Margherita Polacci

Coupled textural and compositional characterization of basaltic scoria: Insights into the transition from Strombolian to fire fountain activity at Mount Etna, Italy

Strombolian and fire fountain activities represent a common expression of explosive basaltic eruptions. However, the transition between these two eruptive styles and their source mechanisms are still debated. We use textural and compositional studies to characterize pyroclastic material from both the Strombolian and Hawaiian-style fire fountain phases of the January–June 2000 Etna activity. We find that basaltic scoria presents distinctive textural and compositional features that reflect different modes of magma vesiculation and crystallization in the two eruptive regimes. Overall, magma that forms Strombolian scoria is far more crystallized, less vesicular, and more evolved, indicating strong volatile depletion and longer residence time before being erupted. Fire fountain scoria indicates a fast-rising magma with evidence of moderate syneruptive volatile exsolution. The new textural and compositional data set is integrated with previous volcanological and geophysical investigations to provide further insights into the dynamics of fire fountains, and to frame the transition from Strombolian explosions to fire fountain activity into a model that may apply to future eruptions at Mount Etna as well as other active basaltic volcanoes.

Three-dimensional investigation of volcanic textures by X-ray microtomography and implications for conduit processes

[1] Synchrotron X-ray computed microtomography (μCT) was applied for the first time to clasts of pumice and scoria generated by active, explosive volcanoes characterized by a range of eruptive styles (mild Strombolian to Plinian) and magmatic composition (basaltic to trachytic). The obtained two-dimensional (2D) tomographic images, corresponding to sample views at different rotation angles, were processed to reconstruct three-dimensional (3D) volumes and then used to make 3D measurements of vesicularity, vesicle number density, volume and connectivity for quantitative characterization of the investigated tephras. The results indicate a positive correlation between vesicle number density and eruption intensity that is used to investigate modes of magma degassing in explosive eruptions. In addition, the vesicle geometry affecting the connected gas flow pathways in pyroclasts from Strombolian eruptions is determined and related to the known permeabilities. Implications on the dynamics of explosive eruptions is discussed and, ultimately, used to quantitatively discriminate between different eruptive styles.

Vesiculation in magmas from Stromboli and implications for normal Strombolian activity and paroxysmal explosions in basaltic systems

[1] We performed a series of X-ray tomographic experiments and lattice Boltzmann permeability simulations on pyroclastic products from explosive activity at Stromboli between December 2004 and May 2006. We reconstructed the 3-D textures of vesicles to investigate the relationship between the nature of vesiculation in the erupted products and the dynamics of gas transport in the shallow conduit in order to derive implications for the eruptive behavior of basaltic volcanoes. Scoriae from normal Strombolian explosions display remarkably consistent vesicle volume distributions fit by power laws with an exponent of 1 (±0.2). We ascribe the origin of such distributions to the combined effect of coalescence and continuous nucleation events in the steady state, shallow magma system that supplies normal Strombolian activity. Volume distributions and textures of vesicles in pumice clasts from the 5 April 2003 and 15 March 2007 paroxysmal activity are markedly different from those in the scoriae. Besides a power law function with a higher exponent, portions of these distributions can be also fit by an exponential function, suggesting the attempt of the system to reach near-equilibrium conditions. The investigated pumice clasts also lack the large, connecting vesicles responsible for the development of degassing pathways in the Stromboli magma that erupts the scoriae. This testifies to a decreased degassing efficiency of the magma associated with paroxysmal explosions and potential overpressure buildup at depth. By comparison with degassing experiments on basaltic melts, we derive a time constraint on the order of minutes to hours for the incubation of paroxysms at Stromboli.

The evolution of lava flows from ephemeral vents at Mount Etna: Insights from vesicle distribution and morphological studies

Abstract Lava flow units from ephemeral vents at Mount Etna are characterized by a cross-flow subdivision into zones with different surface morphology, symmetrically distributed with respect to the centerline of the flow. These zones are: (1) a central zone (cp zone) with a relatively smooth surface near the vent; (2) a lateral zone (lp zone) covered by a carpet of clinkers overlying the massive lava body; and (3) a lateral levee zone (ll zone). In many cases the cp and lp zones are separated by an inward-dipping groove, and the cp zone is from some decimeters up to more than 1 m thicker than the adjacent lp zone. In sections perpendicular to the flow direction, the vesicles corresponding with the cp zone are systematically distributed in a ring-like region and are characterized by a sub-elliptical shape with their minor axes along a direction radial to the center of such region. Vesicles corresponding with the lp zone are still elliptical and their major axes tend to be parallel to the nearest cooling surface. Vesicularity is the lowest at the center of the ring-like region where vesicles approach a spherical shape. In the upper part of the ring-like region, vesicle accumulation and coalescence below the crust produces one or more gas-rich layers which act as levels of preferential detachment for the formation of lava tubes. The observed morphological characteristics and reconstructed vesicle distribution patterns are consistent with a radial distribution of velocity within the lava flow unit and suggest the existence of an inner plug flow region where the velocity gradient is zero. A simple evolution scheme is proposed in which a lava flow unit from ephemeral vent invariably evolves to a lava tube through the downward migration of the plug flow region. The progressive reduction of the cross-flow section due to cooling produces a pressure increase within the flowing lava body and the thickening of the active central portion of the lava flow unit in order to satisfy mass conservation.

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

Dynamics of magma ascent and fragmentation in trachytic versus rhyolitic eruptions

Abstract We have performed a parametric study on the dynamics of trachytic (alkaline) versus rhyolitic (calc-alkaline) eruptions by employing a steady, isothermal, multiphase non-equilibrium model of conduit flow and fragmentation. The employed compositions correspond to a typical rhyolite and to trachytic liquids from Phlegrean Fields eruptions, for which detailed viscosity measurements have been performed. The investigated conditions include conduit diameters in the range 30–90 m and total water contents from 2 to 6 wt%, corresponding to mass flow rates in the range 106–108 kg/s. The numerical results show that rhyolites fragment deep in the conduit and at a gas volume fraction ranging from 0.64 to 0.76, while for trachytes fragmentation is found to occur at much shallower levels and higher vesicularities (0.81–0.85). An unexpected result is that low-viscosity trachytes can be associated with lower mass flow rates with respect to more viscous rhyolites. This is due to the non-linear combined effects of viscosity and water solubility affecting the whole eruption dynamics. The lower viscosity of trachytes, together with higher water solubility, results in delayed fragmentation, or in a longer bubbly flow region within the conduit where viscous forces are dominant. Therefore, the total dissipation due to viscous forces can be higher for the less viscous trachytic magma, depending on the specific conditions and trachytic composition employed. The fragmentation conditions determined through the simulations agree with measured vesicularities in natural pumice clasts of both magma compositions. In fact, vesicularities average 0.80 in pumice from alkaline eruptions at Phlegrean Fields, while they tend to be lower in most calc-alkaline pumices. The results of numerical simulations suggest that higher vesicularities in alkaline products are related to delayed fragmentation of magmas with this composition. Despite large differences in the distribution of flow variables which occur in the deep conduit region and at fragmentation, the flow dynamics of rhyolites and trachytes in the upper conduit and at the vent can be very similar, at equal conduit size and total water content. This is consistent with similar phenomenologies of eruptions associated with the two magma types.

The contribution of synchrotron X-ray computed microtomography to understanding volcanic processes

A series of computed microtomography experiments are reported which were performed by using a third-generation synchrotron radiation source on volcanic rocks from various active hazardous volcanoes in Italy and other volcanic areas in the world. The applied technique allowed the internal structure of the investigated material to be accurately imaged at the micrometre scale and three-dimensional views of the investigated samples to be produced as well as three-dimensional quantitative measurements of textural features. The geometry of the vesicle (gas-filled void) network in volcanic products of both basaltic and trachytic compositions were particularly focused on, as vesicle textures are directly linked to the dynamics of volcano degassing. This investigation provided novel insights into modes of gas exsolution, transport and loss in magmas that were not recognized in previous studies using solely conventional two-dimensional imaging techniques. The results of this study are important to understanding the behaviour of volcanoes and can be combined with other geosciences disciplines to forecast their future activity.

Novel interpretation for shift between eruptive styles in some volcanoes

The transition from effusive, low mass flow rate to explosive, high mass flow rate eruptive behavior is a common aspect of the activity of calc-alkaline volcanoes. However, the process driving the shift between the two eruptive styles is at present debatable and represents a topical theme in the volcanological literature. The main challenge is to understand the mechanism that allows a high mass flow rate when eruptions of highly porphyritic (40 vol % crystals) and viscous magmas (≥106 Pas, pascal second) occur. In this article, volcanological, compositional, and textural observations are used to demonstrate that viscous dissipation, a process that develops heating within flowing magma in a boundary layer near the conduit walls due to friction, is responsible for the long-lasting, sustained explosive phases of this eruption type, as well as for the transition from effusive to explosive behavior.

Toward continuous quantification of lava extrusion rate: Results from the multidisciplinary analysis of the 2 January 2010 eruption of Piton de la Fournaise volcano, La Réunion

The dynamics of the 2–12 January 2010 effusive eruption at Piton de la Fournaise volcano were examined through seismic and infrasound records, time-lapse photography, SO2 flux measurements, deformation data, and direct observations. Digital elevation models were constructed for four periods of the eruption, thus providing an assessment of the temporal evolution of the morphology, the volume and the extrusion rate of the lava flow. These data were compared to the continuous recording of the seismic and infrasonic waves, and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate. This relationship is supported by data from three other summit eruptions of Piton de la Fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies. We can therefore provide an estimate of the lava extrusion rate for the January 2010 eruption with a very high temporal resolution. We found an average lava extrusion rate of 2.4 m3s−1 with a peak of 106.6 m3s−1 during the initial lava fountaining phase. We use the inferred average lava extrusion rate during the lava fountaining phase (30.23 m3s−1) to estimate the value of the initial overpressure in the magma reservoir, which we found to range from 3.7×106 Pa to 5.9×106 Pa. Finally, based on the estimated initial overpressure, the volume of magma expelled during the lava fountaining phase and geodetic data, we inferred the volume of the magma reservoir using a simple Mogi model, between 0.25 km3 and 0.54 km3, which is in good agreement with previous studies.

MeMoVolc consensual document: a review of cross-disciplinary approaches to characterizing small explosive magmatic eruptions

A workshop entitled “Tracking and understanding volcanic emissions through cross-disciplinary integration: a textural working group” was held at the Université Blaise Pascal (Clermont-Ferrand, France) on the 6–7 November 2012. This workshop was supported by the European Science Foundation (ESF). The main objective of the workshop was to establish an initial advisory group to begin to define measurements, methods, formats and standards to be applied in the integration of geophysical, physical and textural data collected during volcanic eruptions. This would homogenize procedures to be applied and integrated during both past and ongoing events. The workshop comprised a total of 35 scientists from six countries (France, Italy, Great Britain, Germany, Switzerland and Iceland). The four main aims were to discuss and define: standards, precision and measurement protocols for textural analysis; identification of textural, field deposit, chemistry and geophysical parameters that can best be measured and combined; the best delivery formats so that data can be shared between and easily used by different groups; and multi-disciplinary sampling and measurement routines currently used and measurement standards applied, by each community. The group agreed that community-wide, cross-disciplinary integration, centred on defining those measurements and formats that can be best combined, is an attainable and key global focus. Consequently, we prepared this paper to present our initial conclusions and recommendations, along with a review of the current state of the art in this field that supported our discussions.