Arnau Folch

FALL3D: A computational model for transport and deposition of volcanic ash

FALL3D is a 3-D time-dependent Eulerian model for the transport and deposition of volcanic ashes and lapilli. The model solves the advection-diffusion-sedimentation (ADS) equation on a structured terrain-following grid using a second-order finite differences (FD) explicit scheme. Different parameterizations for the eddy diffusivity tensor and for the particle terminal settling velocities can be used. The code, written in FORTRAN 90, is available in both serial and parallel versions for Windows and Unix/Linux/Mac X operating systems (OS). A series of pre- and post-process utility programs and OS-dependent scripts to launch them are also included in the FALL3D distribution package. Although the model has been designed to forecast volcanic ash concentration in the atmosphere and ash loading at ground, it can also be used to model the transport of any kind of airborne solid particles. The model inputs are meteorological data, topography, grain-size distribution, shape and density of particles, and mass rate of particle injected into the atmosphere. Optionally, FALL3D can be coupled with the output of the meteorological processor CALMET, a diagnostic model which generates 3-D time-dependent zero-divergence wind fields from mesoscale forecasts incorporating local terrain effects. The FALL3D model can be a tool for short-term ash deposition forecasting and for volcanic fallout hazard assessment. As an example, an application to the 22 July 1998 Etna eruption is also presented.

The generation of overpressure in felsic magma chambers by replenishment

Abstract Evidence of magma mixing is common in the products of explosive felsic eruptions and it is generally accepted as a common mechanism for triggering such events. In order to quantify the potential for magma mixing to trigger explosive eruptions, we have developed a simple analytical model, based on previous models of magma chamber replenishment, which considers the injection of volatile-rich mafic magma into a chamber occupied by a homogeneous, volatile-rich felsic magma. We assume that the overpressure caused by the injection of new magma is not sufficient to trigger an eruption, for which additional overpressure is required. Two mixing-related mechanisms have traditionally been considered to have the potential to generate the additional overpressure: (1) exsolution of volatiles from the felsic magma during forced convection, and (2) exsolution of volatiles from the mafic magma by oversaturation during cooling and subsequent crystallization. Our calculations suggest that exsolution of volatiles from the felsic magma is not an effective mechanism to generate additional overpressure. However, significant overpressure can be achieved by volatile exsolution from the mafic magma during its cooling and crystallization. The time scale between intrusion and eruption considered in our model is of the order of a few days to a few months, which coincides with petrological and geophysical evidence obtained from magma mixing related eruptions. We also suggest that replenishment of shallow felsic magma chambers by mafic magma in most cases does not lead to large scale mixing, as eruption will occur before thermal equilibrium between the two magmas is reached, so that density and viscosity contrasts between the two magmas will remain significant.

Quantifying volcanic ash dispersal and impact of the Campanian Ignimbrite super‐eruption

[1] We apply a novel computational approach to assess, for the first time, volcanic ash dispersal during the Campanian Ignimbrite (Italy) super-eruption providing insights into eruption dynamics and the impact of this gigantic event. The method uses a 3D time-dependent computational ash dispersion model, a set of wind fields, and more than 100 thickness measurements of the CI tephra deposit. Results reveal that the CI eruption dispersed 250–300 km 3 of ash over 3.7 million km 2 . The injection of such a large quantity of ash (and volatiles) into the atmosphere would have caused a volcanic winter during the Heinrich Event 4, the coldest and driest climatic episode of the Last Glacial period. Fluorine-bearing leachate from the volcanic ash and acid rain would have further affected food sources and severely impacted Late Middle-Early Upper Paleolithic groups in Southern and Eastern Europe. Citation: Costa, A., A. Folch, G. Macedonio, B. Giaccio, R. Isaia, and V. C. Smith (2012), Quantifying volcanic ash dispersal and impact of the Campanian Ignimbrite super-eruption, Geophys. Res. Lett., 39, L10310, doi:10.1029/2012GL051605.

A model for wet aggregation of ash particles in volcanic plumes and clouds: 1. Theoretical formulation

[1] We develop a model to describe ash aggregates in a volcanic plume. The model is based on a solution of the classical Smoluchowski equation, obtained by introducing a similarity variable and a fractal relationship for the number of primary particles in an aggregate. The considered collision frequency function accounts for different mechanisms of aggregation, such as Brownian motion, ambient fluid shear, and differential sedimentation. Although model formulation is general, here only sticking efficiency related to the presence of water is considered. However, the different binding effect of liquid water and ice is discerned. The proposed approach represents a first compromise between the full description of the aggregation process and the need to decrease the computational time necessary for solving the full Smoluchowski equation. We also perform a parametric study on the main model parameters and estimate coagulation kernels and timescales of the aggregation process under simplified conditions of interest in volcanology. Further analyses and applications to real eruptions are presented in the companion paper by Folch et al.

Unrest at Campi Flegrei: A contribution to the magmatic versus hydrothermal debate from inverse and finite element modeling

[ 1] We present results from the modeling of ground deformation and microgravimetric data recorded at Campi Flegrei in order to assess the causative phenomena of caldera unrest between 1981 and 2001. We find that residual gravity changes during ground uplift ( 1982 - 1984) are indicative of mass changes in a hybrid of magmatic and hydrothermal sources. During deflation between 1985 and 2001, the inversion of gravity residuals for a single source does not provide convincing results. We then performed the joint inversion of gravity and deformation data for multiple spherical sources and refined source parameters by finite element modeling in order to mitigate against limitations of the analytical solutions. The data recorded during inflation and rapid deflation may be best explained by mass and pressure changes in a deep magmatic source at about 5 km depth and a shallow ( 2 km deep) hydrothermal source. Both sources contribute equally to the gravity changes observed between 1982 and 1984; the contemporary uplift appears to be mainly caused by the shallow source. The subsequent deflation is dominated by a pressure decrease in the hydrothermal source; the magmatic source contributes chiefly to the observed gravity changes. Pressure and density variations within multiple shallow-seated hydrothermal sources provide acceptable fits to the deflation and accompanying gravity changes recorded since 1988. These shallow level dynamics also appear to trigger spatially and temporarily random short-term reversals of the overall mode of ground subsidence since 1985. Our analysis does not support the idea of magmatic contributions to these short-lived periods of inflation.

Pressure evolution during explosive caldera-forming eruptions

Abstract Caldera-forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber. Field studies of caldera-forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions. Pressure evolution during explosive caldera-forming eruptions is investigated through a simple model that describes the first-order quantitative behaviour of the chamber. We consider a piston-like model that assumes a coherent block subsiding along circular, sub-vertical, ring faults into the magma chamber. This subsidence occurs after significant decompression of the chamber by an initial central vent eruption. We assume that the initial pressure distribution in the chamber is magmastatic. Once collapse has begun the chamber roof is supported by the magma, so that magma pressure at the chamber roof increases to lithostatic. We suggest that pressure variations during caldera-forming eruptions are mainly controlled by variations in magma volatile content. Regardless of what induces the formation of ring faults, the model suggests that the occurrence of explosive caldera-forming events depends on the strength of the chamber walls, and the depth, water content and aspect ratio of the magma chamber. No significant differences exist between model results for a cylindrical, or a more realistic elliptical magma chamber geometry of comparable aspect ratio. Assuming a constant strength of the host rock, the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges, for deep, gas-poor chambers, from a few percent up to 40% for shallow, gas-rich chambers. The model suggests that zoned chambers tend to collapse earlier than homogeneous chambers. Dike-shaped chambers will erupt less magma than sill-like chambers before caldera collapse initiates, although dike-like geometries are not associated with stress fields appropriate to create ring faults. The model suggests that once initiated, caldera collapse will tend to force out most or all of the volatile-rich magma from the chamber. For volatile-rich magma chambers, the total volume of erupted magma during caldera-forming event is of the same order as the chamber volume. The model also explains the variation in the erupted mass during the different phases of explosive caldera-forming eruptions, and is in good agreement with natural examples.

Geometrical and mechanical constraints on the formation of ring-fault calderas

Ash-flow, plate-subsidence (piston-like) calderas are bounded by a set of arcuated sub-vertical collapse faults named ring-faults. Experimental studies on caldera formation, performed mostly using spherical or cylindrical magma chamber geometries, find that the resulting ring-faults correspond to steeply outward dipping reverse faults, and show that pre-existing fractures developed during pre-eruptive phases of pressure increase may play a major role in controlling the final collapse mechanism, a situation that should be expected in small to medium sized ring-fault calderas developed on top of composite volcanoes or volcanic clusters. On the other hand, some numerical experiments indicate that large sill-like, elongated magma chambers may induce collapse due to roof bending without fault reactivation, as seems to occur in large plate-subsidence calderas formed independently of pre-existing volcanoes. Also, numerical experiments allow the formation of nearly vertical or steeply inward dipping normal ring-faults, in contrast with most of the analogue models. Using a thermoelastic model, we investigate the geometrical and mechanical conditions to form ring-fault calderas, in particular the largest ones, without needing a previous crust fracturing. Results are given in terms of two dimensionless geometrical parameters, namely λ and e. The former is the chamber extension to chamber depth ratio, whereas the latter stands for the chamber eccentricity. We propose that the (λ,e) pair determinates two different types of ring-fault calderas with different associated collapse regimes. Ring-fault region A is related to large plate-subsidence calderas (i.e. Andean calderas or Western US calderas), for which few depressurisation is needed to set up a collapse initially governed by flexural bending of the chamber roof. In contrast, ring-fault region B is related to small to moderate sized calderas (i.e. composite volcano calderas), for which much depressurisation is needed. Our opinion is that collapse requires, in the latter case, reactivation of pre-existing fractures and it is therefore more complex and history dependent.

A shallow-layer model for heavy gas dispersion from natural sources: Application and hazard assessment at Caldara di Manziana, Italy

Several nonvolcanic sources in central Italy emit a large amount of carbon dioxide (CO2). Under stable atmospheric conditions and/or in the presence of topographic depressions, the concentration of CO2, which has a molecular mass greater than that of air, can reach high values that are lethal to humans or animals. Several episodes of this phenomenon were recorded in central Italy and elsewhere. In order to validate a model for the dispersion of a heavy gas and to assess the consequent hazard, we applied and tested the code TWODEE-2, an improved version of the established TWODEE model, which is based on a shallow-layer approach that uses depth-averaged variables to describe the flow behavior of dense gas over complex topography. We present results for a vented CO2 release at Caldara di Manziana in central Italy. We find that the model gives reliable results when the input quantity can be properly defined. Moreover, we show that the model can be a useful tool for gas hazard assessment by evaluating where and when lethal concentrations for humans and animals are reached.

A Parallel CFD Model for Wind Farms

We present a Computational Fluid Dynamics (CFD) modeling strategy for onshore wind farms aimed at predicting and opti- mizing the production of farms using a CFD model that includes meteorological data assimilation, complex terrain and wind turbine effects. The model involves the solution of the Reynolds-Averaged Navier-Stokes (RANS) equations together with a k-ɛ turbulence model specially designed for the Atmospheric Boundary Layer (ABL). The model involves automatic meshing and generation of boundary conditions with atmospheric boundary layer shape for the entering wind flow. As the integration of the model up to the ground surface is still not viable for complex terrains, a specific law of the wall including roughness effects is implemented. The wake effects and the aerodynamic behavior of the wind turbines are described using the actuator disk model, upon which a volumetric force is included in the momentum equations. The placement of the wind turbines and a mesh refinement for the near wakes is done by means of a Chimera method. The model is implemented in Alya, a High Performance Computing (HPC) multi physics parallel solver based on finite elements and developed at Barcelona Supercomputing Center.

Reconstructing the plinian and co-ignimbrite sources of large volcanic eruptions: A novel approach for the Campanian Ignimbrite

The 39 ka Campanian Ignimbrite (CI) super-eruption was the largest volcanic eruption of the past 200 ka in Europe. Tephra deposits indicate two distinct plume forming phases, Plinian and co-ignimbrite, characteristic of many caldera-forming eruptions. Previous numerical studies have characterized the eruption as a single-phase event, potentially leading to inaccurate assessment of eruption dynamics. To reconstruct the volume, intensity, and duration of the tephra dispersal, we applied a computational inversion method that explicitly accounts for the Plinian and co-ignimbrite phases and for gravitational spreading of the umbrella cloud. To verify the consistency of our results, we performed an additional single-phase inversion using an independent thickness dataset. Our better-fitting two-phase model suggests a higher mass eruption rate than previous studies, and estimates that 3/4 of the total fallout volume is co-ignimbrite in origin. Gravitational spreading of the umbrella cloud dominates tephra transport only within the first hundred kilometres due to strong stratospheric winds in our best-fit wind model. Finally, tephra fallout impacts would have interrupted the westward migration of modern hominid groups in Europe, possibly supporting the hypothesis of prolonged Neanderthal survival in South-Western Europe during the Middle to Upper Palaeolithic transition.