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Project Title
DECADE - Gas Chemistry at Poas Volcano, Costa Rica
Start DateEnd Date
2014-04-01 2017-07-01
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The chemical and isotopic composition of fumarole gas discharges were collected at Poás Volcano, Costa Rica from 2001 to 2014, covering a period during which the volcano experienced a series of phreatic eruptions (2006 to present).  The relative abundances of Poás C-S-H-O gas species are controlled by reactions involving the SO2-H2S and So – SO2 gas buffers indicating magmatic temperatures of up to 800°C.  Although fumarole outlet temperatures are < 120°C for most samples, SO2 is the dominant sulfur gas and HCl contents are relatively high.  Gas compositional changes within the magma-lake-hydrothermal system likely result from a combination of several processes, including: 1) The injection of new and undegassed magma in late 2000 – early 2001, 2) the heating of the hydrothermal system, accompanied by gas pressure build-up, and 3) hydrofracturing through 2006. These processes culminated in the phreatic eruptions of 2006 and 2008. Since 2005, the lake level has declined and is now (2014) at the lowest level (10 m) since the last period that it dried out completely (1989 -1994).  The most recent data of 2014 show a high level of degassing from the dome fumaroles, and the release of HCl – rich and CO2-poor gases implies that the magma injected in late 2000 continues to supply volatiles. Time series sampling of fumarole gases provides important insights to better understand magmatic and hydrothermal processes at active volcanoes and also to potentially forecast phreatic eruptions.  In addition, these data provide valuable insights into CO2 content variations at active volcanoes and that carbon can be controlled by the sulfur buffer system.
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Reporting Year 2016 Click to expand

  • Update 2016: DECADE - Gas Chemistry at Poas Volcano, Costa Rica - submitted on May 23, 2016

    Update Details:

    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.

    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 [1]. 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, SOis 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.


    Report contributed by J. Maarten de Moor.


    1. de Moor JM, Aiuppa A, Pacheco J, Avard G, Kern C, Liuzzo M, Martínez M, Giudice G, Fischer TP (2016) Short-period volcanic gas precursors to phreatic eruptions: Insights from Poás Volcano, Costa Rica.Earth and Plentary Science Letters doi:10.1016/j.epsl.2016.02.056

    Author contributions: JMdM, AA, and TPF conceived of the study. JMdM, JP, GA, ML, MM, and GG collected the data. ML and GG built the Multi-GAS instruments and provided technical expertise with instrument installation, maintenance, and data processing. JMdM, JP, CK, GA, and MM processed the data.  JMdM, AA, JP, CK, TPF and GA wrote the paper, with important contributions and comments from ML and MM.


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