Laura Pioli

The 15 March 2007 explosive crisis at Stromboli volcano, Italy: Assessing physical parameters through a multidisciplinary approach

Basaltic volcanoes are dominated by lava emission and mild explosive activity. Nevertheless, many basaltic systems exhibit, from time to time, poorly documented and little-understood violent explosions. A short-lived, multiblast explosive crisis (paroxysmal explosion) occurred on 15 March 2007 during an effusive eruptive crisis at Stromboli (Italy). The explosive crisis, which started at 20:38:14 UT, had a total duration of ∼5 min. The combined use of multiparametric data collected by the permanent instrumental networks (seismic, acoustic, and thermal records) and a field survey carried out immediately after the event enabled us to constrain the eruptive dynamics and quantify physical parameters. The eruption consisted of three major pulses: In the first, lithic blocks and ash were ejected at speeds of 100–155 m/s and 130–210 m/s, respectively. The high solid load of the eruptive jet resulted in the partial collapse of the column with the formation of a small-volume pyroclastic density current. The second, 12 s long pulse emitted 2.2–2.7 × 107 kg of tephra (mass discharge rate = 1.9–2.3 × 106 kg/s), forming a 3 km high convective plume, dispersing tephra up to the west coast, and a dilute density current with limited dispersal downslope of the craters. A final, 30 s long phase formed a scoria flow with a volume of 1.5–1.7 × 104 m3 (mass discharge rate = 5.9–6.7 × 105 kg/s), a total runout of ∼200 m, and a velocity of 45 m/s. The total gas volume involved in the explosion was 1.3–1.9 × 104 m3 with an initial overpressure of 7.9 ± 0.4 MPa. We compared the 15 March 2007 event with historical paroxysms, in particular with that of 5 April 2003, which was remarkably similar.

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.

Determination of the largest clast sizes of tephra deposits for the characterization of explosive eruptions: a study of the IAVCEI commission on tephra hazard modelling

The distribution of clasts deposited around a volcano during an explosive eruption typically contoured by isopleth maps provides important insights into the associated plume height, wind speed and eruptive style. Nonetheless, a wide range of strategies exists to determine the largest clasts, which can lead to very different results with obvious implications for the characterization of eruptive behaviour of active volcanoes. The IAVCEI Commission on Tephra Hazard Modelling has carried out a dedicated exercise to assess the influence of various strategies on the determination of the largest clasts. Suggestions on the selection of sampling area, collection strategy, choice of clast typologies and clast characterization (i.e. axis measurement and averaging technique) are given, mostly based on a thorough investigation of two outcrops of a Plinian tephra deposit from Cotopaxi volcano (Ecuador) located at different distances from the vent. These include: (1) sampling on a flat paleotopography far from significant slopes to minimize remobilization effects; (2) sampling on specified-horizontal-area sections (with the statistically representative sampling area depending on the outcrop grain size and lithic content); (3) clast characterization based on the geometric mean of its three orthogonal axes with the approximation of the minimum ellipsoid (lithic fragments are better than pumice clasts when present); and (4) use of the method of the 50th percentile of a sample of 20 clasts as the best way to assess the largest clasts. It is also suggested that all data collected for the construction of isopleth maps be made available to the community through the use of a standardized data collection template, to assess the applicability of the new proposed strategy on a large number of deposits and to build a large dataset for the future development and refinement of dispersal models.

The Paroxysmal event and its deposits

The 5 April 2003 eruption of Stromboli volcano (Italy) was the most violent of the past 50 years. It was also the best documented due to the accurate geophysical monitoring of the ongoing effusive eruption. Detailed field studies carried out a few hours to a few months after the event provided further information that were coupled with visual documentation to reconstruct the explosive dynamics. The eruption consisted of an 8-min-long explosive event preceded by a short-lived precursory activity that evolved into the impulsive ejection of gas and pyroclasts. Meter-sized ballistic blocks were launched to altitudes of up to 1400 m above the craters falling on the volcano flanks and on the village of Ginostra, about 2 km far from the vent. The vertical jet of gas and pyroclasts above the craters fed a convective plume that reached a height of 4 km. The calculated erupted mass yielded values of 1.1―1.4 × 10 8 kg. Later explosions generated a scoria flow deposit, with an estimated mass of 1.0―1.3 × 10 7 kg. Final, waning ash explosions closed the event. The juvenile fraction consisted of an almost aphyric, highly vesicular pumice mingled with a shallow-derived, crystal-rich, moderately vesicular scoria. Resuming of the lava emission a few hours after the paroxysm indicate that the shallow magmatic system was not significantly modified during the explosions. Combination of volume data with duration of eruptive phases allowed us to estimate the eruptive intensity: during the climactic explosive event, the mass discharge rate was between 10 6 and 10 7 kg/s, whereas during the pyroclastic flow activity, it was 2.8―3.6 ― 10 5 kg/s. Strong similarities with other historical paroxysms at Stromboli suggest similar explosion dynamics.

Transient explosions at open-vent volcanoes: The case of Stromboli (Italy)

Stromboli is a persistently active, open-vent basaltic volcano whose activity is controlled by the balance between magma supply, outgassing, and eruptive rates, and is characterized by low-intensity, regular Strombolian explosions. However, two types of large, transient, violent explosive eruptions suddenly occur with no clear precursory activity. These explosions, called “major” and “paroxysmal” depending on size, cover a large variability in intensity and mag- nitude, but are all marked by short duration. Paroxysms have significantly larger intensities (>106 kg/s) than major explosions (104 kg/s) and fundamental differences in the characteris- tics (composition, crystallinity, vesicularity) of the erupted tephra, suggesting that different sources feed these two eruption types. Paroxysms are generated by the explosive fragmen- tation of low-porphyricity (LP) magma mingled with high-porphyricity resident magma in the shallow reservoir, whereas major eruptions are likely associated with destabilization of the lower portion of the shallow magmatic system, continuously hybridized by the arrival of LP magma. In general, the intensity of these explosions is related to the amount of the LP magma erupted (>107 kg in paroxysms and 104–105 kg in major explosions), suggesting that the magma plays a major role in the fragmentation mechanism. Despite its primary impor- tance in the hazards of Stromboli, the total amount of magma erupted in these events in the past 10 years is less than 1% of the total mass erupted by the volcano.

The eruptive activity of 28 and 29 December 2002

At 1820 UT of 28 December 2002, an eruptive vent opened on the NE flank of the Sciara del Fuoco (SdF) at 600 m above sea level, marking the onset of the 2002–2003 eruptive crisis of Stromboli volcano. The first eruptive hours were characterized by mild spattering and effusive activity from the new vent and the summit vent at crater 1. Gravitational instability processes also determined the partial collapse of NE walls of the summit cone (crater 1). Pyroclastic material partly accumulated on the NE part of the SdF and partly flowed downslope and reached the sea at Spiaggia dei Gabbiani, forming a ~4-m-thick, reddish avalanche, that was soon covered by a lava flow emitted in the following hours. In this paper, we describe the first hours of activity through eyewitnesses’ reports, geophysical monitoring, field and laboratory studies, of the erupted pyroclastic material and lava flows. Daily temperature measurements were carried out on the avalanche deposit formed by the flow of scoria along the SdF, using a handheld thermal camera mainly during helicopter surveys. A fast cooling rate was typical of the deposit surface, and a slow cooling rate was representative of its inner portion.

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.

Outgassing and eruption of basaltic magmas: The effect of conduit geometry

The flow dynamics of magmas is controlled by several parameters, including magma rheology, density, surface tension, gas and liquid flow rate, and the geometry of the flow field, which is mainly regulated by the conduit shape. For this reason, magmatic flows along fissures and dikes are significantly different from those along axisymmetric conduits. Basaltic eruptions are typically fed by fractures or dike systems that reach the surface, giving rise to fissure eruptions. Scaled experiments show that in these low-viscosity systems, gas transport and segregation (i.e., the outgassing dynamics) are deeply controlled by the fracture geometry. Rapid bubble clustering and formation of bubble plumes determine the formation of lateral magma convection cells involving up to 70 vol% of the melt. In analogy with outgassing in cylindrical conduits, the average vesicularity and size of bubbles increase with increasing gas flow rate and melt viscosity and density, which also control the lateral extent of the bubble plume and convection cells.