Abstract
Visually unattainable magmatic processes in volcanic conduits, such as degassing, are closely linked to eruptive styles at the surface, but their roles are not completely identified and understood. To gain insights, a multi-parametric experiment at Stromboli volcano (Aeolian Islands, Italy) was installed in July 2016 focusing on the normal explosive activity and persistent degassing. During this experiment, gas-dominated (type 0) and particle-loaded (type 1) explosions, already defined by other studies, were clearly identified. A FLIR thermal camera, an Ultra-Violet SO₂ camera and a scanning Differential Optical Absorption Spectroscopy were deployed to record pyroclast and SO2 masses emitted during individual explosions, as well as persistent SO₂ fluxes, respectively. An ASHER instrument was also deployed in order to collect ash fallouts and to measure the grain size distribution of the samples. SO2 measurements confirm that persistent degassing was far greater than that emitted during the explosions. Further, we found that the data could be characterized by two periods. In the first period (25–27 July), activity was mainly characterized by type 0 explosions, characterized by high velocity jets. Pyroclast mass fluxes were relatively low (280 kg/event on average), while persistent SO2 fluxes were high (274 t/d on average). In the second period (29–30 July), activity was mainly characterized by type 1 explosions, characterized by low velocity jets. Pyroclast mass fluxes were almost ten times higher (2,400 kg/event on average), while persistent gas fluxes were significantly lower (82 t/d on average). Ash characterization also indicates that type 0 explosions fragments were characterized by a larger proportion of non-juvenile material compared to type 1 explosions fragments. This week-long field experiment suggests that, at least within short time periods, Stromboli’s type 1 explosions can be associated with low levels of degassing and the mass of particles accompanying such explosive events depends on the volume of a degassed magma cap sitting at the head of the magma column. This could make the classic particle-loaded explosions of Stromboli an aside from the true eruptive state of the volcano. Instead, gas-dominated explosions can be associated with high levels of degassing and are indicative of a highly charged (with gas) system. We thus suggest that relatively deep magmatic processes, such as persistent degassing and slug formation can rapidly influence the superficial behavior of the eruptive conduit, modulating the presence or absence of degassed magma at the explosion/fragmentation level.
Highlights
Characterizing physical processes that drive explosive eruptions, as well as post-fragmentation transport and deposition of the eruptive products, are mandatory if we are to understand the associated volcanic hazards and risks (e.g., Barberi, 1993; Nave et al, 2010; Rosi et al, 2013)
Summit-based observations made between 25 July and July 30, 2016 permitted to identify two significantly different periods in terms of explosion types (Figure 4A): the first between 25 and 27 July being dominated by Type 0 explosions, and the second between 29 and 30 July being dominated by Type 1 events
Weather conditions were relatively stable during all field measurements, with low wind speed, stable direction and no precipitation, but sometimes with summit cloud cover
Summary
Characterizing physical processes that drive explosive eruptions, as well as post-fragmentation transport and deposition of the eruptive products, are mandatory if we are to understand the associated volcanic hazards and risks (e.g., Barberi, 1993; Nave et al, 2010; Rosi et al, 2013). A number of modern geophysical and sampling techniques allow to define key eruptive parameters that inform on those physical processes These include gas flux and composition ( Aiuppa et al, 2009; Burton et al, 2009; Barnie et al, 2015; Delle Donne et al, 2017; Pering et al, 2020), pyroclast dynamics and size distributions (e.g., Patrick et al, 2007; Harris et al, 2012; Taddeucci et al, 2012), and the physical and chemical characteristics of the eruptive products (e.g., D’Oriano et al, 2010; Pioli et al, 2014; Gurioli et al, 2015). In-spite of the persistence of activity and commonality of source process, normal
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