Franck Donnadieu

Experiments on the indentation process during cryptodome intrusions: New insights into Mount St. Helens deformation

Scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome. As the magma analogue is injected vertically into a sand cone, asymmetric deformation is caused by the formation of a curved major shear fault, which dips inward from one side of the cone to the opposite edge of the intrusion. The path of the ascending silicone deviates to follow the trajectory of the fault, and a lateral bulge grows slowly from the footwall of the fault. The oblique push makes the bulge migrate outward, causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault. We suggest that the pattern of internal deformation within Mount St. Helens prior to the May 18, 1980, eruption was similar to that observed in our scaled models.

Mass estimations of ejecta from Strombolian explosions by inversion of Doppler radar measurements

[1] We present a new method for estimating particle loading parameters (mass, number, volume) of eruptive jets by inversion of echo power data measured using a volcano Doppler radar (VOLDORAD) during typical Strombolian activity from the southeast (SE) crater of Mount Etna on 4 July 2001. Derived parameters such as mass flux, particle kinetic and thermal energy, and particle concentration are also estimated. The inversion algorithm uses the complete Mie (1908) formulation of electromagnetic scattering by spherical particles to generate synthetic backscattered power values. In a first data inversion model (termed the polydisperse model), the particle size distribution (PSD) is characterized by a scaled Weibull function. The mode of the distribution is inferred from particle terminal velocities measured by Doppler radar for each explosion. The distribution shape factor is found to be 2.3 from Chouet et al.’s (1974) data for typical Strombolian activity, corresponding to the lognormal PSDs commonly characteristic of other Strombolian deposits. The polydisperse model inversion converges toward the Weibull scale factor producing the best fit between synthetic and measured backscattered power. A cruder, alternative monodisperse model is evaluated on the basis of a single size distribution assumption, the accuracy of which lies within 25% of that of the polydisperse model. Although less accurate, the monodisperse model, being much faster, may be useful for rapid estimation of physical parameters during real-time volcano monitoring. Results are illustrated for two explosions at Mount Etna with contrasted particle loads. Estimates from the polydisperse model give 58,000 and 206,000 kg as maxima for the total mass of pyroclasts, 26,400 and 73,600 kg s � 1 for mass flux rates, 38 and 135 m 3 (22 and 76 m 3 equivalent magma volume) for the pyroclast volumes, and 0.02–0.4 and 0.06–0.12 kg m � 3 for particle concentrations, respectively. The time-averaged kinetic energy released is found to be equal to 4.2 � 10 7 and 3.9 � 10 8 J, and thermal energy is estimated at 8.4 � 10 10 and 3 � 10 11 J.

Remotely monitoring volcanic activity with ground‐based Doppler radar

Concern about hazards that volcanic plumes pose, especially to aviation safety, has led scientists for about two decades to use satellite sensors in different wavelengths for the detection and study of volcanic activity. Together with ground-based meteorological radars, these techniques now enable tracking the ascent and dispersal of large eruptive clouds, making reflectivity mapping, determining plume heights, measuring gas (SO2) and aerosols content, and estimating particle sizes and total mass of gas and fine ash [e.g., Harris and Rose, 1983]. However, there is still a crucial need for direct measurements of particle velocities, especially near an emission vent, to constrain physical and numerical models of eruption dynamics, which in turn should improve our predictive capacity regarding plume behavior.

Digital photogrammetry as a tool in analogue modelling: applications to volcano instability

Abstract Three techniques of digital photogrammetry have been applied successfully to laboratory analogue models to study surface displacements caused by various volcano deformation types. Firstly, side-perspective videos are used to differentiate profile displacements between cryptodome intrusion models and models deforming by ductile inner-core viscous flow. Both models show similar morphologic features including a bulged flank and an asymmetric upper graben. However, differences in displacement trajectories of the bulge crest reflect upward intrusion push contrasting with essentially downward displacement vectors of weak core models. The other two techniques use vertical views correlated automatically either as time-sequence monoscopic views or as coeval stereoscopic pairs. This exploits to a maximum the method’s potential by imaging surface displacements over the whole model. Successive monoscopic photograms, because they suffer only moderate numerical processing for topographic effect removal, can detect very small displacements occurring early in deformation processes. As illustrated by analysis of intrusion models, the monoscopic method allows prediction of fault locations and main displacement locations. It can also anticipate the principal strain directions, and separate different deformation stages. On the other hand, the stereo-photogrammetry technique, although more complicated, provides topography and volume changes, as well as pictures of surface displacements in three dimensions. Results are presented for the spreading of volcano models on a ductile substratum and viscous cored cones. We have found digital photogrammetry to be a useful tool for analogue modelling, because it provides quantitative data on surface displacements, including movement invisible to the eye, as well as topographic changes. It is a good method for investigating and comparing different deformation mechanisms. It is especially useful for interpretation of displacement patterns obtained from monitoring of natural active volcanoes. In fact, results of the methods used in the laboratory can be directly compared with field data from geodetic or photogrammetric surveys, as at Mount St. Helens in 1980.

Geometrical constraints of the 1980 Mount St. Helens intrusion from analogue models

The 1980 Mount St. Helens cryptodome intrusion has been simulated by scaled experiments. Silicone, used as a magma analogue, was intruded into a cohesive cone that models the edifice. Measuring surface deformation features associated with variations of injection rate and depth of cryptodome initiation revealed linear relationships with the intrusion characteristics. Scaled to Mount St. Helens dimensions, the 1980 cryptodome is found to have had a maximum E-W width of 850 m, an initial depth of 680 to 835 m. It had an ascending velocity of 5.7–7.3 m/day that corresponds to a vertical displacement of 330–380 m toward the surface within the two months of deformation. This brought the intrusion to less than 250 m below the crater floor at the point when the north flank failed on May 18, 1980.

Volcanological Applications of Doppler Radars: A Review and Examples from a Transportable Pulse Radar in L-Band

Many types of radar systems have been applied to the study of a wide range of volcanic features. Fields of application commonly include volcano deformation by interferometric synthetic aperture radar (InSAR), mainly satellite-based (e.g. Froger et al., 2007) but also ground-based like LISA (Casagli et al., 2009), digital elevation model generation using satellite or airborne InSAR measurements, surface products mapping by amplitude images of satellite radars, characterization of unexposed deposits by ground-penetrating radars (Russell & Stasiuk, 1997), monitoring of active lava domes and flows by either ad-hoc ground-based radars (e.g. Malassingne et al., 2001; Macfarlane et al., 2006; Wadge et al., 2005, 2008) or commercial ones (Hort et al., 2006; Voge & Hort, 2008, 2009; Voge et al., 2008), and quantitative characterization of explosive activity by means of fixed weather radars (large ash plumes) and transportable radars (Strombolian activity, weak ash plumes). A thorough review of all radar applications in volcanology is beyond the scope of this chapter which, instead, focuses on recent investigations of explosive eruptive regimes enhanced by the developments of dedicated transportable ground-based radars, by the recent advances made in signal interpretation using eruption models, and by the important concerns raised by ash plume hazards.

Indentation of volcanic edifices by the ascending magma

Abstract The process by which magma ascends into and deforms a volcanic edifice is studied by analogue modelling. A control experiment is conducted with a wooden piston moving vertically into a sand cone. This reveals a well-defined fault pattern that makes it possible to draw the main compressive stress trajectory within the cone during the ascent of the piston. This makes it possible to show that the deformational process is that of indentation of the cone by the rigid piston. Experiments with an indenter that is viscous, as in nature, show that the motion of the viscous body is controlled by the first fault created in the cone. This fault serves as a structural guide, making the viscous body deviate from the vertical and resulting in deformation of the flank of the cone, which bulges out. Other major shear faults that were observed in the control experiment are then inhibited and do not form. This result emphasizes that the structural evolution of an indentation process within a brittle cone and at low rate depends on the rheology of the indenter.

Modern Multispectral Sensors Help Track Explosive Eruptions

Due to its massive air traffic impact, the 2010 eruption of Eyjafjallajokull was felt by millions of people and cost airlines more than U.S.

Mass Eruption Rates of Tephra Plumes During the 2011–2015 Lava Fountain Paroxysms at Mt. Etna From Doppler Radar Retrievals

1.7 billion. The event has, thus, become widely cited in renewed efforts to improve real-time tracking of volcanic plumes, as witnessed by special sections published last year in Journal of Geophysical Research, (117, issues D20 and B9).

Erratum to: Lava discharge during Etna’s January 2011 fire fountain tracked using MSG-SEVIRI

Real-time estimation of eruptive source parameters during explosive volcanic eruptions is a major challenge in terms of hazard evaluation and risk assessment as these inputs are essential for tephra dispersal models to forecast the impact of ash plumes and tephra deposits. Between 2011 and 2015, Etna volcano has produced 49 paroxysms characterized by lava fountains generating tephra plumes that reached up to 15 km a.s.l.. We analyzed these paroxysms using the 23.5 cm wavelength Doppler radar (VOLDORAD 2B) signals along with visible camera images of the monitoring network of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo. Range gating of the radar beam allows the identification of the active summit craters in real-time, no matter the meteorological conditions. The radar echoes help to mark (i) the onset of the paroxysm when unstable lava fountains, progressively taking over Strombolian activity, continuously supply the developing tephra plume, then (ii) the transition to stable fountains (climax), and (iii) the end of the climax with a waning phase, therefore providing paroxysm durations. We developed a new methodology to retrieve in real-time a Mass Eruption Rate (MER) proxy from the radar echo power and maximum Doppler velocity measured near the emission source. The increase in MER proxies is found to precede by several minutes the time variations of plume heights inferred from visible and X-Band radar imagery. A calibration of the MER proxy against ascent models based on observed plume heights leads to radar-derived climax MER from 2.96 × 104 to 3.26 × 106 kg s-1. The total erupted mass (TEM) of tephra was computed by integrating over beam volumes and paroxysm duration, allowing quantitative comparisons of the relative amounts of emitted tephra among the different paroxysms. When the climactic phase can be identified, it is found to frequently release 76% of the TEM. Calibrated TEMs are found to be larger than those retrieved by satellite and X-band radar observations, deposit analyses, ground-based infrared imagery or dispersion modeling. The radar-derived mass load parameters therefore represent a very powerful all-weather tool for the quantitative monitoring and real-time hazard assessment of tephra plumes at Etna.