Recycled ejecta modulating Strombolian explosions

Abstract

Two main end-members of eruptive regimes are identified from analyses of high-speed videos collected at Stromboli volcano (Italy), based on vent conditions: one where the vent is completely clogged by debris, and a second where the vent is open, without any cover. By detailing the vent processes for each regime, we provide the first account of how the presence of a cover affects eruptive dynamics compared to open-vent explosions. For clogged vents, explosion dynamics are controlled by the amount and grain size of the debris. Fine-grained covers are entirely removed by explosions, favouring the generation of fine ash plumes, while coarse-grained covers are only partially removed by the explosions, involving minor amounts of ash. In both fine-and coarse-grained cases, in-vent ground deformation of the debris reflect variations in the volumetric expansion of gas in the conduit, with rates of change of the deformation comparable to ground inflation related to pre-burst conduit pressurization. Eruptions involve the ejection of relatively slow and cold bombs and lapilli, and debris is observed to both fall back into the vent after each explosion and to gravitationally accumulate between explosions by rolling down the inner crater flanks to produce the cover itself. Part of this material may also contribute to the formation of a more degassed, crystallized and viscous magma layer at the top of the conduit. Conversely, open-vent explosions erupt with hotter pyroclasts, with higher exit velocity and with minor or no ash phase involved.

Evidences of Plug Pressurization Enhancing Magma Fragmentation During the September 2016 Basaltic Eruption at Piton de la Fournaise (La Réunion Island, France)

“…As a conclusion, we suggest that the sideromelane and tachylite textures are inherited from a progressive subsurface cooling, inducing advanced degassing and secondary crystallization of the magma, which may occur either by (i) recycling material within active vents as already proposed for Etna (D'Oriano et al, 2014) and/or (ii) by directly cooling of the magma at its free surface facilitated by the slight decrease in the lava flux (Figures 2a and 2b, clogging of Vent B) as already observed on Stromboli (Gurioli et al, 2014) and/or (iii) by collapse of inner parts of the vent as we observed some collapse and instabilities features within the eruptive vents ( Figure 3) again observed at Stromboli (Capponi et al, 2016;Patrick et al, 2007). In each case, this degassed, crystallized, and viscous magma, represented by the sideromelane and mainly the tachylite component, formed a relatively impermeable magmatic layer at the fragmentation level within Vent B, during its decrease in activity mainly between 16 and 17 September.…”

The Initial Development of Transient Volcanic Plumes as a Function of Source Conditions

“…A short acceleration exists but is not observed here due to geometrical constrains. Second and most relevant, almost all our explosions featured not one but multiple ejection pulses, also from more than one vent (Capponi et al, ; Gaudin et al, ; Scharff et al, ; Taddeucci et al, ). Our observations focus on the initial development of plumes in a region, which is relatively close to the vent area, and it remains open to discussion how much of the complexity we observe is preserved in the morphology of plumes at later moments and higher elevations above the vent.…”

“…A short acceleration exists but is not observed here due to geometrical constrains. Second and most relevant, almost all our explosions featured not one but multiple ejection pulses, also from more than one vent (Capponi et al, 2016;Gaudin et al, 2014;Scharff et al, 2015;Taddeucci et al, 2012). Our observations focus on the initial development of plumes Note.…”

“…In the first stage, the evolution of the plume is dominated by the initial momentum of an eruptive vent (gas-thrust phase), while at a later stage plume rise is dominated by buoyancy originating from the entrainment and heating of the surrounding atmosphere (buoyancy-driven phase). In eruptions with transient plumes, pulsating behavior of ejection at the vent has been observed repeatedly (Capponi et al, 2016;Gaudin et al, 2017Gaudin et al, , 2014Harris et al, 2012;Scharff et al, 2015;Taddeucci et al, 2012). Such unsteady vent discharge is expected to induce large changes on the resulting morphology and dynamics of plumes (Chojnicki et al, 2014(Chojnicki et al, , 2015a(Chojnicki et al, , 2015bClarke et al, 2002;.…”

Rapid crystallization during recycling of basaltic andesite tephra: timescales determined by reheating experiments

“…An extreme example comes from mild submarine eruptions observed at NW Rota-1 (Mariana arc), where eruption plumes are suppressed by the overlying water column9. Abundant pyroclast recycling has also been described at Stromboli volcano (Italy)11. In both cases, the extent of vent clogging affects the subsequent eruption intensity12 and grain size distribution9.…”