Volcanoes Worldwide Research Articles

Multi-year Satellite Observations of Sulfur Dioxide Gas Emissions and Lava Extrusion at Bagana Volcano, Papua New Guinea

Bagana, arguably the most active volcano in Papua New Guinea, has been in a state of near-continuous eruption for over 150 years, with activity dominated by sluggish extrusion of thick blocky lava flows. If current extrusion rates are representative, the entire edifice may have been constructed in only 300–500 years. Bagana exhibits a remarkably high gas flux to the atmosphere, with persistent sulfur dioxide (SO2) emissions of several thousand tons per day. This combination of apparent youth and high outgassing fluxes is considered unusual among persistently active volcanoes worldwide. We have used satellite observations of SO2 emissions and thermal infrared radiant flux to explore the coupling of lava extrusion and gas emission at Bagana. The highest gas emissions (up to 10 kt/day) occur during co-extrusive intervals, suggesting a degree of coupling between lava and gas, but gas emissions remain relatively high (~2,500 t/d) during inter-eruptive pauses. These passive emissions, which clearly persist for decades if not centuries, require a large volume of degassing but non-erupting magma beneath the volcano with a substantial exsolved volatile phase to feed the remarkable SO2 outgassing: an additional ~1.7–2 km3 basaltic andesite would be required to supply the excess SO2 emissions we observe in our study interval (2005 to present). That this volatile phase can ascend freely to the surface under most conditions is likely to be key to Bagana's largely effusive style of activity, in contrast with other persistently active silicic volcanoes where explosive and effusive eruptive styles alternate.

TROPOMI enables high resolution SO2 flux observations from Mt. Etna, Italy, and beyond

The newly launched imaging spectrometer TROPOMI onboard the Sentinel-5 Precursor satellite provides atmospheric column measurements of sulfur dioxide (SO2) and other gases with a pixel resolution of 3.5 × 7 km2. This permits mapping emission plumes from a vast number of natural and anthropogenic emitters with unprecedented sensitivity, revealing sources which were previously undetectable from space. Novel analysis using back-trajectory modelling of satellite-based SO2 columns allows calculation of SO2 flux time series, which would be of great utility and scientific interest if applied globally. Volcanic SO2 emission time series reflect magma dynamics and are used for risk assessment and calculation of the global volcanic CO2 gas flux. TROPOMI data make this flux time series reconstruction approach possible with unprecedented spatiotemporal resolution, but these new data must be tested and validated against ground-based observations. Mt. Etna (Italy) emits SO2 with fluxes ranging typically between 500 and 5000 t/day, measured automatically by the largest network of scanning UV spectrometers in the world, providing the ideal test-bed for this validation. A comparison of three SO2 flux datasets, TROPOMI (one month), ground-network (one month), and ground-traverse (two days) shows acceptable to excellent agreement for most days. The result demonstrates that reliable, nearly real-time, high temporal resolution SO2 flux time series from TROPOMI measurements are possible for Etna and, by extension, other volcanic and anthropogenic sources globally. This suggests that global automated real-time measurements of large numbers of degassing volcanoes world-wide are now possible, revolutionizing the quantity and quality of magmatic degassing data available and insights into volcanic processes to the volcanological community.

Metal transport in volcanic plumes: A case study at White Island and Yasur volcanoes

We report new data on trace metal emissions at White Island volcano (2015–2016), the first estimates from Yasur volcano (2016), and a brief review of global volcanic metal fluxes. Bi, Cd, Tl, Se, Sb, Te and Au are the most enriched metals and metalloids in the particulate phase of volcanic plumes. Observations of the particulate phase from the volcanic plumes of White Island, Yasur and Etna reveal abundant silicates and sulfates, sulfuric acid and more interestingly Zn oxide containing variable amounts of Cu. Thermodynamic speciation calculations show chloride species to be very important in terms of gaseous metal transport. Complexation with sulfur, in contrast, appears significant only in S-rich and Cl-poor gases, at high (>700 °C) temperature. Bromine, despite being a minor component of volcanic gases, is very efficient at complexing with metals, and especially with Cu, Pb, Bi, Zn and Cr. Metal emission rates in the study period at White Island are one to two orders of magnitude lower than reported previously, with ~6–10 kg/d of Cu, As, Se and Zn, and <2 kg/d of Pb, Bi, Tl, Sb, Te and Cd. Higher values were measured at Yasur volcano, ranging from ~175 kg/d of Cu, ~30 kg/d of As, Zn and Pb, and ~3 kg/d of Tl and Cd. Such fluxes of toxic metals are significant, representing a potential threat for local populations. We compare our new data to previously reported metal emissions from volcanoes worldwide, and show that high halogen emissions appear as the critical parameter controlling metal emissions, explaining the high metal fluxes measured at some intraplate volcanoes.

Bridging the gap: 40Ar/39Ar dating of volcanic eruptions from the ‘Age of Discovery’

Many volcanoes worldwide still have poorly resolved eruption histories, with the date of the last eruption often undetermined. One such example is Ascension Island, where the timing of the last eruption, and consequently, the activity status of the volcano, is unclear. Here, we use the 40Ar/39Ar dating technique to resolve ages of the three youngest lava flows on the island, which are hawaiites and mugearite with 1.5 – 1.9 wt% K2O. In dating these lavas, we provide the first evidence of Holocene volcanic activity on Ascension (0.51 ± 0.18 ka; 0.55 ± 0.12 ka; 1.64 ± 0.37 ka), determining that it should be classed as an active volcanic system. In addition, we demonstrate that the 40Ar/39Ar method can reproducibly date mafic lava flows younger than 1 ka, decreasing the gap between recorded history and geological dating. These results offer new prospects for determining patterns of late-Holocene volcanic activity; critical for accurate volcanic hazard assessment.

Globally Significant CO2 Emissions From Katla, a Subglacial Volcano in Iceland

AbstractVolcanoes are a key natural source of CO2, but global estimates of volcanic CO2 flux are predominantly based on measurements from a fraction of world's actively degassing volcanoes. We combine high‐precision airborne measurements from 2016 and 2017 with atmospheric dispersion modeling to quantify CO2 emissions from Katla, a major subglacial volcanic caldera in Iceland that last erupted 100 years ago but has been undergoing significant unrest in recent decades. Katla's sustained CO2 flux, 12–24 kt/d, is up to an order of magnitude greater than previous estimates of total CO2 release from Iceland's natural sources. Katla is one of the largest volcanic sources of CO2 on the planet, contributing up to 4% of global emissions from nonerupting volcanoes. Further measurements on subglacial volcanoes worldwide are urgently required to establish if Katla is exceptional, or if there is a significant previously unrecognized contribution to global CO2 emissions from natural sources.

Top–Down Precursory Volcanic Seismicity: Implications for ‘Stealth’ Magma Ascent and Long-Term Eruption Forecasting

Volcanic eruptions occur when a conduit forms to connect a crustal magma reservoir to Earth’s surface. Conduit formation is generally assumed to be a ‘bottom-up’ process and a major driver of precursory volcanic seismicity, which is the most commonly monitored parameter at volcanoes worldwide. If both assumptions are true, initial precursory seismicity should coincide spatially with petrologically-estimated magma reservoir depths. A review of well-constrained case studies of volcanoes that erupt after repose intervals of decades indicates that, to the contrary, initial precursory seismicity is consistently several kilometers shallower than the magma reservoir. We propose a model involving a three-phase process of unrest and eruption. Initial conduit formation occurs during a ‘staging’ phase, either aseismically or long before the onset of the immediate precursory run-up to eruption. Staging may involve slow ascent rates and/or small volumes. A destabilization phase then coincides with the onset of precursory seismicity, leading to an eruption phase. This model implies that, most critically, it may be possible to detect precursory magma ascent well before the onset of seismic activity by continuous monitoring of the state of stress in the mid to shallow crust.

Prevalence of Seismic Rate Anomalies Preceding Volcanic Eruptions in Alaska

Seismic rate increases often precede eruptions at volcanoes worldwide. However, many eruptions occur without such precursors, limiting their usefulness for forecasting in some cases. Additionally, identifying seismic rate increases near volcanoes with high levels of background seismicity is non-trivial and many periods of elevated seismicity occur without ensuing eruptions. Although these issues are commonly known, efforts to quantify them are limited. In this study, we consistently apply a common statistical tool, the -statistic, to seismically monitored eruptions in Alaska of various styles to determine the overall prevalence of seismic rate anomalies immediately preceding eruptions. We find that 6 out of 14 (43%) Volcanic Explosivity Index (VEI) ≥2 eruptions have statistically significant precursory seismic rate increases. Of these 6 eruptions, 4 of them occur at volcanoes with relatively felsic compositions, repose periods greater than 10 years, and VEI ≥ 3. Overall, our results confirm that seismic rate increases are common prior to larger eruptions at long dormant, ‘closed-system’ volcanoes, but uncommon preceding smaller eruptions at more frequently active, ‘open-system’ volcanoes with more mafic magmas. We also explore the rate of other anomalies not precursory to eruptions and investigate their origins. Some of these non-eruptive anomalies can be explained by aftershocks of regional seismic events, magmatic activity that did not lead to eruption, or unrest at other nearby volcanoes within our search radius. Some open-system volcanoes have high non-eruptive anomaly rates and low pre-eruptive anomaly rates and are thus not amenable to forecasting with this method. In this study, we find that 26% of anomalies lead to eruption, while 74% do not. With continued calibration at more volcanoes, the statistical tool that we apply may be used more broadly to analyze future periods of seismic unrest at other volcanoes, properly placing such episodes into the context of the long-term background rate. These results and this tool may be useful for informing future eruption forecasts around the world.

The Magmatic Gas Signature of Pacaya Volcano, With Implications for the Volcanic CO2 Flux From Guatemala

AbstractPacaya volcano in Guatemala is one of the most active volcanoes of the Central American Volcanic Arc (CAVA). However, its magmatic gas signature and volatile output have received little attention to date. Here, we present novel volcanic gas information from in‐situ (Multi‐GAS) and remote (UV camera) plume observations in January 2016. We find in‐plume H2O/SO2 and CO2/SO2 ratios of 2‐20 and 0.6‐10.5, and an end‐member magmatic gas signature of 80.5 mol. % H2O, 10.4 mol. % CO2, and 9.0 mol. % SO2. The SO2 flux is evaluated at 885 ± 550 tons/d. This, combined with co‐acquired volcanic plume composition, leads to H2O and CO2 fluxes of 2,230 ± 1,390 and 700 ± 440, and a total volatile flux of ∼3,800 tons/d. We use these results in tandem with previous SO2 flux budgets for Fuego and Santiaguito to estimate the total volcanic CO2 flux from Guatemala at ∼1,160 ± 600 tons/day. This calculation is based upon CO2/total S (St) ratios for Fuego (1.5 ± 0.75) and Santiaguito (1.4 ± 0.75) inferred from a gas (CO2/St ratio) versus trace‐element (Ba/La ratio) CAVA relationship. The H2O‐poor and low CO2/St ratio (∼1.0‐1.5) signature of Pacaya gas suggests dominant mantle‐wedge derivation of the emitted volatiles. This is consistent with 3He/4He ratios in olivine hosted fluid inclusions (FIs), which range between 8.4 and 9.0 Ra (being Ra the atmospheric 3He/4He ratio) at the upper limit of MORB range (8 ± 1 Ra). These values are the highest ever measured in CAVA and among the highest ever recorded in arc volcanoes worldwide, indicating negligible 4He contributions from the crust/slab.

Constraints on the geomorphological evolution of the nested summit craters of Láscar volcano from high spatio-temporal resolution TerraSAR-X interferometry

Small-scale geomorphological changes that are associated with the formation, development, and activity of volcanic craters and eruptive vents are often challenging to characterize, as they may occur slowly over time, can be spatially localized, and difficult, or dangerous, to access. Using high-spatial and high-temporal resolution synthetic aperture radar (SAR) imagery collected by the German TerraSAR-X (TSX) satellite in SpotLight mode in combination with precise topographic data as derived from Pleiades-1A satellite data, we investigate the surface deformation within the nested summit crater system of Lascar volcano, Chile, the most active volcano of the central Andes. Our aim is to better understand the structural evolution of the three craters that comprise this system, to assess their physical state and dynamic behavior, and to link this to eruptive activity and associated hazards. Using multi-temporal SAR interferometry (MT-InSAR) from ascending and descending orbital geometries, we retrieve the vertical and east-west components of the displacement field. This time series indicates constant rates of subsidence and asymmetric horizontal displacements of all summit craters between June 2012 and July 2014, as well as between January 2015 and March 2017. The vertical and horizontal movements that we observe in the central crater are particularly complex and cannot be explained by any single crater formation mechanism; rather, we suggest that short-term activities superimposed on a combination of ongoing crater evolution processes, including gravitational slumping, cooling and compaction of eruption products, as well as possible piston-like subsidence, are responsible for the small-scale geomorphological changes apparent in our data. Our results demonstrate how high-temporal resolution synthetic aperture radar interferometry (InSAR) time series can add constraints on the geomorphological evolution and structural dynamics of active crater and vent systems at volcanoes worldwide.

The September 14, 2015 phreatomagmatic eruption of Nakadake first crater, Aso Volcano, Japan: Eruption sequence inferred from ballistic, pyroclastic density current and fallout deposits

An explosive eruption occurred at Nakadake first crater, Aso Volcano in central Kyushu, southwestern Japan, on September 14, 2015. The sequence and causes of the eruption were reconstructed from the distribution, textures, grain-size, component and chemical characteristics of the related deposits, and video record. The eruptive deposits are divided into ballistics, pyroclastic density current and ash-fall deposits. A large number of ballistic clasts (mostly <10cm in diameter; maximum size 1.6m) are scattered within about 500m from the center of the crater. Almost half of the ballistics appear as fresh and unaltered basaltic andesite rocks interpreted to be derived from a fresh batch of magma, while the rest is weakly to highly altered clasts. A relatively thin ash derived from pyroclastic density currents covered an area of 2.3km2 with the SE-trending main axis and two minor axes to the NE and NW. The pyroclastic density current deposit (maximum thickness<10cm even at the crater rim) is wholly fine grained, containing no block-sized clasts. Based on the isopach map, the mass of the pyroclastic density current deposit was estimated at ca. 5.2×104tons. The ash-fall deposit is finer grained and clearly distributed to about 8km west of the source crater. The mass of the ash-fall deposit was calculated at about 2.7×104tons. Adding the mass of the pyroclastic density current deposit, the total discharged mass of the September 14, 2015 eruption was 7.9×104 tons. The September 14 pyroclastic density current and ash-fall deposits consist of glass shards (ca. 30%), crystals (20–30%) and lithic (40–50%) grains. Most glass shards are unaltered poorly crystallized pale brown glasses which probably resulted from quenching of juvenile magma. This suggests that the September 14, 2015 event at the Nakadake first crater was a phreatomagmatic eruption. Similar phreatomagmatic eruptions occurred at the same crater on September 6, 1979 and April 20, 1990 whose eruptive masses were one order larger than that of the September 14, 2015 eruption. These events highlight the potential hazard from phreatic or phreatomagmatic eruptions at Nakadake first crater, and provide useful information that will assist in preventing or mitigating future disasters at other similar volcanoes worldwide.

Lava flow hazard prediction and monitoring with UAS: a case study from the 2014\u20132015 P\u0101hoa lava flow crisis, Hawai\u2018i

Accurately predicting lava flow path behavior is critical for active crisis management operations. The advance and emplacement of pāhoehoe flows modifies and inverts pre-existing topography, prompting the need for rapid and accurate updates to the topographic models used to forecast flow paths. The evolution and velocity of pāhoehoe flows are dependent on macro and micro topography, the slope of the descent path, effusion rate, and rheology. During the 2014–2015 Pāhoa crisis on the island of Hawai‘i, we used a low-altitude unmanned aerial system (UAS) to quickly and repeatedly image the active front of a slowly advancing pāhoehoe lava flow. This imagery was used to generate a series of 1 m resolution bare-earth digital elevation models (DEMs) and associated paths of steepest descent over the study area. The spatial resolution and timeliness of these UAS-derived models are an improvement over the existing topographic data used by managers during the crisis. Results from a stepwise resampling experiment suggest that the optimum DEM resolution for generating accurate pāhoehoe flow paths through lowland tropical forest environments is between 1 and 3 m. Our updated models show that future flows in this area will likely be deflected by these newly emplaced flows, possibly threatening communities not directly impacted by the original 2014–2015 lava flow. We demonstrate the value of deploying UAS during a dynamic volcanic crisis and suggest that this technology can fill critical monitoring gaps for Kīlauea and other active volcanoes worldwide.

A new approach to investigate an eruptive paroxysmal sequence using camera and strainmeter networks: Lessons from the 3–5 December 2015 activity at Etna volcano

Explosive sequences are quite common at basaltic and andesitic volcanoes worldwide. Studies aimed at short-term forecasting are usually based on seismic and ground deformation measurements, which can be used to constrain the source region and quantify the magma volume involved in the eruptive process. However, during single episodes of explosive sequences, integration of camera remote sensing and geophysical data are scant in literature, and the total volume of pyroclastic products is not determined. In this study, we calculate eruption parameters for four powerful lava fountains occurring at the main and oldest Mt. Etna summit crater, Voragine, between 3 and 5 December 2015. These episodes produced impressive eruptive columns and plume clouds, causing lapilli and ash fallout to more than 100 km away. We analyse these paroxysmal events by integrating the images recorded by a network of monitoring cameras and the signals from three high-precision borehole strainmeters. From the camera images we calculated the total erupted volume of fluids (gas plus pyroclastics), inferring amounts from 1.9×109 m3 (first event) to 0.86×109 m3 (third event). Strain changes recorded during the first and most powerful event were used to constrain the depth of the source. The ratios of strain changes recorded at two stations during the four lava fountains were used to constrain the pyroclastic fraction for each eruptive event. The results revealed that the explosive sequence was characterized by a decreasing trend of erupted pyroclastics with time, going from 41% (first event) to 13% (fourth event) of the total erupted pyroclastic volume. Moreover, the volume ratio fluid/pyroclastic decreased markedly in the fourth and last event. To the best of our knowledge, this is the first time ever that erupted volumes of both fluid and pyroclastics have been estimated for an explosive sequence from a monitoring system using permanent cameras and high precision strainmeters. During future explosive paroxysmal sequences this new approach might help in monitoring their evolution also to understand when/if they are going to finish. Knowledge of the total gas and pyroclastic fractions erupted during each lava fountain episode would improve our understanding of their processes and eruptive behaviour.

Crystal-rich lava dome extrusion during vesiculation: An experimental study

Lava dome-forming eruptions represent a common eruptive style and a major hazard at numerous active volcanoes worldwide. The extrusion mechanics of crystal-rich lava domes and the influence of volatiles on the transition from viscous to brittle behaviour during lava dome extrusion remain unclear. Understanding how gas exsolution and crystallinity control effusive versus explosive eruption behaviour is essential. Here, we present new experimental results on the rheology of synthesised, crystal-rich (50 to 80vol% quartz crystals), hydrous (4.2wt% H2O in the glass) dacite samples, which vesiculate from 5 to 27vol% gas bubbles at high temperatures (from glass transition temperature to 797°C) during deformation conducted in a parallel plate viscometer (constant stress at 0.63–0.64MPa, and variable strain-rates ranging from 8.32·10−8 to 3.58·10−5s−1). The experiments reproduce certain aspects of lava dome deformation in volcanic conduits during vesiculation of the residual melt, instigated in the experiments by increasing temperature. During gas exsolution (i.e. nucleation and growth of gas-pressurised bubbles) and volume inflation, we find that the rheological lubrication of the system during deformation is strongly dictated by the initial crystallinity. At crystal contents <60vol%, gas bubbles form and coalesce during expansion and viscous deformation, favouring strain localisation and gas permeability within shear bands, which control the overall sample rheology. At crystallinities of 60 to 70vol%, gas exsolution generates pressurisation (i.e. pore pressure increase) within the bubbles trapped in the solid crystal clusters, and embryonic formation of microscopic fractures through melt and crystals drives the system to a brittle behaviour. At higher crystallinity (80vol%) vesiculation leads to large pressurisation, which then triggers extensive brittle fragmentation. Through macroscopic fractures, outgassing determines the rheological stalling of the system. In the light of these results we propose a rheological description of crystal-rich lava dome mechanics. The contrasting experimental behaviours at different crystallinities have implications for the style of slow-ascending dome-forming eruptions. All other factors being equal, our experiments suggest that crystal-poor magmas will undergo efficient outgassing, reducing the potential for an explosive eruption. Conversely, crystal-rich magmas may experience limited outgassing and larger gas overpressures during vesiculation, therefore increasing the potential for an explosive eruption.

Dome-like behaviour at Mt. Etna: The case of the 28 December 2014 South East Crater paroxysm

On the 28 December 2014, a violent and short paroxysmal eruption occurred at the South East Crater (SEC) of Mount Etna that led to the formation of huge niches on the SW and NE flanks of the SEC edifice from which a volume of ~3 × 106 m3 of lava was erupted. Two basaltic lava flows discharged at a rate of ~370 m3/s, reaching a maximum distance of ~5 km. The seismicity during the event was scarce and the eruption was not preceded by any notable ground deformation, which instead was dramatic during and immediately after the event. The SO2 flux associated with the eruption was relatively low and even decreased few days before. Observations suggest that the paroxysm was not related to the ascent of volatile-rich fresh magma from a deep reservoir (dyke intrusion), but instead to a collapse of a portion of SEC, similar to what happens on exogenous andesitic domes. The sudden and fast discharge eventually triggered a depressurization in the shallow volcano plumbing system that drew up fresh magma from depth. Integration of data and observations has allowed to formulate a novel interpretation of mechanism leading volcanic activity at Mt. Etna and on basaltic volcanoes worldwide.

What happens to in‐soil Radon activity during a long‐lasting eruption? Insights from Etna by multidisciplinary data analysis

AbstractWe analyze short‐ to long‐term changes (from days to months) in Radon (Rn) activity measured nearby (&lt;2 km) the eruptive fractures that fed a lava effusion at Mt. Etna, Italy, between 13 May 2008 and 6 July 2009. The N120‐140°E eruptive fractures opened between 3050 and 2620 m above sea level before a dike‐forming intrusion fed the ∼14 month‐long lava emission. Our high‐rate data streams include: Rn, ambient parameters (barometric pressure and soil temperature), and seismic data (earthquakes and volcanic tremor) recorded from January 2008 to July 2009. The analysis highlights repeated episodes of rock‐fracturing related to seismic swarms, and vigorous gas pulses and peak values in Rn emissions (maximum ∼4.1×105 Bq/m3 on 16 November 2008), which we interpreted in a conceptual model as the response to inputs from the magmatic system during the eruption. This multidisciplinary study: (i) provides evidence of a close relationship between Rn emission at a fumarole near the summit active craters and local earthquakes, and (ii) enables exploring the important role of the volcanic source on the temporal development of the Rn flux, which may account for the much higher (≫94 m/d) ascent speed of the Rn carrier (vapor) than diffusion. The close location of Rn probes to the active conduits, along with the application of our multidisciplinary approach, may shed new light on the internal dynamics of other active volcanoes worldwide.

Modelling ground deformation patterns associated with volcanic processes at the Okataina Volcanic Centre

The Okataina Volcanic Centre (OVC) is one of two large active rhyolite centres in the modern Taupo Volcanic Zone (TVZ) in the North Island of New Zealand. It is located in a complex section of the Taupo rift, a tectonically active section of the TVZ. The most recent volcanic unrest at the OVC includes the ~1315CE Kaharoa and 1886 Tarawera eruptions. Current monitoring activity at the OVC includes the use of continuous GPS receivers (cGPS), lake levelling and seismographs. The ground deformation patterns preceding volcanic activity the OVC are poorly constrained and restricted to predictions from basic modelling and comparison to other volcanoes worldwide. A better understanding of the deformation patterns preceding renewed volcanic activity is essential to determine if observed deformation is related to volcanic, tectonic or hydrothermal processes. Such an understanding also means that the ability of the present day cGPS network to detect these deformation patterns can also be assessed.The research presented here uses the finite element (FE) modelling technique to investigate ground deformation patterns associated with magma accumulation and diking processes at the OVC in greater detail. A number of FE models are produced and tested using Pylith software and incorporate characteristics of the ~1315CE Kaharoa and 1886 Tarawera eruptions, summarised from the existing body of research literature. The influence of a simple ring fault structure at the OVC on the modelled deformation is evaluated. The ability of the present-day continuous GPS (cGPS) GeoNet monitoring network to detect or observe the modelled deformation is also considered. The results show the modelled horizontal and vertical displacement fields have a number of key features, which include prominent lobe based regions extending northwest and southeast of the OVC. The results also show that the ring fault structure increases the magnitude of the displacements inside the caldera, in particular in the vicinity of the southern margin. As a result, some of the cGPS stations in the vicinity of the OVC are more important for measuring deformation related to volcanic processes than others. The results have important implications for how any future observed deformation at the OVC is observed and interpreted.

Using a process-based model of pre-eruptive seismic patterns to forecast evolving eruptive styles at Sinabung Volcano, Indonesia

Most volcanoes worldwide are not monitored in real-time; for those that are, patterns of pre-eruptive earthquakes coupled with conceptual models of magma ascent enable short-term forecasting of eruption onset. Basic event locations, characterization of background seismicity, and recognition of changes in earthquake types and energy release are most important to successful eruption forecasting. During renewed activity at Sinabung volcano, Indonesia, this approach was used by the Center for Volcanology and Geological Hazards Mitigation (CVGHM) and the USGS Volcano Disaster Assistance Program to forecast eruption onset, identify changes in eruptive styles and raise or lower alert levels and extend or contract evacuation zones. After >400years of quiescence, Sinabung began erupting in August 2010. The volcano was unmonitored at the onset of these eruptions, which were phreatic, but soon after a monitoring network was installed by CVGHM. Increasing swarms of high-frequency volcano tectonic (VT) earthquakes were used to forecast continuing phreatic eruptions. Volcanic activity decreased in mid-September 2010, while additional intrusions at depth (inferred from continued distal VT swarms) continued through September 2013, when explosive phreatic eruptions recurred. Explosive eruptions were forecast based on increases in real-time seismic amplitude measurement (RSAM) and VT seismicity. Seismicity changed markedly in late November and early December 2013 with the occurrence of deep earthquakes and an overall transition from low-frequency (LF) dominated and irregular (in time and magnitude) earthquakes to more regular LF and hybrid seismicity – a transition that accompanied the continued rise, eventual emergence and growth of a lava dome in the summit crater. This lava dome was first observed on 18 December. In late December 2013 to early January 2014, the eruptive style changed again as additional ascending magma deformed the summit and the dome grew beyond the capacity of the summit crater, resulting in the en masse collapse of the lava dome (2Mm3) on 11 January and the largest pyroclastic flow to date. The collapse was forecast on the basis of a several order of magnitude increase in RSAM, continued strong distal VT seismicity, an increase in proximal seismicity, and large-scale observed deformation of the summit area. Similarly, a later collapse of a second summit lava dome on 1 February 2014 was forecast on the basis of increased distal seismicity. Here, we demonstrate how a process-based volcano seismicity model was used in combination with real-time data to forecast the time and magnitude of eruptions, as well as changes in eruption style.

Characteristics of puffing activity revealed by ground-based, thermal infrared imaging: the example of Stromboli Volcano (Italy)

Puffing, i.e., the frequent (1 s ca.) release of small (0.1–10 m3), over-pressurized pockets of magmatic gases, is a typical feature of open-conduit basaltic volcanoes worldwide. Despite its non-trivial contribution to the degassing budget of these volcanoes and its recognized role in volcano monitoring, detection and metering tools for puffing are still limited. Taking advantage of the recent developments in high-speed thermal infrared imaging, we developed a specific processing algorithm to detect the emission of individual puffs and measure their duration, size, volume, and apparent temperature at the vent. As a test case, we applied our method at Stromboli Volcano (Italy), studying “snapshots” of 1 min collected in the years 2012, 2013, and 2014 at several vents. In all 3 years, puffing occurred simultaneously at three or more vents with variable features. At the scale of the single vent, a direct relationship links puff temperature and radius, suggesting that the apparent temperature is mostly a function of puff thickness, while the real gas temperature is constant for all puffs. Once released in the atmosphere, puffs dissipate in less than 20 m. On a broader scale, puffing activity is highly variable from vent to vent and year to year, with a link between average frequency, temperature, and volume from 136 puffs per minute, 600 K above ambient temperature, 0.1 m3, and the occasional ejection of pyroclasts to 20 puffs per minute, 3 K above ambient, 20 m3, and no pyroclasts. Frequent, small, hot puffs occur at random intervals, while as the frequency decreases and size increases, an increasingly longer minimum interval between puffs, up to 0.5 s, appears. These less frequent and smaller puffs also display a positive correlation between puff volume and the delay from the previous puff. Our results suggest an important role of shallow bubble coalescence in controlling puffing activity. The smaller and more frequent puffing at “hotter” vents is in agreement with the rapid rise of small gas pockets with only limited coalescence, while slower rise, allowing more time for coalescence, leads to larger but less frequent puffing at “colder” vents. This link between puffing style and vent thermal state points to a feedback between gas flux and magma temperature (and viscosity), where higher gas flux stirs and heats the magma, which, by getting less viscous, becomes a preferential way for bubble rise. Such a link has implications for the monitoring of the state of the shallow conduit at open-vent volcanoes as well as for determination of their total gas budget, relevant for hazard forecast and environmental studies.

Seismicity patterns during a period of inflation at Sierra Negra volcano, Galápagos Ocean Island Chain

Basaltic shield volcanoes of the western Galápagos islands are among the most rapidly deforming volcanoes worldwide, but little was known of the internal structure and brittle deformation processes accompanying inflation and deflation cycles. A 15-station broadband seismic array was deployed on and surrounding Sierra Negra volcano, Galápagos from July 2009 through June 2011 to characterize seismic strain patterns during an inter-eruption inflation period and to evaluate single and layered magma chamber models for ocean island volcanoes. We compare precise earthquake locations determined from a 3D velocity model and from a double difference cluster method. Using first-motion of P-arrivals, we determine focal mechanisms for 8 of the largest earthquakes (ML≤1.5) located within the array. Most of the 2382 earthquakes detected by the array occurred beneath the broad (∼9 km-wide) Sierra Negra caldera, at depths from surface to about 8 km below sea level. Although outside our array, frequent and larger magnitude (ML≤3.4) earthquakes occurred at Alcedo and Fernandina volcano, and in a spatial cluster beneath the shallow marine platform between Fernandina and Sierra Negra volcanoes. The time–space relations and focal mechanism solutions from a 4-day long period of intense seismicity June 4–9, 2010 along the southeastern flank of Sierra Negra suggests that the upward-migrating earthquake swarm occurred during a small volume intrusion at depths 5–8 km subsurface, but there was no detectable signal in InSAR data to further constrain geometry and volume. Focal mechanisms of earthquakes beneath the steep intra-caldera faults and along the ring fault system are reverse and strike-slip. These new seismicity data integrated with tomographic, geodetic, and petrological models indicate a stratified magmatic plumbing system: a shallow sill beneath the large caldera that is supplied by magma from a large volume deeper feeding system. The large amplitude inter-eruption inflation of the shallow sill beneath the Sierra Negra caldera is accompanied by only very small magnitude earthquakes, although historical records indicate that larger magnitude earthquakes (Mw <6) occur during eruptions, trapdoor faulting episodes without eruptions, and large volume flank intrusions.

What factors control superficial lava dome explosivity?

Dome-forming eruption is a frequent eruptive style and a major hazard on numerous volcanoes worldwide. Lava domes are built by slow extrusion of degassed, viscous magma and may be destroyed by gravitational collapse or explosion. The triggering of lava dome explosions is poorly understood: here we propose a new model of superficial lava-dome explosivity based upon a textural and geochemical study (vesicularity, microcrystallinity, cristobalite distribution, residual water contents, crystal transit times) of clasts produced by key eruptions. Superficial explosion of a growing lava dome may be promoted through porosity reduction caused by both vesicle flattening due to gas escape and syn-eruptive cristobalite precipitation. Both processes generate an impermeable and rigid carapace allowing overpressurisation of the inner parts of the lava dome by the rapid input of vesiculated magma batches. The relative thickness of the cristobalite-rich carapace is an inverse function of the external lava dome surface area. Explosive activity is thus more likely to occur at the onset of lava dome extrusion, in agreement with observations, as the likelihood of superficial lava dome explosions depends inversely on lava dome volume. This new result is of interest for the whole volcanological community and for risk management.