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Volcano seismicity is one of the key parameters to understand magma dynamics of erupting volcanoes. However, the physical process at the origin of the resulting complex and broadband seismic signals remains unclear. Syn-eruptive very long period (VLP) seismic signals have been explained in terms of the sudden expansion of gas pockets rising in the liquid melt. Their origin is linked to a magma dynamics which triggers the explosive process occurring before the explosive onset. We provide evidence based on acoustic, thermal, and ground deformation data to demonstrate that VLP signals at Stromboli are generated at the top of the magma column mainly after the explosion onset. We show that VLP amplitude and duration scale with the eruptive flux which induces a decompression of 103–104 Pa involving the uppermost ~ 250 m of the feeding conduit. The seismic VLP source represents the final stage of a ~ 200 s long charge and discharge mechanism the magma column has to release excess gas accumulated at the base of a denser and degassed magma mush. The position of the VLP seismic source coincides with the centroid of the shallow mush plug and tracks elevation changes of the magma free surface. [ABSTRACT FROM AUTHOR]

• Block resurgence following crater subsidence was observed at Nyamulagira volcano. • Lava-flow dynamics was inferred by thermal modelling constrained by satellite data. • Subsidence results from the compaction of a mix of molten and solidified lava layers. • The new unconventional streaky model explains the pit-crater depth fluctuations. The rise and fall of magma columns is a process commonly observed in volcanoes hosting lava lakes and serves as proxy of magmatic reservoir pressure and warning for potential eruptions. This dynamic process typically involves the filling and emptying of a "pit crater" - a cylindrical depression usually formed by one or more collapses of a structure lying above an emptied surficial reservoir. Between 2016 and 2020, the 300 m deep pit crater located within Nyamulagira volcano (Democratic Republic of Congo) underwent an unusual filling dynamic. In May 2019 while filled to about 90% the crater floor collapsed by 90 m within two months, followed by the rapid emergence of a sub-circular solidified block overlooking the crater rim in less than 6 months. Using numerical simulations of models based on thermal energy and constrained by multiparametric data, we account for the incremental filling of the crater by successive intra-crateric lava flows and the subsequent collapse of the crater floor. We further characterize this unconventional filling mode based on thermal budget considerations. [ABSTRACT FROM AUTHOR]

Formation of secondary sulfate minerals during the reaction between volcanic gases and rocks modulates the composition and flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (MI MIII (XVIO 4) 2 with MI = NH 4 +, Na+, K+; MIII = Al3+, Fe3+ and XVI = S6+, Mo6+) in the 330 °C fumaroles of the Lascar volcano (Chile). The alum occurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rock cations within the crust. The average growth rate is estimated at ca. 0.3 μm/day, based on the 19-year-long activity of the degassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreases towards crust rim. The crust selectively concentrates Tl, V and Te (thousands of μg/g) and to a lesser extent Mo (hundreds of μg/g). The uptake of gaseous Tl, V and Mo is due to the possibility for these elements to enter the MI, MIII and XVI sites of alum, respectively. The process of Te uptake must be related to the incorporation of Tl and V with which Te tightly correlates. Extensive substitution of Tl, V and Te occurs at the surface of the crust where the supply of rock cations is the lowest. Such surface enrichment does not occur for Mo, because Mo substitutes for S, another element from the gas. These findings suggest that the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lack of rock-derived cations. Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that the partition coefficients of Tl (3.3 × 107), V (1.1 × 107) and Te (0.6 × 107) between crust surface and gases are two to four orders of magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alums lose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano (5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrous alums. This study suggests that anhydrous alums significantly immobilize Tl, V and Te in the ground of quiescent volcanoes, reducing the atmospheric emissions of these three elements. [ABSTRACT FROM AUTHOR]

This study focuses on the chemical compositions of volcanic gases and aerosols emitted by Lascar volcano (Northern Chile). The sustained volcanic plume was sampled in April 2009, April 2011 and November 2012 through filter packs and analyzed for major gaseous species (SO 2 , HCl, and HF) and trace elements. During fieldwork, SO 2 flux measurements were also performed by UV spectrometry (DOAS). The Lascar volcano is a significant and sustained emitter of SO 2 (between 150 and 940 t/d), HCl (between 170 t/d and 210 t/d) and HF (up to 100 t/d). Combining the SO 2 -normalized trace element concentrations and the SO 2 emission rates, we evaluate that the quiescent degassing of Lascar is an important local source of trace metals to the atmosphere with contributions to global volcanic fluxes generally less than 2%. Our data were used to infer the origin of the gas emitted at Lascar. Two major degassing sources were identified: a deep magmatic reservoir and a shallow hydrothermal system. The variable interaction between these two sources is the most likely scenario for explaining the compositional ranges in acid gases but also in a volatile and fluid–mobile trace element such as B. These variations are related to changes in volcanic activity: an extensive interaction between the hot ascending magmatic gases and the shallow aquifer occurred in 2009, possibly due to a long period of quiescence, before magmatic activity renewed in 2011 and 2012. Our study highlights, therefore, that filter-pack measurements may be used to study changes in subsurface processes that probably play a key role in triggering volcanic eruptions. [ABSTRACT FROM AUTHOR]