Near-surface magma flow instability drives cyclic lava fountaining at Fagradalsfjall, Iceland

Lava fountains are a common manifestation of basaltic volcanism. While magma degassing plays a clear key role in their generation, the controls on their duration and intermittency are only partially understood, not least due to the challenges of measuring the most abundant gases, H2O and CO2. The 2021 Fagradalsfjall eruption in Iceland included a six-week episode of uncommonly periodic lava fountaining, featuring ~ 100-400 m high fountains lasting a few minutes followed by repose intervals of comparable duration. Exceptional conditions on 5 May 2021 permitted close-range (~300 m), highly time-resolved (every ~ 2 s) spectroscopic measurement of emitted gases during 16 fountain-repose cycles. The observed proportions of major and minor gas molecular species (including H2O, CO2, SO2, HCl, HF and CO) reveal a stage of CO2 degassing in the upper crust during magma ascent, followed by further gas-liquid separation at very shallow depths (~100 m). We explain the pulsatory lava fountaining as the result of pressure cycles within a shallow magma-filled cavity. The degassing at Fagradalsfjall and our explanatory model throw light on the wide spectrum of terrestrial lava fountaining and the subsurface cavities associated with basaltic vents.

Validation of a novel Multi-Gas sensor for volcanic HCl alongside H2S and SO2 at Mt. Etna

Volcanic gas emission measurements inform predictions of hazard and atmospheric impacts. For these measurements, Multi-Gas sensors provide low-cost in situ monitoring of gas composition but to date have lacked the ability to detect halogens. Here, two Multi-Gas instruments characterized passive outgassing emissions from Mt. Etna's (Italy) three summit craters, Voragine (VOR), North-east Crater (NEC) and Bocca Nuova (BN) on 2 October 2013. Signal processing (Sensor Response Model, SRM) approaches are used to analyse H₂S/SO₂ and HCl/SO₂ ratios. A new ability to monitor volcanic HCl using miniature electrochemical sensors is here demonstrated. A "direct-exposure" Multi-Gas instrument contained SO₂, H₂S and HCl sensors, whose sensitivities, cross-sensitivities and response times were characterized by laboratory calibration. SRM analysis of the field data yields H₂S/SO₂ and HCl/SO₂ molar ratios, finding H₂S/SO₂ = 0.02 (0.01-0.03), with distinct HCl/SO₂ for the VOR, NEC and BN crater emissions of 0.41 (0.38-0.43), 0.58 (0.54-0.60) and 0.20 (0.17-0.33). A second Multi-Gas instrument provided CO₂/SO₂ and H₂O/SO₂ and enabled cross-comparison of SO₂. The Multi-Gas-measured SO₂-HCl-H₂S-CO₂-H₂O compositions provide insights into volcanic outgassing. H₂S/SO₂ ratios indicate gas equilibration at slightly below magmatic temperatures, assuming that the magmatic redox state is preserved. Low SO₂/HCl alongside low CO₂/SO₂ indicates a partially outgassed magma source. We highlight the potential for low-cost HCl sensing of H₂S-poor HCl-rich volcanic emissions elsewhere. Further tests are needed for H₂S-rich plumes and for long-term monitoring. Our study brings two new advances to volcano hazard monitoring: real-time in situ measurement of HCl and improved Multi-Gas SRM measurements of gas ratios.

New geochemical insights into volcanic degassing

Magma degassing plays a fundamental role in controlling the style of volcanic eruptions. Whether a volcanic eruption is explosive, or effusive, is of crucial importance to approximately 500 million people living in the shadow of hazardous volcanoes worldwide. Studies of how gases exsolve and separate from magma prior to and during eruptions have been given new impetus by the emergence of more accurate and automated methods to measure volatile species both as volcanic gases and dissolved in the glasses of erupted products. The composition of volcanic gases is dependent on a number of factors, the most important being magma composition and the depth of gas-melt segregation prior to eruption; this latter parameter has proved difficult to constrain in the past, yet is arguably the most critical for controlling eruptive style. Spectroscopic techniques operating in the infrared have proved to be of great value in measuring the composition of gases at high temporal resolution. Such methods, when used in tandem with microanalytical geochemical investigations of erupted products, are leading to better constraints on the depth at which gases are generated and separated from magma. A number of recent studies have focused on transitions between explosive and effusive activity and have led to a better understanding of gas-melt segregation at basaltic volcanoes. Other studies have focused on degassing during intermediate and silicic eruptions. Important new results include the recognition of fluxing by deep-derived gases, which buffer the amount of dissolved volatiles in the melt at shallow depths, and the observation of gas flow up permeable conduit wall shear zones, which may be the primary mechanism for gas loss at the cusp of the most explosive and unpredictable volcanic eruptions. In this paper, I review current and future directions in the field of geochemical studies of volcanic degassing processes and illustrate how the new insights are beginning to change the way in which we understand and classify volcanic eruptions.

Optical sensing of volcanic gas and aerosol emissions

Volcanic gas and aerosol surveillance yield important insights into magmatic, hydrothermal, and atmospheric processes. A range of optical sensing and sampling techniques has been applied to measurements of the composition and fluxes of volcanic emissions. In particular, the 30-year worldwide volcanological service record of the Correlation Spectrometer (COSPEC) illustrates the point that robust, reliable, straightforward optical techniques are of tremendous interest to the volcano observatory and research community. This chapter reviews the field, in particular the newer and more versatile instruments capable of augmenting or superseding COSPEC, with the aim of stimulating their rapid adoption by the volcanological community. It focuses on sensors that can be operated from the ground, since they generally offer the most flexibility and sensitivity. The success of COSPEC underlines the point, however, that such devices should be comparatively cheap, and easy to use and maintain, if they are to be widely used.

Dynamics of magma degassing

Gas exsolution and segregation are fundamental controls on eruption dynamics and magma genesis. Basaltic magma loses gas relatively easily because of its low viscosity. However, bubbles grown by decompression and diffusion during magma ascent are too small to segregate. Coalescence, however, can create bubbles big enough for gas to escape from the rising basalt magma. In evolved magmas, such as andesite and rhyolite, high viscosity prevents bubbles rising independently through the magma. The original gas content of magma erupted as lava is commonly the same as that erupted explosively, so that a gas separation mechanism is required. A permeable magma foam can form to allow gas escape once bubbles become interconnected. Magma permeabilities can be much higher than wall-rock permeabilities, and so vertical gas loss can be an important escape path, in addition to gas loss through the conduit walls. This inference is consistent with observations from the Soufrière Hills Volcano, Montserrat, where gas escapes directly from the dome, and particularly along shear zones (faults) related to the conduit wall. Dynamical models of magma ascent have been developed which incorporate gas escape. The magma ascent rate is sensitive to gas escape, as the volume proportion of gas affects density, magma compressibility and rheology, resulting in both horizontal and vertical pressure gradients in the magma column to allow gas escape. Slight changes in gas loss can make the difference between explosive and effusive eruption, and multiple steady-state flow states can exist. In certain circumstances, there can be abrupt jumps between effusive and explosive activity. Overpressures develop in the ascending magma, caused primarily by the rheological stiffening of magma as gas exsolves and crystals grow. A maximum overpressure develops in the upper parts of volcanic conduits. The overpressure is typically several MPa and increases as permeability decreases. Thus, the possibility of reaching conditions for explosions increases as permeability decreases, both due to overpressure increase and the retention of more gas. Models of magma ascent from an elastic magma chamber, combined with concepts of permeability and overpressure linked to degassing, provide an explanation for the periodic patterns of dome growth with short-lived explosive activity, as in the 1980–1986 activity of Mount St Helens. Degassing of magma in conduits can also cause strong convective circulation between deep magma reservoirs and the Earth’s surface. Such circulation not only allows degassing to occur from deep reservoirs, but may also be a significant driving force for crystal differentiation.