Strombolian activity is characterized by repeated, low energy, explosions and is named after the volcano where such activity has persisted for around 2000 years, i.e., Stromboli (Aeolian Islands, Italy). Stromboli represents an excellent laboratory where measurements of such explosions can be made from safe, but close, distances. During a field campaign in 2008, two 15 cm diameter bombs were quenched and collected shortly after a normal explosion. The bombs were characterized in terms of their textural, chemical, rheological, and geophysical signatures. The vesicle and crystal size distribution of the samples, coupled with the glass chemistry, enabled us to quantify variations in the degassing history and rheology of the magma resident in the shallow (i.e., in last 250 m of conduit length). The different textural facies observed in these bombs showed that fresh magma was mingled with batches of partially to completely degassed, oxidized, high-crystallinity, high-viscosity, evolved magma. This magma sat at the top of the conduit and played only a passive role in the explosive process. The fresh, microlite-poor, vesiculated batch, however, experienced a response to the explosive event, by undergoing rapid decompression. Integration of geophysical measurements with sample analyses indicates that popular bubble-bursting models may not fit this case. We suggest that the degassed, magma forms a plug, or rheological layer, at the top of the conduit, through which the fresh magma bursts. In this model we need to consider the paradox of a slug ascending too fast through a magma of varying viscosity and yield strength.

International audienceTextural and chemical analyses of bombs quenched directly from a normal explosion at Stromboli volcano (Italy) were integrated with coincident seismic, acoustic, and thermal data. The data set defines a new gas-dominated type of “strombolian” eruption, named type 0. These events are characterized by high-velocity emission (150–250 m s−1) of a few relatively small, juvenile particles, with entrained non-juvenile clasts that previously fell back into the vent to be re-erupted. For the studied event, the explosion depth was more than 250 m deep, and the particles showed little residence time in the shallow system. Slug ascent velocities over the final 20–35 m of magma-filled conduit, and the low viscosity of the resident magma, are all consistent with simple bubble burst in a “clean” conduit. This conduit type and eruption style likely fit popular slug ascent and burst models used to explain “strombolian” eruptions. In contrast, the ballistic-dominated type of explosions (type 1) are associated with larger proportions of stagnant material in the shallow system magma mix. We argue that the additional volume of this stagnant material pushes the free surface upward. Because of the larger volume of material available for entrainment into a type 1 slug burst, which has to fragment through a thick cap of degassed material, type 1 events tend to be rich in particles. In contrast, the less spectacular, gas-rich (type 0) events have little material to entrain, thus being poor in lapilli and bombs

International audienceWe propose a novel approach to studying a ballistic bomb deposit. Favorable circumstances, a unique dispersal axis, an operational thermal video camera, and application of an innovative methodology allowed estimates of volume and mass erupted, and definition of mass partitioning between bombs of various sizes. This allowed the creation of a multidisciplinary database for a single major explosion at Stromboli volcano (Italy), the type locality of Strombolian eruptions. The dispersion and direction of the deposit were consistent with a major explosion on 21 January 2010. Field data comprised 780 mapped bomb locations and sizes, and were organized into a GIS with a lidar-derived digital elevation model as its base. This allowed us to define the landing distribution and flight parameters for erupted bombs. The data defined discontinuous deposition to build a cluster-dominated bomb field, with a total deposit volume of ∼10 m3, a mass of ∼2 × 104 kg, and a grain size dominated by large bombs (1 m in diameter). The parameters defined here for a major eruption at Stromboli show that the Strombolian style of volcanism, and its deposits, need to be treated carefully, and a different approach is needed in the future to truly characterize and classify such small (but globally common) explosive eruptions. The recognition that sedimentation from such eruptions will be uneven leads to the important conclusion that isopachs and isopleths cannot be used to estimate eruption volumes for such explosions

Explosive activity at Stromboli is explained in terms of dynamics of large gas bubbles that ascend in the magma conduit and burst at the free surface generating acoustic pressure that propagates as infrasonic signals in the atmosphere. The rate and the amplitude of the infrasonic activity is directly linked to the rate and the overpressure of the bursting gas bubbles and thus reflects the rate at which magma column degasses under non-equilibrium pressure conditions. We investigate the link between explosive degassing and magma vesiculation by comparing the rate of infrasonic activity with the bubble size distributions (BSDs) of scoria clasts collected during several days of explosive activity at Stromboli. BSDs of scoria show a characteristic power law distribution, which reflect a gas bubble concentration mainly controlled by a combined process of bubble nucleation and coalescence. The cumulative distribution of the infrasonic pressure follows two power laws, indicating a clear separation between the frequent, but weak, bursting of small gas bubbles (puffing) and the more energetic explosions of large gas slugs. The exponents of power laws derived for puffing and explosive infrasonic activity show strongly correlated (0.96) changes with time indicating that when the puffing rate is high, the number of energetic explosions is also elevated. This correlation suggests that both puffing and explosive activity are driven by the same magma degassing dynamics. In addition, changes of both infrasonic power law exponents are very well correlated (0.92 with puffing and 0.87 with explosions) with variations of the BSD exponents of the scoria clasts, providing evidence of the strong interplay between scoria vesiculation and magma explosivity. Our analysis indicates that variable magma vesiculation regimes recorded in the scoria correlate with the event number and energy of the explosive activity. We propose that monitoring infrasound on active volcanoes may be an alternative way to look at the vesiculation process in open conduit systems.

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