Turmoil at Turrialba Volcano (Costa Rica): Degassing and eruptive processes inferred from high‐frequency gas monitoring

• Published in: Journal of geophysical research. Solid earth Vol. 121; no. 8; pp. 5761 - 5775
• Main Authors: Moor, J. Maarten, Aiuppa, A., Avard, G., Wehrmann, H., Dunbar, N., Muller, C., Tamburello, G., Giudice, G., Liuzzo, M., Moretti, R., Conde, V., Galle, B.
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01-08-2016; John Wiley and Sons Inc
• Subjects: Airports; Carbon dioxide; Chemistry and Physics of Minerals and Rocks/Volcanology; Composition; Costa Rica; Degassing; Emissions; Eruptions; explosive eruption; Explosive Volcanism; Gas composition; Gas monitoring; Gases; Geochemistry; Geological; Geophysics; High temperature; Hydrogen sulfide; hydrothermal system; Hydrothermal systems; Lava; Magma; Marine Geology and Geophysics; Mineralogy and Petrology; Monitoring; Natural Hazards; phreatic eruption; phreatomagmatic eruption; Reservoirs; Seismology; Subduction Zone Processes; Sulfur; Sulfur dioxide; Sulphur; Tectonophysics; Volatiles; Volatilization; Volcanic eruptions; Volcanic Gases; Volcano Monitoring; Volcano Seismology; Volcanoes; Volcanology; Washing; Costa Rica; ASW, Costa Rica; phreatic eruption; phreatomagmatic eruption; hydrothermal system; explosive eruption; volcano monitoring; volcanic gases
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2016JB013150

Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high‐frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO2‐rich gas (CO2/Stotal > 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2 weeks before eruptions, which are accompanied by shallowly derived sulfur‐rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is ~8–10 km deep, whereas the shallow magmatic gas source is at ~3–5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (>3000 T/d SO2 and H2S/SO2 > 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2 < 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high‐temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.
Key Points
A gas composition precursor to eruptions is identified
Changes in gas compositions are associated with transitions in eruptive processes
Magma depth and volume are constrained