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It is generally accepted, but not experimentally proven, that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data. By discussing the results of two years of real-time observation of H 2 O, CO 2 , and SO 2 in volcanic gases from Mount Etna volcano, we unambiguously demonstrate that increasing CO 2 /SO 2 ratios can allow detection of the pre-eruptive degassing of rising magmas. Quantitative modeling by the use of a saturation model allows us to relate the pre-eruptive increases of the CO 2 /SO 2 ratio to the refi lling of Etna's shallow conduits with CO 2 -rich deep-reservoir magmas, leading to pressurization and triggering of eruption. The advent of real-time observations of H 2 O, CO 2 , and SO 2 , combined with well-constrained models of degassing, represents a step forward in eruption forecasting.

Explosive eruptions are the most powerful and destructive type of volcanic activity. These eruptions are characterized by magma fragmentation, the process through which a bubbly or foamy magma is transformed into a gas-pyroclast dispersion. Although magma fragmentation has been investigated both experimentally and theoretically, and the basic transport phenomena that occur in a volcanic conduit have been modelled, the underlying mechanism responsible for magma fragmentation is still poorly understood. This lack of understanding seriously limits our ability to forecast volcanic hazards, preventing reliable discrimination between conditions that lead to explosive and effusive eruptions. Here I develop a model in which a fragmentation criterion, based on a rate-limited crossing of the glass transition, , is incorporated into a multiphase fluid-dynamic description of magma ascent. The numerical results of this model demonstrate the feasibility of strain-induced brittle fragmentation of magma in volcanic eruptions, and reconcile experimental with theoretical studies as well as with the observed volcanic products of large explosive eruptions.

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The climactic event of Mount Pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism. We have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits, and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions. White pumice has higher vesicularity, deformed and highly coalesced vesicles with thin walls, euhedral phenocrysts and microlite-free groundmass. Gray pumice shows lower vesicularity, wider ranges in vesicle number density, limited coalescence, vesicles with thick walls that are less deformed, phenocrysts and microphenocrysts with abundant solution pitting, and groundmass containing ubiquitous microlites and crystal fragments. The presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors. In particular, the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls, a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety.