C. Cigolini

Intra-crater activity, aa-block lava, viscosity and flow dynamics: Arenal Volcano, Costa Rica

Abstract Arenal Volcano has exhibited an almost continuous effusion of lava since September 19th, 1968. Present intra-crater activity is characterized by a relatively quiet rate of effusion accompanied by rhythmic pulsing of the lava due to degassing phenomena. At the vent, the main lava type is aa, which is seldom associated with lava aplastada (flattened lava). This latter lava is formed within the zone of major activity and exhibits smooth, slightly curved, reddish-brown oxidized surfaces which develop irregular cracks on cooling. The genesis of this lava type is due to gas oxidation and rhythmic pulsing. The additional stresses produced by the latter phenomenon (combined with surface tension forces) would hinder the formation of aa lava and generate flattened lava. The dynamic development of lava flows can be explained in terms of three continuous cyclic phases: early, mature and collapsing. A single flow can be divided into four structural units: the upper part (unit a), the middle-upper vesiculated part (unit b), the nucleus (unit c), and the fragmental base (unit d). Rheologically these lavas behave like a Bingham plastic characterized in the field by plug flow. While the 1968–1973 lava flows are characterized by an advanced but incomplete transition from aa to block lava, present flows generally undergo the entire process. In this case the transition zone for a mature flow is located within the middle-lower part of the volcanic cone, reaching an approximate length of 300 m. Transition occurring during the mature phase has been investigated in detail. It is related to the progressive brittle deformation of the middle-upper part (unit b). Fracturing takes place along the side of the plug where the shear strain is higher as well as within the central part where it is due to topographic discontinuities systematically reflected in the mechanical behavior of the rigid plug. Viscosities calculated on the basis of field measurements at 50–200 m from the crater give values of 108 poise. These values are consistent with experimental results obtained in the range of extrusion temperatures (1100–1150°C) and are in good agreement with experiments on suspension rheology in basalts for a similar temperature range and degree of crystallinity. The high viscosity of Arenal lavas can be explained in terms of their high crystallinity (70–80%), lower extrusion temperatures and higher silica content in respect to basalts. The existence of two interstitial immiscible melts also contributes to increase viscosity.

High‐resolution radon monitoring and hydrodynamics at Mount Vesuvius

A yearlong high-resolution radon survey has been carried on at Mount Vesuvius, starting in May 1998. Radon activities were acquired by exposing charcoal canisters and track-etch detectors. Sampling stations were deployed along two major summit faults and around the caldera bottom. Volcanically-related earthquakes, with MD ≥ 2.5, may be discriminated from regional seismic events since their cumulative radon anomalies are recorded from stations located along all the above structural features. On the contrary, radon anomalies correlated to regional earthquakes, with MD ≥ 4, are essentially recorded by the sampling sites located along the two summit faults (whose roots extend deeper into the Tertiary basement rocks that underlay the volcano). Radon migration to the surface is ruled by convection within a porous medium of relatively low porosity (ϕ ≈ 10−5), suggesting that fluid motion is strongly localised along fractures. It is suggested that fluid pressure build up, followed by fluid release and migration during incipient fracturing of the porous medium, precede the onset of volcanically-induced earthquakes.

Probing Stromboli volcano from the mantle to paroxysmal eruptions

Abstract We investigated the plumbing system of Stromboli volcano from the upper mantle to the surface. Thermobarometric estimates indicate that the deeper detected part of the plumbing system is located in the upper mantle, at approximately 34–24 km depth where, during their ascent, primitive Stromboli basalts (HKCA to shoshonitic) interact with peridotitic materials. In this region magma flow is probably channelled along fracture zones that may converge into a feeder dyke that crosscuts the Moho at about 17 km depth. During their ascent, basaltic magmas will interact with lower crust materials represented by cumulates of earlier Stromboli-type basalts at 13–10 km depth. This zone is also the section of the plumbing system where the feeder dyke is entering the chamber. Thermobarometric estimates, obtained by constructing a grid of selected reactions, indicate that current primitive Stromboli basalts equilibrate at 0.3–0.15 GPa and temperatures approaching 1200 °C, and progressively crystallize and degas before being erupted. Crystal size distributions on lavas and juvenile tephra erupted in 2002–2003 give very variable residence times. Based on average bubble distances, the estimated times for the exsolution of the gaseous phases range from 2–7 days to 45 min for the lavas and scorias, down to about 15 h to 12 min for the pumices erupted during paroxysmal explosions. Estimated syneruptive viscosities range from 102 Pa s for the anhydrous basaltic pumices at 1200 °C, to 103–104 Pa s for lavas approaching their effusion temperatures (1100–1150 °C). In turn, viscosities for the hydrous basaltic melt that led to the formation of the basaltic pumices may be around 10 Pa s or lower. In the light of the above, we discuss the possible shapes and volumes of Stromboli magma chamber by considering a sphere, an ellipsoid (geometrically concordant with the regional stress distribution) and a feeder dyke, the last two being more likely. In the light of volcanological, structural and geophysical data on conduit thickness, we propose an alternative model that takes into account the volumes of recently erupted lavas. This model consists of a convective ellipsoidal magma chamber ‘injected’ by an active feeder dike of undegassed magma of higher temperature, lower density and lower viscosity. This dyke will evolve into a magma column inside the chamber and will separate the reservoir into two lateral, nearly symmetric convective regions. Crystallization would occur preferentially in the proximity of the wallrocks, particularly where the chamber is entering the conduit. The onset of paroxysmal explosions during major effusive cycles may be explained by a drastic increase in the intrusion rates at the base of the chamber that will produce a progressive inflation of the magma column dynamically transferred to the chamber walls. The ceasing of ‘anomalous’ intrusion rates at the base of the chamber, coupled with higher discharge rates, will progressively depressurize the chamber to a critical threshold, until the stress transferred to the walls is dynamically released: at this point the walls themselves will undergo a nearly instantaneous elastic rebound and contract in the attempt to recover their original pre-eruptive geometry. These dynamics will squeeze up portions of the undegassed magma column, triggering a paroxysmal explosion with the ejection of ‘golden pumices’.

Enhanced volcanic hot-spot detection using MODIS IR data: results from the MIROVA system

Abstract We describe a new volcanic hotspot detection system, named Middle InfraRed Observation of Volcanic Activity (MIROVA), based on the analysis of infrared data acquired by the Moderate Resolution Imaging Spectroradiometer sensor (MODIS). MIROVA uses the middle infrared radiation (MIR), measured by MODIS, in order to detect and measure the heat radiation deriving from volcanic activity. The algorithm combines spectral and spatial principles, allowing the detection of heat sources from 1 megawatt (MW) to more than 10 gigawatt (GW). This provides a unique opportunity to: (i) recognize small-scale variations in thermal output that may precede the onset of effusive activity; (ii) track the advance of large lava flows; (iii) estimate lava discharge rates; (iv) identify distinct effusive trends; and, lastly, (v) follow the cooling process of voluminous lava bodies for several months. Here we show the results obtained from data sets spanning 14 years recorded at the Stromboli and Mt Etna volcanoes, Italy, and we investigate the above aspects at these two persistently active volcanoes. Finally, we describe how the algorithm has been implemented within an operational near-real-time processing chain that enables the MIROVA system to provide data and infrared maps within 1–4 h of the satellite overpass.

An integrated dynamic model for the volcanic activity at Poas volcano, Costa Rica

AbstractA dynamic model for the activity of Poas Volcano, Costa Rica, is proposed. Data collected during a three-year period show that the volcanic processes occur within a small hydrothermal system. Heat is supplied by a magma body in the conduit and is transferred to the surface by fluid convection. Within a given volume of rock, pore pressure builds up due to the upward motion of fluids and the increase in vapor pressure when the temperature rises above the boiling point. Ultimately, the system becomes unstable when the pore pressure overcomes the total pressure. This leads to the assumption that the kinetic and thermal energies are proportional to the depth at which the mechanical equilibrium is disturbed. Laboratory experiments were performed by heating samples of the crater lake deposits. The preliminary results of these experiments show significant analogies with the low-energy activity of the volcano. Following this model we estimated that a phreatic explosion which reaches 200 m in height (comparable to the one observed by Francis et al. in 1978) originates at a depth of 70 m and a temperature of 180° C; these values agree with those reported in the literature. In addition, “magmatic” sulfur, which partitions into the rising hydrothermal fluids, reacts at lower temperature and higher

Modelling satellite-derived magma discharge to explain caldera collapse

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Tracking dynamics of magma migration in open-conduit systems

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Hot-spot detection and characterization of strombolian activity from MODIS infrared data

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