Description: Active lava lakes represent a variety of open-vent volcanism in which a sizable body of lava accumulates at the top of the magma column, constrained by the vent and/or crater geometry. The longevity of lava lakes reflects a balancing of cooling and outgassing occurring at the surface and input of hot and gas-rich magma from below. Due to their longevity and relative accessibility, lava lakes provide a natural laboratory for studying fundamental volcanic processes such as degassing, convection and cooling. This article examines all seven lakes that existed at the time of writing in 2018, located in the Pacific, Antarctica, Africa, and South and Central America. These lakes span all tectonic environments, and a range of magma compositions. We focus on analysis of the lake surface motion using image velocimetry, which reveals both similarities and contrasts in outgassing and lake dynamics when comparing the different lakes. We identify two categories of lake behavior: Organized (Erta’Ale, Nyiragongo, Kīlauea after 2011, and Erebus) and Chaotic (Villarrica, Masaya, Marum). This division does not map directly to lake size, viscosity, gas emission rate, or temperature. Instead, when examined together, we find that the lakes follow a linear relationship between average surface speed and the ratio of total gas flux to lake surface area. This relationship points to the combined importance of both flux and lake size in addition to the total volume of gas emission, and suggests that a shared deep mechanism controls the supply of heat and gas to all lakes. On the other hand, the differences between Chaotic and Organized lakes highlight the important role of the geometry of the conduit-lake transition, which superimposes a shallow signal on that of the deep circulation. The spatial patterns of surface motion we document suggest that the release of gas bubbles at Chaotic lakes is more efficient (i.e., bubbles are less likely to be retained and recycled) compared with Organized lakes. In addition, the data presented here indicate that the solidified crust of Organized lakes plays a role in regulating convection and outgassing in lava lakes.

Description: Published

Description: 16-31

Description: 5V. Processi eruttivi e post-eruttivi

Description: JCR Journal

Description: Open vent basaltic volcanoes account for a substantial portion of the global atmospheric outgassing flux, largely through passive degassing and mild explosive activity. We present volcanic gas flux and composition data from Yasur Volcano, Vanuatu collected in July 2018. The average volcanic plume chemistry is characterised by a mean molar CO2/SO2 ratio of 2.14, H2O/SO2 of 148 and SO2/HCl of 1.02. The measured mean SO2 flux in the period of 6th to 9th July is 4.9 kg s−1 . Therefore, the mean fluxes of the other species are 7.5 kg∙s −1 CO2, 208 kg∙s −1 H2O and 4.8 kg∙s −1 HCl. The degassing regime at Yasur volcano ranges from ‘passive’ to ‘active’ styles, with the latter including Strombolian activity and spattering. Gases emitted during active degassing are enriched in SO2 over HCl and CO2 over SO2 relative to passive degassing, with CO2/SO2 ratios of 2.85 ± 0.17, SO2/HCl of 1.6 ± 0.22, and H2O/SO2 of 315 ± 78.8. Gases emitted during passive degassing have CO2/SO2 ratios of 1.96 ± 0.12, SO2/ HCl of 0.50 ± 0.07 and H2O/SO2 of 174 ± 43.5. We use a model of volatile degassing derived from melt inclusion studies (Metrich et al., 2011), combined with our observations of chemical variations in the outgassing bubbles to propose a mechanism for magma degassing in the conduit at Yasur. We envisage a shallow conduit filled with crystal-rich magma, forming a viscous and mobile plug that develops an effective yield strength from the surface to a depth of at least 2000 m, in which bubbles are trapped, grow, ascend towards the surface and burst in a typical Strombolian eruption. Deeper bubbles released during active degassing are enriched in CO2 and SO2 compared to bubbles released during ‘passive degassing’, which are sourced from close to the surface, and are, consequently, HCl-rich.

Description: Published

Description: 106869

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: JCR Journal