Poás volcano (Costa Rica) is one of the most chemically extreme environments on Earth, hosting an ultra-acidic crater lake (pH ~0, T ~50°C) as well as high temperature fumaroles (up to ~800°C in recent years). The lake was the site of intense phreatic eruptive behavior between 2006 and 2014. Volcanic eruptions involving interaction with water are particularly energetic, causing a disproportionate number of human casualties. Phreatic eruptions are also exceedingly difficult to forecast, often occurring with little or no geophysical precursors.
A new article published in Earth and Planetary Science Letters by a group of Deep Carbon Observatory scientists led by Maarten de Moor (Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional, Heredia, Costa Rica) reports the results from a DECADE (Deep Earth Carbon Degassing initiative) project to investigate gas emissions at Poás . The team measured gas emissions from the crater lake in situ using a fixed multiple gas analyzer station (Multi-GAS) during a two month period of phreatic activity in 2014. The gas composition data show significant variations in the ratio between SO2 and CO2, which are statistically correlated with both the occurrence and the size of phreatic eruptions. The authors found that the composition of gas emitted directly from the lake approaches that of magmatic gas days before large phreatic eruptions. These promising results show that high-frequency gas monitoring may provide an effective means of forecasting phreatic eruptions. The biggest challenge to this monitoring approach is maintaining the Multi-GAS instrument in extremely harsh conditions. Peripheral components of the station were destroyed by a large eruption on 2 June 2014, which spelled the end of the lake gas emission experiment. However, the instrument survived and is currently monitoring changes in fumarolic gas composition.
The behavior of CO2 in the Poás hydrothermal system played a pivotal role in understanding the observed variations in gas composition. In contrast to other major volcanic gas species, CO2 is essentially inert in ultra-acidic conditions and therefore passes through the hydrothermal system and acid lake with minimal modification. In contrast, SO2 is partially removed from the gas phase by hydrothermal reactions producing aqueous bisulfate and liquid/solid native sulfur. Gas flux measurements conducted using mini-DOAS (differential optical absorption spectroscopy) show that high emission rates of SO2 from the lake occur during eruptive activity and are also associated with high SO2/CO2. The team therefore argued that the efficiency of S removal from the gas is inhibited with increasing gas flux through the hydrothermal system, resulting in increasing SO2/ CO2. Importantly, the results suggest that short-period pulses of magmatic gas and heat are directly responsible for generating individual phreatic eruptions. Furthermore, the amount of energy need to produce phreatic eruptions is quantifiable by integrating gas flux and composition measurements, seismicity, and webcam footage. Ultimately, excess energy transferred to, and stored in, the sublimnic zone by magmatic gas primes the hydrothermal system for eruption. This energy is catastrophically released as spectacular phreatic explosions.
Movie showing examples of phreatic eruptions at Poás volcano, Costa Rica. Edited by Geoffroy Avard.
Report contributed by J. Maarten de Moor.
Image: Maarten de Moor, lead author of the study, at Poás crater lake with a portable Multi-GAS during a period of relative calm.