An increasing number of studies are being presented demonstrating that volcanic glasses can be heterogeneous at the nanoscale. These nano-heterogeneities can develop both during viscosity measurements in the laboratory and during magma eruptions. Our multifaceted study identifies here total transition metal oxide content as a crucial compositional factor governing the tendency of basalt melts and glasses towards nanolitization: at both anhydrous and hydrous conditions, an undercooled trachybasalt melt from Mt. Etna readily develops nanocrystals whose formation also hampers viscosity measurements, while a similar but FeO- and TiO2-poorer basalt melt from Stromboli proves far more stable at similar conditions. We therefore outline a procedure to reliably derive pure liquid viscosity without the effect of nanocrystals, additionally discussing how subtle compositional differences may contribute to the different eruptive styles of Mt. Etna and Stromboli.

Volcanic materials can experience up to eleven orders of magnitude of cooling rate (qc) starting from 10–5 K s−1. The glassy component of volcanic material is routinely measured via differential scanning calorimeter (DSC) to obtain qc through the determination of the glass fictive temperature (Tf). Conventional DSC (C-DSC), which has been employed for decades, can only access a relatively small range of qc (from ~ 10–2 to ~ 1 K s−1). Therefore, extrapolations up to six orders of magnitude of C-DSC data are necessary to derive qc of glasses quenched both at extremely low and high qc. Here, we test the reliability of such extrapolations by combining C-DSC with the recently introduced flash calorimetry (F-DSC). F-DSC enables to extend the qc exploration up to 104 K s−1. We use three synthetic glasses as analogs of volcanic melts. We first apply a normalization procedure of heat flow data for both C-DSC and F-DSC to derive Tf as a function of experimental qc, following the “unified area-matching” approach. The obtained Tf–qc relationship shows that Arrhenius models, widely adopted in previous studies, are only valid for qc determination within the calibration range. In contrast, a non-Arrhenius model better captures qc values, especially when a significant extrapolation is required. We, therefore, present a practical “how-to” protocol for estimating qc using DSC.

The 2021 Tajogaite eruption of Cumbre Vieja (La Palma, Spain) was typified by the emission of low viscosity lavas that flowed at high velocities and inundated a large area. We experimentally investigated the rheological evolution of melt feeding the eruption through concentric cylinder viscometry to understand the exceptional flowing ability of these lavas and constrain its emplacement dynamics. We conducted a set of cooling deformation experiments at different cooling rates (from 0.1 to 10 °C/min), and isothermal deformation experiments at subliquidus dwell temperatures between 1225 and 1175°C. All experiments were conducted at a shear rate of 10 s−1. Results show that disequilibrium crystallization and its timescale fundamentally control the rheological evolution of the melt, resulting in different rheological response to deformation of the crystal‐bearing magmatic suspension. Integrating rheological data with field observations allows us to shed light on the mechanisms that govern the high flowability of these lavas.

The thermal evolution of pyroclastic deposits is the primary factor controlling the rheology of secondary deformation during pyroclastic density currents (PDC) transport and deposition, influencing processes such as welding and rheomorphism (G. Giordano & Cas, 2021). Welding is defined as the adhesion and plastic compaction of hot pyroclasts and can occur gradually during the deposition of an ignimbrite (

<p>The silicic flows and domes can impose mechanical and thermal stress on the underlying substrate causing mineralization and lithification of granular bodies. In addition, the released water from the permeable substrate as dominant volatile species can contribute to the glass hydration of the flow. This fluid-lava interaction can be directly studied in ancient successions with exposed contacts. The Lebuj flow (Tokaj Mountains, Hungary) developed in a Miocene caldera setting, where the erosion revealed its basal zone including lava-substrate interaction textures. The main textural units comprise (1) a rhyolitic lava flow (F<sub>1</sub>: perlitic glass with obsidian marekanite, F<sub>2</sub>: microcrystalline-glass transition and F<sub>3</sub>: a basal breccia layer) and (2) the underlying mixed substrate unit (S<sub>1</sub>: massive rhyolite and breccia S<sub>2</sub>: enclosed partially sintered rhyolite tuff). The thin section textural analyses were completed by BSE imaging, Raman mapping (SiO<sub>2</sub> polymorphs) and FTIR spot measurement (perlite H<sub>2</sub>O, clays). Glass transition temperature (T<sub>g</sub>) was estimated using the chemical based GRD model.</p><p>The flow margin contacted with underlying volcanoclastic deposits along a steeply inclined (50-75°) plane with subordinate fragmentation. The substrate suffered re-heating by the flow where porosity loss and welding (solid-state sintering) occurred. The silica polymorphs are observed growing into open pore spaces and fractures and interpreted as precipitates from vapor phase fluids passing through the permeable lithologies. The smectite group minerals typically record acidic type alteration, where the water-rock interaction commonly produces glass replacement minerals. The FTIR-identified clays (mixed layer kaolinite/montmorillonite or beidellite) indicate low-to medium alteration degree (estimated temperature between 50-100 °C).</p><p>The lithophysae, spherulites and microcrystalline bands in the flow unit are textural evidence for prolonged groundmass crystallization above T<sub>g</sub>. The relict obsidian grains in the glass are proofs of an incomplete hydration process. The FTIR and BSE investigations demonstrate the presence of sharp transitions from the hydrated ~3 wt.% perlitic rims to non-hydrated obsidian cores.</p><p>Textural and mineralogical evidence suggest that the following series of events occurred as the consequences of the lava-substrate interaction: a) a viscous rhyolite flow advanced on an irregular topography; b) shear and brittle fracturing occurred at the contact; c) groundmass crystallization (above T<sub>g</sub>, ~ 690-715 °C) and hydration (below T<sub>g</sub>) acted in the flow; d) low temperature mineralization and variable scale sintering occurred in the substrate (below T<sub>g</sub>). According to the fluid exchange model beneath silicic lava domes (Ball et al. 2015), the water – rock interaction resulted in weak hydrothermal alteration of the substrate and water flux to the quenched glass (flow). As an interaction of the two processes, the increased sintering and mineralization reduced the porosity of the substrate which probably restricted further water uptake for hydration. Thus the obsidian results from a ‘quenched’ hydration front (Bindeman and Lowenstern 2016).</p><p> </p><p>Ball J.L., Stauffer P.H., Calder E.S., Valentine G.A. (2015). Bull Volcanol 77:1–16.</p><p>Bindeman I.N., Lowenstern J.B.  (2016).  Contrib to Mineral Petrol 171:89</p><p><strong> </strong></p><p><strong>Aknowledgements</strong></p><p>This research has been funded by the Hungarian–Italian MTA-CNR bilateral research project 2019–2022. The research was also supported by Development and Innovation Office–NKFIH No. 131869 OTKA project. RL was supported by the Bolyai János Research Fellowship.</p>