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Borgia A.,EDRA | Borgia A.,Rutgers University | Aubert M.,University Blaise Pascal | Merle O.,University Blaise Pascal | Van Wyk De Vries B.,University Blaise Pascal
Special Paper of the Geological Society of America | Year: 2010

The definition of a volcano is discussed, and a new encompassing version is provided. The discussion focuses on the observations that volcanism is a self-similar process that ranges many orders of magnitude in space and time scales, and that all kinds of geologic processes act on volcanoes. Former definitions of volcano, such as that from the Glossary of Geology (1997, p. 690)-"a vent in the surface of the Earth through which magma and associated gases and ash erupt" or "the form or structure, usually conical, that is produced by the ejected material" are clearly insufficient. All definitions that we encountered tend to consider volcanoes from the point of view of a single discipline, each of them neglecting relevant aspects belonging to other disciplines. For the two cases mentioned above a volcano is seen only from the point of view of eruptive activity or of morphology. We attempt to look at volcano holistically to provide a more comprehensive definition. We define a volcano as a geologic environment that, at any scale, is characterized by three elements: magma, eruption, and edifice. It is sufficient that only one of these elements is proven, as long as the others can be inferred to exist, to have existed, or to have the potential to exist in the future. © 2010 The Geological Society of America. All rights reserved. Source

Borgia A.,EDRA | Borgia A.,Rutgers University | Murray J.B.,Open University Milton Keynes
Special Paper of the Geological Society of America | Year: 2010

A theoretical similarity derived for spreading volcanoes remains valid for over three orders of magnitude in size, from small slumping volcanoes to large ocean plates. This similarity means that Mount Etna, Sicily, can be used as a terrestrial analogue of Tharsis Rise on Mars. A quantitative comparison of surface structures suggests that Tharsis has been spreading outward as a result of movement of plates at a planetary scale, producing rifting through the summit of the Rise, folding around its periphery, and with radial tear-fault systems connecting the summit rifts to the basal folds. The tear-fault systems form the fossae, of which Valles Marineris is the largest, and decouple the various plates of the Rise, so Tharsis appears to be analogous to a terrestrial mid-ocean ridge, but without any corresponding subduction zone. Instead, like volcanoes, the Tharsis plates are obducted over the Martian crust. The maximum viscosity at the bottom of the Martian lithosphere, derived from our spreading analysis, is in the order of 1021 Pa s, so if the mantle of Mars is hot and ductile, we suggest that the spreading of Tharsis could still be occurring. © 2010 The Geological Society of America. All rights reserved. Source

Mazzoldi A.,Universidad Michoacana de San Nicolas de Hidalgo | Borgia A.,EDRA | Ripepe M.,University of Florence | Marchetti E.,University of Florence | And 3 more authors.
Journal of Volcanology and Geothermal Research | Year: 2015

Seismogenic structures such as faults play a primary role in geothermal system generation, recharge and output. They are also the most susceptible to release seismic energy over fluid injection/extraction operations during anthropic exploitation. We describe the microseismic activity recorded in 2000-2001 in the Piancastagnaio geothermal field, on the SE flank of Mt. Amiata volcano, southern Tuscany, Italy. From our field observations we find that a relatively high percentage (i.e. about 5%) of the recorded events are of hydro-fracturing origin and have a distinct waveform seismic signature when compared to the recorded events of tectonic shear-fracturing origin. While hydrofracturing events are mostly concentrated around the geothermal fields, the spatial distribution of hypocenters shows a deepening and a density increase of the micro-seismic activity from the volcanic axis toward the exploited geothermal reservoir, suggesting that volcanic spreading at Amiata is still active. The study of different data-sets from different time periods together with the knowledge from Terzaghi's law that production of large quantity of pore-fluid with the associated fluid pressure reduction could augment the stress normal to faults' surfaces (and thus their resistance to slip), make us argue that the process of volcanic spreading affecting the edifice of Amiata may allow augmented accumulation of stresses on faults, eventually leading to the release of higher stress drops, once ruptures occur. The Gutenberg-Richter magnitude-frequency distribution shows that the strongest events on record have a local magnitude in the 5-5.5ML range, for 100-year recurrence time. In conclusions, we infer that geothermal exploitation at Mt. Amiata should be closely monitored in order to understand how fluid injection/production is responsible for the hydrofracturing seismic activity and affects stress accumulation on and rupture of faults within and in the neighborhood of the geothermal fields. This understanding may allow a geothermal field management that will hopefully reduce the risk for inducing larger seismic events in the area. © 2015 Elsevier B.V. Source

Borgia A.,EDRA | Borgia A.,University of Milan | Mazzoldi A.,EDRA | Mazzoldi A.,Universidad Michoacana de San Nicolas de Hidalgo | And 6 more authors.
Journal of Volcanology and Geothermal Research | Year: 2014

We made a stratigraphic, structural and morphologic study of the Amiata Volcano in Italy. We find that the edifice is dissected by intersecting grabens that accommodate the collapse of the higher sectors of the volcano. In turn, a number of compressive structures and diapirs exist around the margin of the volcano. These structures create an angular drainage pattern, with stream damming and captures, and a set of lakes within and around the volcano. We interpret these structures as the result of volcanic spreading of Amiata on its weak substratum, formed by the late Triassic evaporites (Burano Anhydrites) and the Middle-Jurassic to Early-Cretaceous clayey chaotic complexes (Ligurian Complex). Regional doming created a slope in the basement facilitating the outward flow and spreading of the ductile layers forced by the volcanic load.We model the dynamics of spreading with a scaled lubrication approximation of the Navier Stokes equations, and numerically study a set of solutions. In the model we include simple functions for volcanic deposition and surface erosion that change the topography over time. Scaling indicates that spreading at Amiata could still be active. The numerical solution shows that, as the central part of the edifice sinks into the weak basement, diapiric structures of the underlying formations form around the base of the volcano. Deposition of volcanic rocks within the volcano and surface erosion away from it both enhance spreading. In addition, a sloping basement may constitute a trigger for spreading and formation of trains of adjacent diapirs. As a feedback, the hot hydrothermal fluids decrease the shear strength of the anhydrites facilitating the spreading process.Finally, we observe that volcanic spreading has created ideal heat traps that constitute todays' exploited geothermal fields at Amiata. Normal faults generated by volcanic spreading, volcanic conduits, and direct contact between volcanic rocks (which host an extensive fresh-water aquifer) and the rocks of the geothermal field, constitute ideal pathways for water recharge during vapour extraction for geothermal energy production. We think that volcanic spreading could maintain faults in a critically stressed state, facilitating the occurrence of induced and triggered seismicity. © 2014 Elsevier B.V. Source

Borgia A.,EDRA | Borgia A.,University of Milan | Borgia A.,Open University Milton Keynes | Cattaneo L.,EDRA | And 8 more authors.
Computers and Geosciences | Year: 2011

The tides of the Venetian Lagoon generally vary between -0.5 and +0.7. m. asl. Occasionally, they may reach a maximum of 1.5. m (acqua alta) and a minimum of -0.8. m. asl (acqua bassa). Intertidal areas, called "barene," exist all along the coast of the Lagoon. These areas are characterized by canals that concentrate the flow of water (and the deposition of sands) during the rising and waning of the tides, and that inundate and drain the vegetated areas found between canals (where organic-rich clays are deposited). Therefore, since the area is subject to subsidence, in time, sand dykes (the original canals) become juxtaposed to clayey dykes (the original vegetated areas). In addition, the sands form a continuous hydrogeologic network within the clays, very similar to that of a vascular system that effectively drains the whole "barena" deposits. In order to be effective, measures for monitoring, confining, or remediating the transport of pollutants through this kind of environment must explicitly take into account the geologic complexity. The same complexity must be included in the numerical models that support remediation efforts. At the moment, there appears to be no off-the-shelf graphical interface that is able to manage such complexity for TOUGH2. To attempt to solve this problem we have used a calibrated USGS-MODFLOW model, of the barena of "Passo a Campalto" in the Venetian Lagoon, developed with the GMS graphical interface. The model is made of 42 layers, which, apart from the first layer, are 0.5. m thick; the first layer has the thickness distribution of a dump found on top of the barena deposit at Passo a Campalto. Each layer consists of 100×60 square cells, for a total of 252,000 cells, only about half of which are active. Using a FORTRAN routine, we translate this grid, with all the hydrogeologic boundary conditions, into a TOUGH2 input file, and we provide the additional necessary information for running a TOUGH2 simulation. The results are promising, in that we were able to produce TOUGH2 grids with very complex geology and to run the models with success. For visualization, the results can be imported back into GMS as 3D scatter point sets, or they can be plotted with any adequate plotting software such as MatLab. Developing conceptual and numerical models with an elaborate graphical interface such as GMS effectively allows setting up complex problems while concentrating on their physics. © 2011. Source

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