Abstract
Abstract Volcanic eruptions involving interaction with water are amongst the most violent and unpredictable geologic phenomena on Earth. Phreatic eruptions are exceptionally difficult to forecast by traditional geophysical techniques. Here we report on short-term precursory variations in gas emissions related to phreatic blasts at Poas volcano, Costa Rica, as measured with an in situ multiple gas analyzer that was deployed at the edge of the erupting lake. Gas emitted from this hyper-acid crater lake approaches magmatic values of SO2/CO2 1–6 days prior to eruption. The SO2 flux derived from magmatic degassing through the lake is measureable by differential optical absorption spectrometry (sporadic campaign measurements), which allows us to constrain lake gas output and input for the major gas species during eruptive and non-eruptive periods. We can further calculate power supply to the hydrothermal system using volatile mass balance and thermodynamics, which indicates that the magmatic heat flux into the shallow hydrothermal system increases from ∼27 MW during quiescence to ∼59 MW during periods of phreatic events. These transient pulses of gas and heat from the deeper magmatic system generate both phreatic eruptions and the observed short-term changes in gas composition, because at high gas flux scrubbing of sulfur by the hydrothermal system is both kinetically and thermodynamically inhibited whereas CO2 gas is always essentially inert in hyperacid conditions. Thus, the SO2/CO2 of lake emissions approaches magmatic values as gas and power supply to the sub-limnic hydrothermal system increase, vaporizing fluids and priming the hydrothermal system for eruption. Our results suggest that high-frequency real-time gas monitoring could provide useful short-term eruptive precursors at volcanoes prone to phreatic explosions.
Highlights
Volcanic eruptions involving explosive vaporization of meteoric water are common occurrences on our wet planet
We suggest that the short-term variations in SO2 /CO2 in the lake gas emissions are due to varying influx of magmatic gas, which determines whether phreatic eruptions occur or not
Phreatic eruptions occurring through crater lakes represent the wet endmember of a spectrum of “non-magmatic” eruption types (Rouwet et al, 2014a)
Summary
Volcanic eruptions involving explosive vaporization of meteoric water are common occurrences on our wet planet. Eruptions that occur through surface water account for ∼8% of historic eruptions but have claimed a disproportionate number of human lives (∼20% of total deaths) (Mastin and Witter, 2000). Lahars generated by explosive ejection of crater lake water are the deadliest hazard associated with volcanic lakes and have killed >16,000 people in historical times (Mastin and Witter, 2000). Phreatic eruptions are exceptionally challenging to forecast because many do not directly involve the movement of magma, which generates distinct geophysical signals useful in eruption forecasting, and may occur due to subtle changes in shallow hydrothermal systems (Rouwet et al, 2014a). Documented precursors of phreatic activity show long to medium term changes in geophysical (i.e. seismicity, deformation, and gravimetry) or geochemical (i.e. lake water and gas compositions) parameters (Sano et al, 2015; Barberi et al, 1992; Rouwet et al, 2014b), but accurate short-term (days) eruption forecasting remains exception-
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