ to ∼ + 1 ‰) in diffuse volcanic and fumarole gas emissions around the Mediterranean, without the need to invoke unusual mantle source compositions. At Santorini a mixing model involving magmatic CO (with δ C of - 3 ± 2 ‰ and elevated (Rn)/CO ratios ∼ 10 - 10 Bq kg) and CO released from decarbonation of crustal limestone (with (Rn)/CO ∼ 30-300 Bq kg, and δ C of + 5 ‰) can account for the δ C and (Rn)/CO characteristics of the 'high flux' gas source. This model suggests ∼ 60 % of the carbon in the high flux deep CO end member is of magmatic origin. This combination of δ C and (Rn) measurements has potential to quantify magmatic and crustal contributions to the diffuse outgassing of CO in volcanic areas, especially those where breakdown of crustal limestone is likely to contribute significantly to the CO flux. © 2013 Elsevier B.V. All rights reserved.

Cyclical patterns of behaviour in timeseries of seismic and geodetic data at volcanoes are frequently observed during lava dome-building eruptions, and are particularly well-documented from the current eruption of the Soufriere Hills Volcano (SHV), Montserrat. However, the discontinuous nature of many SO 2 measurements often preclude the identification and quantitative analysis of cyclical patterns in degassing data. Here, using a long SO 2 timeseries from SHV, with continuous measurements since 2002, we explore for the first time degassing behaviour at a resolution comparable to that possible for seismic and deformation datasets. Timeseries analysis of flux data spanning 2002–2011 reveals that SO 2 emissions at SHV exhibit complex cyclicity, with dominant cycles evident on both multi-year and multi-week (~50 day) timescales. These cycles persist through phases of both active extrusion and eruptive pause, and show close similarities to periodic components previously identified at SHV in timeseries of seismicity, ground deformation and lava extrusion. The strength of expression or amplitude of degassing cycles, particularly on multi-week timescales, shows distinct temporal variation, and appears to correlate with the occurrence and nature of explosive activity occurring in 2002–2009. This suggests that the amplitude of surface gas flux cycles is modulated by physical conditions within the conduit. Direct quantitative comparison between seismicity, dome growth, and degassing for eruptive Phases 2 (2002–2003) and 3 (2005–2007) reveals that peaks in SO 2 flux appear to correspond broadly to enhanced lava extrusion and elevated seismicity within cycles of 30–50 days. However, time lags of 2, 4 and 7 days are observed between initial low-frequency seismic swarms and peaks in dome growth, SO 2 flux and rockfall event rate respectively. Multi-parameter correlations offer valuable insights into the controls on subsurface gas ascent, but further research is required to fully explore the contributions of permeability and overpressure, as well as other subsurface processes.

Several lines of evidence have previously been used to suggest that ice retreat after the last glacial maximum (LGM) resulted in regionally-increased levels of volcanic activity. It has been proposed that this increase in volcanism was globally significant, forming a substantial component of the post-glacial rise in atmospheric CO₂, and thereby contributing to climatic warming. However, as yet there has been no detailed investigation of activity in glaciated volcanic arcs following the LGM. Arc volcanism accounts for 90% of present-day subaerial volcanic eruptions. It is therefore important to constrain the impact of deglaciation on arc volcanoes, to understand fully the nature and magnitude of global-scale relationships between volcanism and glaciation.The first part of this paper examines the post-glacial explosive eruption history of the Andean southern volcanic zone (SVZ), a typical arc system, with additional data from the Kamchatka and Cascade arcs. In all cases, eruption rates in the early post-glacial period do not exceed those at later times at a statistically significant level. In part, the recognition and quantification of what may be small (i.e. less than a factor of two) increases in eruption rate is hindered by the size of our datasets. These datasets are limited to eruptions larger than 0.1 km³, because deviations from power-law magnitude–frequency relationships indicate strong relative under-sampling at smaller eruption volumes. In the southern SVZ, where ice unloading was greatest, eruption frequency in the early post-glacial period is approximately twice that of the mid post-glacial period (although frequency increases again in the late post-glacial). A comparable pattern occurs in Kamchatka, but is not observed in the Cascade arc. The early post-glacial period also coincides with a small number of very large explosive eruptions from the most active volcanoes in the southern and central SVZ, consistent with enhanced ponding of magma during glaciation and release upon deglaciation.In comparison to non-arc settings, evidence of post-glacial increases in rates of arc volcanism is weak, and there is no need to invoke significantly increased melt production upon ice unloading, as occurred in areas such as Iceland. Non-arc volcanoes may therefore account for a relatively higher proportion of global volcanic emissions in the early post-glacial period than is suggested by the relative contributions of arc and non-arc settings at the present day.The second part of this paper critically examines global eruption records, in an effort to constrain global-scale changes in volcanic output since the LGM. Accurate interpretation of these records relies on correcting both temporal and spatial variability in eruption recording. In particular, very low recording rates, which also vary spatially by over two orders of magnitude, prevent precise, and possibly even accurate, quantitative analysis. For example, if we assume record completeness for the past century, the number of known eruptions (volcanic explosivity index ≥ 2) from some low-latitude regions, such as Indonesia, is approximately 1 in 20,000 (0.005%) for the period 5–20 ka. There is a need for more regional-scale studies of past volcanism in such regions, where current data are extremely sparse. We attempt to correct for recording biases, and suggest a maximum two-fold (but potentially much less) increase in global eruption rates, relative to the present day, between 13 and 7 ka. Although volcanism may have been an important source of CO₂ in the early Holocene, it is unlikely to have been a dominant control on changes in atmospheric CO₂ after the LGM.

[1] A systematic survey of 3 years of L band interferometric synthetic aperture radar (InSAR) measurements of the Central American Volcanic Arc shows a striking lack of magmatic deformation. We make measurements at 20 of the 26 historically active volcanoes and demonstrate that none were deforming magmatically (2007–2010), although we do measure shallow subsidence associated with flow deposits and edifice loading at three volcanoes. The minimum detection rates for our survey, as estimated from the variance in time series of radar path delay, are relatively high due to strong variability of tropospheric water vapor. We compare the average detection threshold (2.4 cm/yr) to published InSAR measurements and show that the majority (~78%) of deformation events would have been measurable with the same level of noise as Central America. We calculate that if magmatic volcano deformation were spread evenly across historically active volcanoes worldwide, the probability of none of Central America's 26 volcanoes deforming would be < 1%. The lack of magmatic deformation in Central America may be indicative of differences in magma storage relative to other well-studied continental arcs. The high proportion of basalts that ascend directly from depth relative to andesites stored in the shallow crust may limit the potential for high magnitude deformation. Magma stored in vertically elongated reservoirs and high parental melt volatile contents that result in bubble-rich, compressible magmas at shallow depths may also reduce surface deformation. We consider the measurement and analysis of a lack of deformation at active volcanoes to be essential for realizing the potential of regional scale InSAR surveys.

Volcanoes are often remote, and have footprints that may extend across many hundreds or thousands of square kilometres. They are generally inaccessible during eruption, and may continue to be inaccessible for extended periods of time after eruption, while their products can be scattered or dispersed over regional or global scales. Consequently, since direct measurement can only provide us with part of the picture of many volcanic processes, remote sensing is now playing an increasingly important role in advancing understanding of the science underlying volcanic behaviour, on this planet and beyond (e.g. Mouginis-Mark et al. 2000; Sparks et al. 2012). Satellite, airborne and ground-based remote sensing are increasingly vital tools for monitoring active or potentially active volcanoes, and assessing their likely, real-time or time-averaged impact (Fig. 1). At the same time, the synoptic-scale surveys that are often well suited to remote-sensing techniques allow us to address questions about the fundamental processes that control volcano behaviour in a way that is not necessarily possible from individual case studies. New research is often driven by technological advancements in the development of novel sensors or launching of new platforms, meaning that the space agencies are increasingly involved in identifying scientific questions and priorities (e.g. Ferruci et al. 2012). Multiple and complementary data streams are increasingly being used both to monitor active volcanoes and advance volcanological science. The key geophysical parameters that comprise monitoring data streams typically include the categories of (i) seismicity, (ii) surface deformation, (iii) thermal measurements and (iv) gas flux and composition data as major components. While measurements of seismicity remain the domain of ground-based seismometers, remote-sensing techniques have made major contributions in each of the others. This Special Publication volume is concerned with the use of remote sensing at volcanoes. It is split into three parts, roughly arranged from the subsurface, to the surface, and then further afield as volcanic products are injected and dispersed into the atmosphere. The papers span a range of applications of remote-sensing techniques to monitor and understand (a) surface deformation, (b) surface thermal anomalies and (c) gas fluxes, as well as tracking ash and gas plumes from eruptions to gain insights into the extent of a volcano’s impacts. Volcanology is driven, in part, by the operational concerns surrounding volcano monitoring and hazard and crisis management but the goal of volcanological science is, at its heart, to understand the processes that underlie volcanic activity. This Special Publication is also concerned with how we go from observations to this deeper understanding, including the progress that can be made by integrating observations and modelling. While this volume focuses mainly on satellite-based remote sensing, integrating datasets from different platforms is also of vital importance, and so papers on airborne remote sensing and measurement from both manned and unmanned aircraft are also included.

Measuring volcano deformation is key to understanding the behaviour of erupting vol- canoes and detecting those in periods of unrest. Satellite techniques provide the opportunity to do so on a global scale but, with some notable exceptions, the deformation of volcanoes in the tropics has been understudied relative to those at higher latitudes, largely due to technical difficulties in applying Interferometric Synthetic Aperture Radar (InSAR). We perform a systematic survey of the Central American Volcanic Arc to investigate the appli- cability of Interferometric Synthetic Aperture Radar (InSAR) to volcanoes in the tropics. Volcano characteristics that may prevent InSAR measurement include: (1) dense vegetation cover; (2) per- sistent activity; and (3) steep slopes. Measurements of deformation are further inhibited by atmos- pheric artefacts associated with: (4) large changes in topographical relief. We present a systematic method for distinguishing between water vapour artefacts and true deformation. Our data show a linear relationship (c .2 cm /km) between the magnitudes of water vapour artefacts and volcano edifice height. For high relief volcanoes (e.g. Fuego, Guatemala, 3763 m a.s.l. (above sea level)) errors are of the order of 4-5 cm across the volcano's edifice but are less than 2 cm for lower relief (e.g. Masaya, Nicaragua, 635 m a.s.l.). Examples such as Arenal, Atitlan and Fuego illustrate that satellite acquisition strategies incorporating ascending and descending tracks are particularly important for studying steep-sided volcanoes. Poor coherence is primarily associated with temporal decorrelation, which is typically more rapid in southern Central America where Evergreen broadleaf vegetation dominates. Land-use classification is a better predictorof decorrelation rate thanvegetationindex.Comparisonof coher- ence for different radar wavelengths match expectations; high resolution X-band radar is best suited to local studies where high-resolution digital elevation models (DEMs) exist, while L-bandwavelengths are necessary for regional surveys.However, this is the first time that relation- ships between phase coherence and time, perpendicular baseline, radar wavelength, and land use have been quantified on the scale of a whole volcanic arc.

The Ozone Monitoring Instrument (OMI) is a satellite-based ultraviolet (UV) spectrometer with unprecedented sensitivity to atmospheric sulphur dioxide (SO2) concentrations. Since late 2004, OMI has provided a high-quality SO2 dataset with near-continuous daily global coverage. In this review, we discuss the principal applications of this dataset to volcano monitoring: (1) the detection and tracking of large eruption clouds, primarily for aviation hazard mitigation; and (2) the use of OMI data for long-term monitoring of volcanic degassing. This latter application is relatively novel, and despite showing some promise, requires further study into a number of key uncertainties. We discuss these uncertainties, and illustrate their potential impact on volcano monitoring with OMI through four new case studies. We also discuss potential future avenues of research using OMI data, with a particular emphasis on the need for greater integration between various monitoring strategies, instruments and datasets.

Caldera-forming eruptions of island volcanoes generate tsunamis by the interaction of different eruptive phenomena with the sea. Such tsunamis are a major hazard, but forward models of their impacts are limited by poor understanding of source mechanisms. The caldera-forming eruption of Santorini in the Late Bronze Age is known to have been tsunamigenic, and caldera collapse has been proposed as a mechanism. Here, we present bathymetric and seismic evidence showing that the caldera was not open to the sea during the main phase of the eruption, but was flooded once the eruption had finished. Inflow of water and associated landsliding cut a deep, 2.0–2.5 km3, submarine channel, thus filling the caldera in less than a couple of days. If, as at most such volcanoes, caldera collapse occurred syn-eruptively, then it cannot have generated tsunamis. Entry of pyroclastic flows into the sea, combined with slumping of submarine pyroclastic accumulations, were the main mechanisms of tsunami production., The Bronze Age eruption of Santorini is known to have generated tsunamis with caldera collapse as the likely mechanism. However, new bathymetric and seismic data presented by Nomikou et al. show that the entry of pyroclastic flows into the sea is the most likely tsunami-generating mechanism at Santorini.

Two basaltic glass samples have been studied, one from the Masaya volcano in Nicaragua and one from the Villarrica volcano in Chile using 57Fe Mössbauer spectroscopy. The iron(II) to iron(III) ratio has been determined for each sample. The 80 K data shows some separation of the sites and is fitted allowing for discrete doublets. © 2006 Springer Science+Business Media, Inc.

The halogens (F, Cl, Br, I) form an important suite of tracers of igneous processes, and may be used to track magmas from their point of origin, through their differentiation, evolution, saturation in vapour and escape to the Earth's atmosphere and hydrosphere. This review summarises the current state of the field, starting with an analysis of what is, and what is not, known about the distribution of halogens in Earth's interior reservoirs, and the principal controls on their behaviour during partial melting, crystallisation and degassing. With recent advances in measurement techniques, there is now a new opportunity to improve the inventories of halogen emissions from the open-vent, continuously-degassing systems that dominate the background contribution of volcanoes to the atmosphere. The different approaches to estimating the fluxes of HCl and HF to the atmosphere from arc and global volcanism are converging, and can now be used to place important constraints on the global cycling of halogens through subduction-zone systems. Arc-related volcanic emissions of halogens dominate the global halogen degassing budgets. Our current best estimates of halogen degassing fluxes from arc volcanoes are 4.3 (± 1) Tg/a (HCl), 0.5 (± 0.2) Tg/a (HF), 5-15 Gg/a (HBr) and 0.5-2 Gg/a (HI). © 2008 Elsevier B.V. All rights reserved.

Previous studies have used thermodynamic (i.e., “equilibrium”) models to simulate the changes in composition occurring as high-T magmatic gases mix and react with atmospheric gases at volcanic vents (or similarly, in lava domes and on lava lakes). There is current interest in using the outputs of these high-T mixture models to initialize low-T kinetic models for volcanic plume chemistry. However, recent studies have indicated that certain species (i.e., H2S) may not re-equilibrate within the high-T mixture. In this work we modify an existing high-T mixture model to treat H2S and/or H2 as inert (i.e., kinetically stable) whilst re-equilibrating all other species. This modification allows us to explore the implications of H2S and H2 kinetic stability for the production of oxidized trace species, and to generate more realistic initializations for low-T kinetic models. Our results show that if H2S and H2 do not re-equilibrate, high concentrations of Br, Cl and OH may be formed in high-T mixtures with smaller amounts of atmospheric gases than previously anticipated. For the average magmatic gas composition considered here, we show that the production of Br, Cl and OH requires a high-T mixture with ~ 1% air if H2S does not re-equilibrate and ~ 6% otherwise. In contrast, the formation of NO in high-T mixtures requires greater amounts (> 1%) of atmospheric gases because of low total N in the mixture. These results suggest that high-T mixtures may play a more significant role in the formation of oxidized Br and Cl species than previously recognized.

An understanding of the influence of anthropogenic and natural point sources of pollution on the compositions of distant airmasses requires the careful collection of atmospheric composition data over extended periods of time. Equally, a proper understanding of the influence of megacities, biomass burning plumes and persistent volcanic emissions on the oxidising capacity of the atmosphere requires analysis of the downstream fate of their associated gas and aerosol plumes. Marquez et al. (2005) recently presented some interesting data on the compositions of ambient atmospheric gases and particles at a sampling site in Mexico (Pico de Orizaba National Park, elevation 3300 m; Fig. 1) in late February and early March 2001. Two notable features of their data were the high concentrations of SO2 (mean value 14 ppb) and particulate sulphate (mean 5 μg m−3). The authors ascribed these features to the arrival of air from urban sources nearby, either Mexico City (200 km to the west, elevation 2240 m) or Puebla City (70 km to the west, elevation 2160 m). However, airmass parcels which arrive at the sampling location from either Mexico City, or Puebla, are likely to have passed close to the active volcano of Popocatepetl, 160 km from the sampling endpoint (Fig. 1). In their original paper, Marquez et al. (2005) dismissed any possible volcanic influence on their sampling on the basis of the absence of any volcanic (ash) plume visible in AVHRR weather satellite data. However, Popocatepetl, like many volcanoes, is a prodigious source of reactive gas and aerosol even in the absence of any visible ash plumes. ‘Passively’ degassing volcanoes are becoming increasingly well known as major contributors to the tropospheric gas and aerosol burden, and in particular that of SO2, sulphate and other particles

The impact of volcanic emissions, especially from passive degassing and minor explosions, is a source of uncertainty in estimations of aerosol indirect effects. Observations of the impact of volcanic aerosol on clouds contribute to our understanding of both present-day atmospheric properties and of the pre-industrial baseline necessary to assess aerosol radiative forcing. We present systematic measurements over several years at multiple active and inactive volcanic islands in regions of low present-day aerosol burden. The time-averaged indirect aerosol effects within 200 km downwind of island volcanoes are observed using Moderate Resolution Imaging Spectroradiometer (MODIS, 2002–2013) and Advanced Along-Track Scanning Radiometer (AATSR, 2002–2008) data. Retrievals of aerosol and cloud properties at Kīlauea (Hawai'i), Yasur (Vanuatu) and Piton de la Fournaise (la Réunion) are rotated about the volcanic vent to be parallel to wind direction, so that upwind and downwind retrievals can be compared. The emissions from all three volcanoes – including those from passive degassing, Strombolian activity and minor explosions – lead to measurably increased aerosol optical depth downwind of the active vent. Average cloud droplet effective radius is lower downwind of the volcano in all cases, with the peak difference ranging from 2–8 μm at the different volcanoes in different seasons. Estimations of the difference in Top of Atmosphere upward Short Wave flux upwind and downwind of the active volcanoes from NASA's Clouds and the Earth's Radiant Energy System (CERES) suggest a downwind elevation of between 10 and 45 Wm−2 at distances of 150–400 km from the volcano, with much greater local (< 80 km) effects. Comparison of these observations with cloud properties at isolated islands without degassing or erupting volcanoes suggests that these patterns are not purely orographic in origin. Our observations of unpolluted, isolated marine settings may capture processes similar to those in the pre-industrial marine atmosphere.

Mercury is a highly volatile, bioaccumulating toxic trace metal with a long (∼1yr) atmospheric residence time. Hg is strongly enriched in volcanic emanations, and volcanoes are the only natural sources of direct Hg emission to the free troposphere and stratosphere. However, there is considerable uncertainty over the annual emission rate of mercury from volcanoes. Previous estimates, based on limited measurements from volcanic plumes, span three orders of magnitude (∼100-103MgHg/yr), or from 103Mg) explosive eruptions overwhelm the total atmospheric burden several times per century, and account for ∼15% of total volcanic Hg emissions. © 2003 Elsevier Ltd. All rights reserved.

Seismic activity has been postulated as a trigger of volcanic eruption on a range of timescales, but demonstrating the occurrence of triggered eruptions on timescales beyond a few days has proven difficult using global datasets. Here, we use the historic earthquake and eruption records of Chile and the Andean southern volcanic zone to investigate eruption rates following large earthquakes. We show a significant increase in eruption rate following earthquakes of MW > 8, notably in 1906 and 1960, with similar occurrences further back in the record. Eruption rates are enhanced above background levels for ~ 12 months following the 1906 and 1960 earthquakes, with the onset of 3-4 eruptions estimated to have been seismically influenced in each instance. Eruption locations suggest that these effects occur from the near-field to distances of ~ 500 km or more beyond the limits of the earthquake rupture zone. This suggests that both dynamic and static stresses associated with large earthquakes are important in eruption-triggering processes and have the potential to initiate volcanic eruption in arc settings over timescales of several months. © 2008 Elsevier B.V. All rights reserved.

Santorini Volcano, the site of the catastrophic Minoan eruption in Greece, exhibits two distinct eruptive styles: small, effusive eruptions occur relatively frequently and build shields and domes of lava, whereas large explosive eruptions occur rarely, at intervals of 10,000-30,000 years. Both types of eruption were thought to incubate in a shallow magma chamber that is continually charged by small batches of melt injected into the chamber from below. However, petrological work suggests that at least 15% of the material ejected during the Minoan explosive eruption arrived in the magma chamber less than 100 years before the eruption. Here we use Satellite Radar Interferometry (InSAR) and Global Positioning System (GPS) measurements of surface deformation at Santorini to show that 10-20 million m 3 of magma have been intruded beneath the volcano since January 2011. This volume is equivalent to 10-50% of the volumes of recorded dome-forming eruptions. GPS and triangulation data show that this is the only volumetrically significant intrusion to have occurred since 1955, shortly after the last eruption. Our observations imply that whether Santorini is in an explosive or dome-forming phase, its shallow magma chamber is charged episodically by high-flux batches of magma. The durations of these events are short in comparison with the intervening periods of repose and their timing is controlled by the dynamics of deeper magma reservoirs.

article i nfo Establishing a time series of deformation is one of the keys to understanding and predicting the magmatic behaviour of active volcanoes. Satellite techniques represent an increasingly useful tool for measuring volcanic deformation over timescales spanning days to decades. Colombia contains numerous young or active volcanoes, many of which are inaccessible. We use L-band (23.6 cm wavelength) radar data acquired between 2006 and 2009, to survey 15 active volcanoes along the Colombian segment of the Northern Volcanic Zone. Analysis of 100 interferograms showed that the majority of volcanoes were not deforming. However, independent interferograms display an average subsidence of 3 cm on the northeast flank of Galeras, coinciding with the January 2008 eruption. We combine InSAR, field measurements and source modelling to determine the origin, size and location of the source of subsidence at Galeras. Our results suggest that this signal was caused by deflation of the magma chamber associated with the January 2008 event. Modelling provides insight into the depth to source (~2 km) and a volume change (−6.5×10 5 m 3 ) which is consistent with that derived from modelling contemporaneous tilts and the volume of material erupted. Previous studies based on various datasets support the existence of a resident/recurring chamber at this location, over a decadal timescale. Our InSAR results corroborate the hypothesis of shallow magma storage beneath Galeras and provide the first piece of evidence that can be linked to a particular eruption.

Lava flows can produce changes in topography on the order of 10s-100s of metres. A knowledge of the resulting volume\ud change provides evidence about the dynamics of an eruption. We present a method to measure topographic changes from the\ud differential InSAR phase delays caused by the height differences between the current topography and a Digital Elevation Model\ud (DEM). This does not require a pre-event SAR image, so it does not rely on interferometric phase remaining coherent during\ud eruption and emplacement. Synthetic tests predicts that we can estimate lava thickness of as little as �9 m, given a minimum of 5\ud interferograms with suitably large orbital baseine separations. In the case of continuous motion, such as lava flow subsidence, we\ud invert interferometric phase simultaneously for topographic change and displacement. We demonstrate the method using data from\ud Santiaguito volcano, Guatemala, and measure increases in lava thickness of up to 140 m between 2000 and 2009, largely associated\ud with activity between 2000 and 2005. We find a mean extrusion rate of 0.43 +/- 0.06 m3/s, which lies within the error bounds of\ud the longer term extrusion rate between 1922-2000. The thickest and youngest parts of the flow deposit were shown to be subsiding\ud at an average rate of �-6 cm/yr. This is the first time that flow thickness and subsidence have been measured simultaneously. We\ud expect this method to be suitable for measurment of landslides and other mass flow deposits as well as lava flows.