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In volcanic regions ascending magma is subject to depressurization and is generally accompanied by exsolution of volatiles. We assume a process in which these volatiles propagate upward across newly fractured and permeable rock layers, bringing a sharp increase of pore pressure and temperature within a thin disc-shaped region (inclusion). Thermo-poro-elastic (TPE) inclusion models provide a mechanism to explain seismicity and deformation induced by p and T changes in absence of new magma emplacement in volcanic contexts. They are also suitable to represent the mechanical effects due to fluid extraction and re-injection in geothermal fields. In the present work analytic solutions are provided for the displacement, strain and stress fields assuming a TPE unbounded medium. Significant deviatoric stress is generated by positive increments of pore pressure and temperature: the stress field is fully deviatoric outside the TPE inclusion, but a strong isotropic stress component is present within, leading to highly heterogeneous faulting mechanisms: if the disc plane is horizontal, thrust faulting mechanisms are favoured within the TPE disc over optimally oriented faults and normal mechanisms above. The model is easily generalized to a vertically thick disc with variable temperature and pore-pressure changes: then, an extensional environment can be obtained even within the TPE inclusion assuming upward decreasing of pore pressure and temperature changes. The supplied analytical solution may be used to model near-field TPE inclusion effects and to validate more complex numerical modelling. [ABSTRACT FROM AUTHOR]

Long-period (LP) seismic events have occurred repeatedly at Galeras volcano, Colombia, during the transition from effusive dome formation to explosive Vulcanian eruptions. Since 1989, two types of LP events have been observed there: one characterized by long-lasting, decaying harmonic oscillations (NLP events) and the other by non-harmonic oscillatory features (BLP events). NLP events are attributed to resonances of a dusty gas-filled crack in the magma plugging the eruptive conduit. Sixteen episodes of NLP events occurred at Galeras during 1992–2010, each characterized by systematic temporal variations in the frequencies and quality factors of NLP events. Our and previous estimates of crack model parameters during three of those NLP episodes indicate that the similar temporal variations in crack geometry and fluid properties can be explained by an increase in the ash content within the crack and a decrease in crack volume. We found that NLP events, associated with low SO2 fluxes, are anticorrelated with BLP events, which are accompanied by high SO2 emissions. From our observations and analytical results, we inferred that BLP events are generated by resonances of open cracks in the uppermost magma plug, corresponding to tuffisite veins, that efficiently transfer volcanic gases. After sufficient degassing and densification, the magma plug effectively seals the conduit. The growing overpressure in the deeper magma is then released through a shear fracture along the conduit margin. The intrusion of deeper, vesiculated magma into the shear fracture depressurizes and fragments the magma, producing a dusty gas and triggering the crack resonances that generate NLP events. Our results thus indicate that the evolution of the properties of the magma plug controls the occurrences of BLP and NLP events at Galeras. Although NLP events do not always precede explosive eruptions, they indicate that an important overpressure is building in the shallow conduit. [ABSTRACT FROM AUTHOR]

Lusi is a sediment-hosted hydrothermal system located near Sidoarjo in Central Java, Indonesia, and has erupted continuously since May 2006. This mud eruption extends over a surface of ∼7 km2, and is framed by high containment dams. The present study investigates the geometry of the subsurface structures using a detailed gravimetric model to visualize in 3-D the Lusi system and surrounding lithologies. The obtained residual Bouguer anomaly map, simulated through geostatistical interpolation methods, supports the results of previous deformation studies. The negative gravity anomaly zones identified at Lusi are interpreted as fractured areas through which fluids can ascend towards the surface. A 3-D detailed geological model of the area was constructed with Geomodeller™ to highlight the main features. This model relies on the structures' density contrasts, the interpreted residual Bouguer anomaly map, and geological data from previous authors. 3-D algorithms were used to calculate the gravity response of the model and validate it by inverse methods. The final output is a gravity constrained 3-D geological model of the Lusi mud edifice. These results provide essential details on the Lusi subsurface and may be useful for possible future geothermal resource exploitation and for the risk mitigation plans related to the maintenance of the man-made framing embankment. [ABSTRACT FROM AUTHOR]

Gas flux in volcanic conduits is often associated with long-period oscillations known as seismic tremor (Lesage et al. ; Nadeau et al.). In this study, we revisit and extend the 'magma wagging'and 'whirling'models for seismic tremor, in order to explore the effects of gas flux on the motion of a magma column surrounded by a permeable vesicular annulus (Jellinek & Bercovici; Bercovici et al. ; Liao et al.). We find that gas flux flowing through the annulus leads to a Bernoulli effect, which causes waves on the magma column to become unstable and grow. Specifically, the Bernoulli effects are associated with torques and forces acting on the magma column, increasing its angular momentum and energy. As the displacement of the magma column becomes large due to the Bernoulli effect, frictional drag on the conduit wall decelerates the motions of the column, restoring them to small amplitude. Together, the Bernoulli effect and the damping effect contribute to a self-sustained wagging-and-whirling mechanism that help explain the longevity of long-period seismic tremor. [ABSTRACT FROM AUTHOR]

Thermal gradients due to magma dynamics in active volcanic areas may affect the emanating power of the substrate and the background level of radon signal. This is particularly effective in subvolcanic substrates where intense hydrothermal alteration and/or weathering processes generally form hydrous minerals, such as zeolites able to store and release great amounts of H2O (up to ~25 wt.%) at relative low temperatures. To better understand the role played by thermally induced devolatilization reactions on the radon signal, a new experimental setup has been developed for measuring in real time the radon emission from a zeolitized volcanic tuff. Progressive dehydration phenomena with increasing temperature produce radon emissions two orders of magnitude higher than those measured during rock deformation, microfracturing and failure. In this framework, mineral devolatilization reactions can contribute significantly to produce radon emissions spatially heterogeneous and non-stationary in time, resulting in a transient state dictated by temperature gradients and the carrier effects of subsurface gases. Results from these experiments can be extrapolated to the temporal and spatial scales of magmatic processes, where the ascent of small magma batches from depth causes volatile release due to dehydration phenomena that increase the radon signal from the degassing host rock material. [ABSTRACT FROM AUTHOR]

The volatile content in magmas is fundamental for the triggering and style of volcanic eruptions. Carbon dioxide, the second most abundant volatile component in magmas after H2O, is the first to reach saturation upon ascent and depressurization. We investigate experimentally CO2-bubble nucleation in trachybasalt and trachyte melts at high temperature and high pressure (HT and HP) through wetting-angle measurements on different (sialic, mafic or oxide) phenocryst phases. The presence of crystals lowers the supersaturation required for CO2-bubble nucleation up to 37 per cent (heterogeneous nucleation, HeN), with a minor role of mineral chemistry. Different from H2O-rich systems, feldspar crystals are effective in reducing required supersaturation for bubble nucleation. Our data suggest that leucite, the dominant liquidus phase in ultrapotassic systems at shallow depth (i.e. <100 MPa), facilitates late-stage, extensive magma vesiculation through CO2HeN, which may explain the shifting of CO2-rich eruptive systems towards an apparently anomalous explosive behaviour. [ABSTRACT FROM AUTHOR]

Artificial neural networks (ANNs) are a valuable and well-established inversion technique for the estimation of geophysical parameters from satellite images; once trained, they help generate very fast results. Furthermore, satellite remote sensing is a very effective and safe way to monitor volcanic eruptions in order to safeguard the environment and the people affected by those natural hazards.This paper describes an application of ANNs as an inverse model for the simultaneous estimation of columnar content and height of sulphur dioxide (SO2) plumes from volcanic eruptions using hyperspectral data from remote sensing.In this study two ANNs were implemented in order to emulate a retrieval model and to estimate the SO2 columnar content and plume height. ANNs were trained using all infrared atmospheric sounding interferometer (IASI) channels between 1000–1200 and 1300–1410 cm−1 as inputs, and the corresponding values of SO2 content and height of plume, obtained from the same IASI channels using the SO2 retrieval scheme by Carboni et al., as target outputs.The retrieval is demonstrated for the eruption of the Eyjafjallajökull volcano (Iceland) for the months of 2010 April and May and for the Grimsvotn eruption during 2011 May.Both neural networks were trained with a time series consisting of 58 hyperspectral eruption images collected between 2010 April 14 and May 14 and 16 images from 2011 May 22 to 26, and were validated on three independent data sets of images of the Eyjafjallajökull eruption, one in April and the other two in May, and on three independent data sets of the Grímsvötn volcanic eruption that occurred in 2011 May. The root mean square error (RMSE) values between neural network outputs and targets were lower than 20 Dobson units (DU) for SO2 total column and 200 millibar (mb) for plume height.The RMSE was lower than the standard deviation of targets for the Grímsvötn eruption. The neural network had a lower retrieval accuracy when the target value was outside the values used during the training phase. [ABSTRACT FROM PUBLISHER]

Angular momentum exchanges between atmosphere and solid Earth are strongly modulated by variations in global atmospheric circulation. Geodetically determined length-of-day (LOD) fluctuations provide an independent resource to investigate climate changes. Here, I evaluate the effects of volcanic and anthropogenic sulphate aerosols on Earth's rotational energy variations. The period analysed, 1980–2002, shows that the strongest seasonal LOD variations are related to sulphate peak concentrations from the El-Chichón 1982, and Pinatubo and Cerro Hudson 1991 volcanic eruptions. The Earth's rotational energy budget implies that radiative forcing alone cannot produce the observed LOD anomalies. Rather, the required amount of atmospheric kinetic energy can be explained only by a strong influence of sulphate aerosols on energy partitioning into the atmosphere, for example, as sulphate aerosols affect latent heat release and transport during condensation–evaporation–freezing cycles. Overall, the effects of sulphate aerosols on Earth's spin changes are faster than those produced by greenhouse gases. [ABSTRACT FROM PUBLISHER]

Volcanic tremor is an important precursor to explosive eruptions and is ubiquitous across most silicic volcanic systems. Oscillations can persist for days and occur in a remarkably narrow frequency band (i.e. 0.5–7 Hz). The recently proposed magma-wagging model of Jellinek & Bercovici provides a basic explanation for the emergence and frequency evolution of tremor that is consistent with observations of many active silicic and andesitic volcanic systems. This model builds on work suggesting that the magma column rising in the volcanic conduit is surrounded by a permeable vesicular annulus of sheared bubbles. The magma-wagging model stipulates that the magma column rattles within the spring like foam of the annulus, and predicts oscillations at the range of observed tremor frequencies for a wide variety of volcanic environments. However, the viscous resistance of the magma column attenuates the oscillations and thus a forcing mechanism is required. Here we provide further development of the magma-wagging model and demonstrate that it implicitly has the requisite forcing to excite wagging behaviour. In particular, the extended model allows for gas flux through the annulus, which interacts with the wagging displacements and induces a Bernoulli effect that amplifies the oscillations. This effect leads to an instability involving growing oscillations at the lower end of the tremor frequency spectrum, and that drives the system against viscous damping of the wagging magma column. The fully non-linear model displays tremor oscillations associated with pulses in gas flux, analogous to observations of audible ‘chugging’. These oscillations also occur in clusters or envelopes that are consistent with observations of sporadic tremor envelopes. The wagging model further accurately predicts that seismic signals on opposite sides of a volcano are out of phase by approximately half a wagging or tremor period. Finally, peaks in gas flux occur at the end of the growing instability several tens of seconds after the largest tremors, which is consistent with observations of a 30- to 50-s lag between major tremor activity and maximum gas release. The extended magma-wagging model, thus, predicts tremor frequency and its evolution before and during an eruption, as well as a driving mechanism to keep the tremor excited for long periods. [ABSTRACT FROM AUTHOR]

We suggest that carbon dioxide exsolved from a mid-crustal basaltic dyke intrusion in Iceland migrated upwards and triggered shallow seismicity by allowing failure on pre-existing fractures under the relatively low elastic stresses (100–200 kPa; 1–2 bar) generated by the dyke inflation. Intense swarms of microseismicity accompanied magmatic intrusion into a dyke at depths of 13–19 km in the crust of Iceland's Northern Volcanic Rift Zone during 2007–2008. Contemporaneously, a series of small normal earthquakes, probably triggered by elastic stresses imposed by the dyke intrusion, occurred in the uppermost 4 km of crust: fault plane solutions from these are consistent with failure along the extensional fabric and surface fissure directions mapped in the area, suggesting that the faults failed along existing rift zone fabric even though the mid-crustal dyke is highly oblique to it. Several months after the melt froze in the mid-crust and seismicity associated with the intrusion had ceased, an upsurge in shallow microseismicity began in the updip projection of the dyke near the brittle–ductile transition at 6–7 km depth below sea level. This seismicity is caused by failure on right-lateral strike-slip faults, with fault planes orientated 23 ± 3°, which are identical with the 24 ± 2° orientation in this area of surface fractures and fissures caused by plate spreading and extension of the volcanic rift zone. However, these earthquakes have T-axes approximately aligned with the opening direction of the dyke, and the right-lateral sense of failure is opposite that of regional strike-slip faults. We suggest that the fractures occurred along pre-existing weaknesses generated by the pervasive fabric of the rift zone, but that the dyke opening in the mid-crust beneath it caused right-lateral failure. The seismicity commenced after a temporal delay of several months and has persisted for over 3 yr. We propose that fluids exsolved from the magma in the dyke, primarily carbon dioxide, percolated updip and to shallower depths predominantly along pre-existing fractures. Increased pore pressure from the volatiles reduced the effective normal compressive stress on faults, increasing the likelihood of failure and allowing the modest stress changes generated by the intrusion to cause failure. Propagation of volatiles through the crust would also account for the observed time delay between the intrusion at depth and the shallow earthquake clusters. A further short-lived cluster of earthquakes at 2–4 km depth beneath the surface exhibits left-lateral strike-slip faulting with epicentres well orientated along a lineation which is identical with other subparallel strike-slip faults in the area that transfer motion between two adjacent spreading segments. These shallow earthquakes lie beyond lobes of significant positive Coulomb stress change caused by the intrusion, implying minimal modifications to the stress field in their vicinity; hence, they continue to respond to the regional stress field rather than the local stress field generated by the dyke intrusion. [ABSTRACT FROM AUTHOR]

The characteristics of the Strombolian-like activity at Erebus volcano (Antarctica) are evidenced by decomposing the wavefield of the explosion-quakes associated with the bursting of gas slugs at the surface. The decomposition is achieved by adopting a non-linear technique operating in time domain, that is, independent component analysis. Although the spectral properties and polarization analysis of the recorded signals provide evidence of an overall complex volcanic structure, the decomposed waves indicate well-polarized phases with high rectilinearity. Slightly different directions in azimuth can be individuated compatible with different and close vibrating solid structures like conduits in the summit volcanic plumbing system. [ABSTRACT FROM AUTHOR]

Among the physical parameters that govern magma flows, gas content plays a major role. In particular, the presence of gas controls the magma viscosity and velocity gradients, which characterize the eruptive style. Depending on whether gas can escape the volcanic conduit or not, the magma pressure will increase (or not), leading to an effusive or an explosive regime. Gas loss is thus a key point for understanding the physics of magma flows. Its role during the magma ascent is studied numerically. In this paper, I introduce, for the first time, both horizontal and vertical magma permeabilities into a 2-D magma flow model. The magma permeability is defined specifically for every point of the conduit as a function of the gas content and the bubble shape. Starting from an (unrealistic) closed system, I calculate iteratively the evolution of a degassing dyke to reach the stationary state for an open system. Gas loss does not occur homogeneously within the conduit, which leads to degassed magma sections presenting both vertical and lateral discrepancies. Its major influence lies within 500 m below the surface, where it leads to the formation of a 30–100 m thick degassed cover at the top of the conduit and the presence of 1–2 m thick degassed material near the conduit walls. Globally, the range of gas content decreases from 60–90 vol. % for a closed system down to 10–60 vol. % for an open system. This distribution in gas content and the associated flow are in good agreement with several field observations. [ABSTRACT FROM AUTHOR]

Searching for small periodic signals, such as the 12 hr (S2) barometric tide, and monitoring their amplitude as a function of time, can provide important clues on the complex processes affecting fluid transport in unsaturated fractured media under multiple influences. Here, first, we show that a modified spectrogram analysis (MSA) is more efficient than simple Fourier transform to reveal weak periodic signals. Secondly, we show how transient periodic signals can be monitored as a function of time using spectrograms. These methods are applied to time-series of radon and carbon dioxide concentration, dripwater flow rates and air temperature measured during several years in the Roselend dead-end tunnel, located in the French Alps near an artificial lake. A weak S2 line is evidenced in radon concentration, with enhanced amplitude during transient radon bursts. Similarly, the S2 line is observed using MSA in dripwater flow rates which sample mainly fracture flow, as suggested by a hydrochemical analysis, while it is not seen in dripwater flow rates sampling matrix flow. In the absence of a strong 24 hr line, the presence of a S2 line suggests sensitivity to barometric pressure, and thus a significant advective contribution in radon and some dripwater transport. No S2 line is observed in the carbon dioxide time-series. The temporal structure of the S2 component, however, is not similar in the radon concentration and the dripwater flow rates, suggesting, in particular, that dripwater does not play a significant role in the generation of radon bursts. Temperature time-series exhibit a significant S2 contribution, induced by atmospheric pressure, spatially organised in the tunnel, decreasing vertically upwards. A remarkable transient temperature inversion during radon bursts suggests that the additional advective air contributions responsible for the radon bursts occur from the non-saturated rocks below the tunnel. [ABSTRACT FROM AUTHOR]

Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s−1. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 106 km3 of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma. [ABSTRACT FROM AUTHOR]

Investigates the influence of pyroclast size distributions and released magma gas contents on the eruption velocities of volcanoes. Reference to the Hawaiian and Plinian eruptions; Estimates of exsolved magma volatile contents; Role of numerical models of eruption dynamics.