During 1998During -2000, the island of Dominica in the Lesser Antilles experienced a major volcanic earthquake swarm spatially associated with volcanic centres in the south of the island. This swarm provided the motivation for a major re-assessment of geologic history and volcanic hazards in this region. This has led to a reinterpretation of the south-western-most corner of the island as a large volcanic complex, termed the Plat Pays volcanic complex, which has exhibited a wide range of past eruptive activity and which, in the Quaternary, experienced a major caldera collapse triggered by a voluminous pyroclastic eruption. Stratigraphy and new 14 C age determinations reveal a complicated history of development. The earliest activity is represented by the formation of the Plat Pays stratovolcano and associated parasitic domes. A major explosive eruptioñ 39,000 years b.p. produced the Grand Bay ignimbrite and triggered a major caldera collapse of the summit and south-western flank of the Plat Pays stratovolcano. Following the major eruption, re-injection of Plat Pays magma resulted in the extrusion of approximately 12 lava domes both within and outside the resulting depression. Caldera collapse was followed by at least one catastrophic flank collapse, but it is unclear whether or not this was triggered by caldera formation. The only on-shore evidence of flank collapse is the breach of the caldera rim and truncation of post-caldera deposits emplaced on or near the caldera rim; we find no evidence of on-shore large-magnitude rock avalanche deposits within the stratigraphic framework of the Plat Pays volcanic com-plex. Frequent seismic swarms of volcanic earthquakes and vigorous geothermal activity indicate that south Dominica is still underlain by an active magma reservoir system. Our new geological observations, combined with an interpretation of recent seismic swarm activity, suggest that an eruption (probably dome-forming) from the Plat Pays volcanic complex is probable within the next 100 years.

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This paper introduces an open source computer code to perform an integrated probabilistic spatio-temporal volcanic hazard assessment in distributed volcanic fields. The program, named MatHaz, is a set of Matlab scripts that follows a sequential methodology. After the user has provided a set of input files, this tool first estimates the spatial probability of future volcanic vents, then the temporal probability of future volcanic events, and finally models up to five volcanic phenomena (pyroclastic density currents, ballistic projectiles, lava flows, lahars, and tephra fallout) following a probabilistic approach. These results can be combined and depicted as an integrated quantitative (and/or qualitative) volcanic hazard map, with weightings of hazard factors chosen by the user. We illustrate the use of this tool by applying it to the Carrán-Los Venados Volcanic Field in southern Chile. The opensource, replicable, and user-friendly nature of the code allows its application to any volcanic region of the world, regardless of its extent, type, and amount of volcano-structural data.

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Hydrothermal alteration by2 SO E -rich fluids governs the physicochemical properties of stratovolcanoes worldwide (Mayer et al., 2016;Zimbelman et al., 2005). As these fluids alter the conduit rocks, inevitable changes in rock porosity and permeability can limit outgassing and promote explosive volcanic behavior. This was observed at Poás volcano (Costa Rica), where the formation of a pressurized hydrothermal seal by secondary mineral precipitation and volatile accumulation limited 2 SO E -emissions for two years and triggered the 2017 phreatomagmatic eruption (de Moor et al., 2019). Similar hydrothermal sealing driven by acid-sulphate alteration also played a role in triggering phreatic and phreatomagmatic eruptions at Soufrière Hills (Montserrat) (Edmonds et al., 2003), and Ontake (Japan) (Stix & de Moor, 2018). At Whakaari-White Island volcano (New Zealand), a partially sealed hydrothermal system is implied to have played a role in the recent 2019, and past eruptions (Burton et al., 2021;Christenson et al., 2017). However, mineralogical and microimaging evidence of such sealing and its evolution for different lithologies within the conduit is lacking. Moreover, the effect of such hydrothermal processes on the fluid flow and elastic properties of conduit-filling rocks remains unconstrained, despite being of paramount importance for accurately inverting geophysical data to understand volcano pressurization.The development of an effective seal requires conduit rocks with low porosity and low permeability that limit outgassing and favor fluid accumulation and pressure build-up (Stix & de Moor, 2018). So far, experimental studies on the effects of alteration on porosity and permeability of volcanic rocks have been mainly based on surface-collected rocks and show conflicting observations. For example, pore and fracture filling secondary minerals in lavas (