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Steam‐driven eruptions, both phreatic and hydrothermal, expel exclusively fragments of non‐juvenile rocks disintegrated by the expansion of water as liquid or gas phase. As their violence is related to the magnitude of the decompression work that can be performed by fluid expansion, these eruptions may occur with variable degrees of explosivity. In this study we investigate the influence of liquid fraction and rock petrophysical properties on the steam‐driven explosive energy. A series of fine‐grained heterogeneous tuffs from the Campi Flegrei caldera were investigated for their petrophysical properties. The rapid depressurization of various amounts of liquid water within the rock pore space can yield highly variable fragmentation and ejection behaviors for the investigated tuffs. Our results suggest that the pore liquid fraction controls the stored explosive energy with an increasing liquid fraction within the pore space increasing the explosive energy. Overall, the energy released by steam flashing can be estimated to be 1 order of magnitude higher than for simple (Argon) gas expansion and may produce a higher amount of fine material even under partially saturated conditions. The energy surplus in the presence of steam flashing leads to a faster fragmentation with respect to gas expansion and to higher ejection velocities imparted to the fragmented particles. Moreover, weak and low permeability rocks yield a maximum fine fraction. Using experiments to unravel the energetics of steam‐driven eruptions has yielded estimates for several parameters controlling their explosivity. These findings should be considered for both modeling and evaluation of the hazards associated with steam‐driven eruptions., Key Points Experimental studies to unravel the steam‐driven eruption energeticsInfluence of liquid fraction and rock petrophysical properties on the explosive energySteam flashing results in a faster fragmentation with respect to gas expansion and into higher ejection velocities of fragmented material

Viscosity has been determined during isothermal crystallization of an andesite from Tungurahua volcano (Ecuador). Viscosity was continuously recorded using the concentric cylinder method and employing a Pt‐sheathed alumina spindle at 1 bar and from 1400°C to subliquidus temperatures to track rheological changes during crystallization. The disposable spindle was not extracted from the sample but rather left in the sample during quenching thus preserving an undisturbed textural configuration of the crystals. The inspection of products quenched during the crystallization process reveals evidence for heterogeneous crystal nucleation at the spindle and near the crucible wall, as well as crystal alignment in the flow field. At the end of the crystallization, defined when viscosity is constant, plagioclase is homogeneously distributed throughout the crucible (with the single exception of experiment performed at the lowest temperature). In this experiments, the crystallization kinetics appear to be strongly affected by the stirring conditions of the viscosity determinations. A TTT (Time‐Temperature‐Transformation) diagram illustrating the crystallization “nose” for this andesite under stirring conditions and at ambient pressure has been constructed. We further note that at a given crystal content and distribution, the high aspect ratio of the acicular plagioclase yields a shear‐thinning rheology at crystal contents as low as 13 vol %, and that the relative viscosity is higher than predicted from existing viscosity models. These viscosity experiments hold the potential for delivering insights into the relative influences of the cooling path, undercooling, and deformation on crystallization kinetics and resultant crystal morphologies, as well as their impact on magmatic viscosity., Key Points: Viscosity determination at subliquidus temperatures of an andesitic sampleUse of disposable Pt‐sheathed alumina spindlesDescription of viscosity and crystallization kinetics under constant stirring

We combine geophysical and experimental observations to interpret preeruptive unrest at Volcán de Colima in 1998. 17,893 volcanic earthquakes were detected between 1 October and 31 December 1998, including 504 clusters. Using seismic ambient noise interferometry, we observe a drop in velocity prior to the eruption linked to damage accumulation during magma ascent. This is supported by experimental observations where static stress causes a velocity decrease prior to failure. Furthermore, we observe acoustic emission clusters during the experiments, with lower porosity samples producing higher numbers of repeaters. This behavior introduces tensile failure as an additional viable mechanism for clusters during magma ascent. The findings suggest that preeruptive magma ascent may be monitored to variable degrees of accuracy via descriptions of damage accumulation and associated seismic velocity changes., Key Points Multidisciplinary seismic and experimental investigation of the 1998 eruption of Volcán de ColimaRelative velocity decrease seen in seismic ambient noise and during experimental Brazil tests prior to eruption/failureEvidence for tensile failure in the volcanic edifice as a source of repeating earthquakes during magma ascent

Raman spectrometers will form a key component of the analytical suite of future planetary rovers intended to investigate geological processes on Mars. In order to expand the applicability of these spectrometers and use them as analytical tools for the investigation of silicate glasses, a database correlating Raman spectra to glass composition is crucial. Here we investigate the effect of the chemical composition of reduced silicate glasses on their Raman spectra. A range of compositions was generated in a diffusion experiment between two distinct, iron‐rich end‐members (a basalt and a peralkaline rhyolite), which are representative of the anticipated compositions of Martian rocks. Our results show that for silica‐poor (depolymerized) compositions the band intensity increases dramatically in the regions between 550–780 cm−1 and 820–980 cm−1. On the other hand, Raman spectra regions between 250–550 cm−1 and 1000–1250 cm−1 are well developed in silica‐rich (highly polymerized) systems. Further, spectral intensity increases at ~965 cm−1 related to the high iron content of these glasses (~7–17 wt % of FeOtot). Based on the acquired Raman spectra and an ideal mixing equation between the two end‐members we present an empirical parameterization that enables the estimation of the chemical compositions of silicate glasses within this range. The model is validated using external samples for which chemical composition and Raman spectra were characterized independently. Applications of this model range from microanalysis of dry and hydrous silicate glasses (e.g., melt inclusions) to in situ field investigations and studies under extreme conditions such as extraterrestrial (i.e., Mars) and submarine volcanic environments., Key Points Raman spectra of glasses are presented for a large range of chemical compositions, analogous to Martian compositionsSpectral intensities are parameterized to derive a model for estimation of the glass composition from Raman spectraThis enables in situ estimations of glass compositions in extraterrestrial or submarine volcanic settings