http://hdl.handle.net/2122/263 2013-05-20T01:43:15Z http://hdl.handle.net/2122/8373 Title: An analytical model for gas overpressure in slug-driven explosions: Insights into Strombolian volcanic eruptions Authors: Del Bello, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Llewellin, E. W.; University of Durham; Taddeucci, J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Scarlato, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Lane, S.; Lancaster University Abstract: Strombolian eruptions, common at basaltic volcanoes, are mildly explosive events that are driven by a large bubble of magmatic gas (a slug) rising up the conduit and bursting at the surface. Gas overpressure within the bursting slug governs explosion dynamics and vigor and is the main factor controlling associated acoustic and seismic signals. We present a theoretical investigation of slug overpressure based on magma-static and geometric considerations and develop a set of equations that can be used to calculate the overpressure in a slug when it bursts, slug length at burst, and the depth at which the burst process begins. We find that burst overpressure is controlled by two dimensionless parameters: V′, which represents the amount of gas in the slug, and A′, which represents the thickness of the film of magma that falls around the rising slug. Burst overpressure increases nonlinearly as V′ and A′ increase. We consider two eruptive scenarios: (1) the “standard model,” in which magma remains confined to the vent during slug expansion, and (2) the “overflow model,” in which slug expansion is associated with lava effusion, as occasionally observed in the field. We find that slug overpressure is higher for the overflow model by a factor of 1.2–2.4. Applying our model to typical Strombolian eruptions at Stromboli, we find that the transition from passive degassing to explosive bursting occurs for slugs with volume >24–230 m3, depending on magma viscosity and conduit diameter, and that at burst, a typical Strombolian slug (with a volume of 100–1000 m3) has an internal gas pressure of 1–5 bars and a length of 13–120 m. We compare model predictions with field data from Stromboli for low-energy “puffers,” mildly explosive Strombolian eruptions, and the violently explosive 5 April 2003 paroxysm. We find that model predictions are consistent with field observations across this broad spectrum of eruptive styles, suggesting a common slug-driven mechanism; we propose that paroxysms are driven by unusually large slugs (large V′). 2012-02-09T23:00:00Z http://hdl.handle.net/2122/8299 Title: Morphometric analysis of lava flow units: Case study over LIDAR-derived topography at Mount Etna, Italy Authors: Tarquini, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; Favalli, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; Mazzarini, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; Isola, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; Fornaciai, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia Abstract: High resolution, LIDAR-derived digital elevation models of volcanic areas can significantly improve knowledge of lava flow morphology and emplacement mechanisms. Here we focus on single flow units, presenting a new semi-automatic procedure which provides a quantitative analysis of their shape. The method relies on the automatic processing of the elevation profiles obtained on transects orthogonal to the flow unit axis. The initial phase of the Mount Etna flank eruption from September 2004 is taken as test case, and the procedure is applied on an active lava flow, which was emplaced on the eastern flank of the volcano. The main topographic dataset used is a 2-m-resolution digital elevation model obtained from a LIDAR survey. Starting from the axis of a lava flow unit, our method yields morphometric data on the flow unit at a 2 m spacing, calculating parameters including flow width, channel width, the heights of the levees, inward and outward slope of levees, and estimating pre-emplacement slope along the axis. The procedure is embedded in a customized GIS, which allows easy processing, handling and displaying of data. The procedure has also been applied to another flow unit emplaced during the October–November 1999 overflow from the Bocca Nuova crater. Results show that the channel width seems to accommodate first‐order trends of the pre-emplacement slope along the flow unit axis, while it is little affected by high frequency changes in slope; in contrast, flow unit width and flow unit thickness are apparently influenced by small‐scale changes in slope. The different emplacement conditions of the two flow units are reflected by the overall contrasting morphologies, as shown by the different average thickness and by the different ratios between (i) flow width vs. channel width and (ii) flow unit section area vs. channel width. The new method provides an enhanced, systematic and thorough morphometric description of flow units, which may improve the understanding of the emplacement mechanisms of lava flows on Earth and other planets. 2012-07-31T22:00:00Z http://hdl.handle.net/2122/8226 Title: The deformation offshore of Mount Etna as imaged by multichannel seismic reflection profiles Authors: Argnani, A.; CNR, ISMAR-Bologna, Bologna, Italy; Mazzarini, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; Bonazzi, C.; CNR, ISMAR-Bologna, Bologna, Italy; Bisson, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; Isola, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia Abstract: Despite the clear evidence of active flank dynamics that is affecting the eastern side of Mount Etna, the contribution of tectonic processes has not been yet understood. So far, the various models proposed to explain the observed flank deformation have been based on onshore structural data, coming from the volcanic edifice. The Ionian offshore of Mount Etna has been only recently investigated using multichannel seismic profiles, and offers the opportunity to image the structural features of the substrate of the unstable flank of the volcano. This contribution aims at describing the deformation located offshore Mount Etna using multichannel seismic profiles recently acquired during three seismic surveys. The onshore flank deformation of Mount Etna appears to be laterally confined by two tectonic guidelines, trending roughly E–W, located to the north and south of the deforming flank; the northern guideline, in particular, takes the surface expression of a sharp fault (Pernicana Fault). Though often assumed that these boundary structures continue offshore as linear features, connected to a frontal thrust ramp, the occurrence of this simple offshore structural system has not been imaged. In fact, seismic data show a remarkable degree of structural complexity offshore Mount Etna. The Pernicana Fault, for instance, is not continuing offshore as a sharp feature; rather, the deformation is expressed as ENE–WSW folds located very close to the coastline. It is possible that these tectonic structures might have affected the offshore of Mount Etna before the Pernicana Fault system was developed, less than 15 ka ago. The southern guideline of the collapsing eastern flank of the volcano is poorly expressed onshore, and does not show up offshore; in fact, seismic data indicate that the Catania canyon, a remarkable E–W-trending feature, does not reflect a tectonic control. Seismic interpretation also shows the occurrence of a structural high located just offshore the edifice of Mount Etna. Whereas a complex deformation affects the boundary of this offshore bulge, it shows only limited internal deformation. Part of the topography of the offshore bulge pre-existed the constructional phase of Mount Etna, being an extension of the Hyblean Plateau. Only in the northern part, the bulge is a recent tectonic feature, being composed by Plio-Quaternary strata that were folded before and during the building of Mount Etna. The offshore bulge is bounded by a thrust fault that can be related to the intrusion of the large-scale magmatic body below Mount Etna. 2012-12-31T23:00:00Z http://hdl.handle.net/2122/8130 Title: Granular pressure at the base of dry flows of angular rock fragments as a function of grain size and flow volume: A relationship from laboratory experiments Authors: Cagnoli, B.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Romano, G. P.; Università La Sapienza Abstract: Experiments are carried out by releasing angular rock fragments down a curved chute and by measuring the basal pressures that are exerted by the granular flows on the basal containing surface (the substrate). The purpose of these experiments is to understand the mechanisms of energy dissipation and interaction with the ground of rock avalanches and dense pyroclastic flows. Our data show that collisions due to particle agitation affect significantly the basal interaction of granular flows. In particular, our experiments reveal that particle agitation per unit of flow mass increases as grain size increases or as flow volume decreases (with all the other features the same). This is so because as grain size increases or as flow volume decreases (with all the other features the same), there are fewer particles in the flows and the agitation due to the interaction with the rough containing boundary surfaces penetrates relatively more inside the flows. The analysis of the experimental data generates a linear relationship between particle agitation (expressed as a dimensionless basal pressure deviation) and a parameter which is directly proportional to the square of grain size and inversely proportional to the cube root of flow volume. This relationship shows the intrinsic ability of the granular flows to dissipate more energy (larger particle agitation per unit of flow mass) or less energy (smaller particle agitation per unit of flow mass) as a function of flow volume and grain size. 2011-12-31T23:00:00Z http://hdl.handle.net/2122/8100 Title: An overview of experimental models to understand a complex volcanic instability: Application to Mount Etna, Italy Authors: Acocella, V.; Dipartimento di Scienze Geologiche, Università Roma tre, Roma, Italy; Neri, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; Norini, G.; Istituto per la Dinamica dei Processi Ambientali, Consiglio Nazionale delle Ricerche, Dalmine, Italy Abstract: Volcanic edifices are often unable to support their own load, triggering the instability of their flanks. Many analogue models have been aimed, especially in the last decade, at understanding the processes leading to volcano flank instability; general behaviors were defined and the experimental results were compared to nature. However, available data at well-studied unstable volcanoes may allow a deeper understanding of the specific processes leading to instability, providing insights also at the local scale. Etna (Italy) constitutes a suitable example for such a possibility, because of its well-monitored flank instability, for which different triggering factors have been proposed in the last two decades. Among these factors, recent InSAR data highlight the role played by magmatic intrusions and a weak basement, under a differential unbuttressing at the volcano base. This study considers original and recently published experimental data to test these factors possibly responsible for flank instability, with the final aim to better understand and summarize the conditions leading to flank instability at Etna. In particular, we simulate the following processes: a) the longterm activity of a lithospheric boundary, as the Malta Escarpment, separating the Ionian oceanic lithosphere from the continental Sicilian lithosphere, below the most unstable east flank of the volcano; b) spreading due to a weak basement, with different boundary conditions; c) the pressurization of a magmatic reservoir, as that active during the 1994–2001 inflation period; d) dike emplacement, as observed during the major 2001 and 2002–2003 eruptions. The experimental results suggest that: 1) the long-term activity of a lithospheric tectonic boundary may create a topographic slope which provides a differential buttressing at the volcano base, a preparing factor to drive longer-term (>105 years) instability on the east flank of the volcano; 2) volcano spreading (b104 years) has limited effect on flank instability at Etna; 3) magmatic intrusions (b101 years), both in the form of Mogi-like sources or dikes, provide the most important conditions to trigger flank instability on the shorter-term. 2012-12-31T23:00:00Z http://hdl.handle.net/2122/7903 Title: Is Coulomb's law suitable to estimate basal friction in rapid mass movements of rock fragments? Authors: Cagnoli, B.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Quareni, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia Abstract: Rapid mass movements of rocks fragments (pyroclastic flows and rock avalanches for example) can be considered among the most hazardous natural phenomena because of their large momentum content. 2009-06-10T22:00:00Z http://hdl.handle.net/2122/7891 Title: Vertical segregation in granular mass flows: A shear cell study Authors: Cagnoli, B.; University of California - Berkeley; Manga, M.; University of California - Berkeley Abstract: Non-fluidised, dry granular mass flows are obtained with rock fragments located on a rough rotating disk. In these flows that develop a quasi-rigid upper layer and a basal layer of colliding particles, dense clasts sink whereas light ones rise when surrounded by particles with intermediate density. Our experiments demonstrate that the presence of a quasi-rigid upper layer in granular mass flows does not prevent vertical segregation and that the formation of coarse-tail grading in pyroclastic flows does not require fluidising gases. High-speed videos reveal that vertical segregation in granular mass flow of rock fragments is generated by inertia differences between segregating clasts and matrix when they are both pushed upward by collisions with the basal layer. Coarse-tail grading occurs because the average segregation velocity of smaller clasts is smaller than that of larger clasts. 2004-12-31T23:00:00Z http://hdl.handle.net/2122/7890 Title: Granular mass flows and Coulomb’s friction in shear cell experiments: Implications for geophysical flows Authors: Cagnoli, B.; University of California - Berkeley; Manga, M.; University of California - Berkeley Abstract: Granular mass flows of rock fragments are studied in the lab by means of a high-speed video camera at 2000 frames per second. These granular flows are generated using beds of pumice fragments positioned on a rough rotating disk, whose angular velocity is controlled by a motor. The experimental apparatus allows an understanding of the arrangement of the particles in granular mass flows with relatively small and relatively large values of the Savage number (the Savage number represents the ratio between grain collision stresses and gravitational grain contact stresses). In particular, these flows develop a basal layer of agitated and colliding particles underneath a relatively rigid upper layer. Our experimental results suggest the validity, on average, of the Coulomb’s relationship between shear and normal forces at the base of granular mass flows irrespective of their Savage number value. In Coulomb’s equation the shear stresses do not depend on the shear rate. We expect the Coulomb friction law to be valid also in moving pyroclastic flows. Our experiments suggest that the collisions and subsequent comminution of pumice fragments in moving pyroclastic flows could provide ash for the overriding ash clouds. In our experiments the amount of ash generated by particle-particle and particle-boundary interactions increases as the value of the Savage number increases. In nature, part of this ash may also simply move toward the base of the flows because of kinetic sieving. 2003-12-31T23:00:00Z http://hdl.handle.net/2122/7665 Title: Implementazione di una nuova procedura per caratterizzare la forma di particelle mediante misure al CAMSIZER e algoritmi di clustering Authors: Lo Castro, M. D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; Andronico, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; Cassisi, C.; Università degli Studi di Catania; Montalto, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; Prestifilippo, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia Abstract: In this work we present the calibration phase of a new procedure for the characterization of the shape of pyroclastic particles. This research has been granted by INGV of Catania, with funds deriving from the “Progetto Giovani”, in collaboration with Retsch Technology in Haan. The innovation of this procedure arises from the use of CAMSIZER (an instrument developed by the German leader company). This instrument permits to obtain very important information both on size and shape parameters of a high number of particles (hundreds of thousands data). Moreover, we used clustering and classification algorithms in order to group particles according to their morphologic characteristics. This calibration phase has been tested only on standard materials with regular geometries such as cubes, spheres and cylinders. In the future we will apply this methodology to volcanic ash particles that, as well-known, are characterized by irregular morphologies. 2010-12-31T23:00:00Z http://hdl.handle.net/2122/7527 Title: Effects of flow volume and grain size on mobility of dry granular flows of angular rock fragments: A functional relationship of scaling parameters Authors: Cagnoli, B.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Romano, G. P.; La Sapienza Abstract: Flows of angular rock fragments are released down a concave upward chute in the laboratory to study their mobility. This mobility is measured as the reciprocal of the apparent coefficient of friction that is equal to the vertical drop of the centre of mass of the granular material divided by its horizontal distance of travel. Our experiments show that the finer the grain size (all the other features the same), the larger is the mobility of the centre of mass. We believe this to be due to the fact that in finer grain size flows there are less agitated particles per unit of flow mass so that these flows dissipate less energy per unit of travel distance. Our experiments show also that the larger the volume (all the other features the same), the larger is the apparent coefficient of friction. We believe this to be so because the frontal portion of a flow reaches the less steep part of a curved slope and stops before the rear portion preventing the rear portion and the centre of mass from travelling further downhill. This phenomenon (which is more prominent in larger volume flows whose rear and frontal ends are more distant) counteracts the decrease of energy dissipation per unit of flow mass, due to the decrease of particle agitation per unit of flow mass, that is expected when the volume of a flow increases (all the other features the same). Our analysis generates a functional relationship between the dimensionless apparent coefficient of friction and a scaling parameter whose numerator is equal to the mean grain size multiplied by the cube root of the deposit volume and whose denominator is the square of the channel width. 2011-12-31T23:00:00Z