1 Purpose
The emission of volcanic gases opens a window into the interior of the volcanic plumbing system. Basically there are three types of information: magnitude of the gas flux, temporal pattern of the gas flux and changes in chemical composition. Traditionally, however gas fluxes and composition measurements were sparsely used as indicators for volcanic activity, mostly due to the difficulty to obtain reliable data on a continuous basis. The advent of the COSPEC instrument during the 1970ties marked a big step forward from in-situ measurements, however truly continuous measurements of chemical composition and gas-fluxes from volcanoes only became possible with the introduction of infrared spectrometers and in particular advanced, compact UV/visible spectrometers (McGonigle et al., 2002, 2003; Galle et al., 2003, 2010).
The scanning dual-beam Mini-DOAS instrument represents a major breakthrough in volcanic gas monitoring, by providing real-time automatic, unattended measurements of gas emission e.g. SO2 from a volcano with less than 5 min time resolution during daylight (Galle et al., 2010). High temporal resolution SO2 emission monitoring over long time scales can provide insight into the presence, volume, and ascent/descent rate of magma bodies prior to, during, and after eruptions (Symonds et al., 1994). Combined with other geophysical data sets, SO2 gas time series can help one to understand the dynamics of magma ascent (Sparks, 2003) and their relationship with conduit and hydrothermal processes (Olmos et al., 2007; Arellano et al., 2008; Edmonds et al., 2003). The emitted volcanic gases have a large environmental impact on local (Baxter, 1990; Delmelle et al., 2002), regional, and global scales (Halmer et al., 2002; Robock, 2002; Intergovernmental Panel on Climate Change, 2007).
- Covariation between SO2 fluxes and RSAM (Real Time Seismic Amplitude Measurements) before, during and after the Santa Ana eruption (El Salvador) on October 1, 2005. (Data courtesy of Servicio Nacional de Estudios Territoriales and University of El Salvador, and Bo Galle, Chalmers TU, Gothenburg).
1.1 Changes in degassing rates and gas chemistry as indicators of volcanic activity
SO2 flux measurements contributed significantly to hazard assessment prior to the 1991 eruption of Mt. Pinatubo (Daag et al., 1996). Immediately prior to the eruption, the measured SO2 flux increased by an order of magnitude over two weeks, mirroring seismic unrest. These observations were interpreted as evidence of shallow intrusion of magma, increasing the estimates of the probability of an impending eruption. More recently, SO2 flux observations have supported conclusions of magmatic unrest prior to eruptions of Soufriere Hills Volcano (Montserrat), Popocateptl (Mexico), and Tungurahua (Ecuador) (Arellano et al., 2008).
Combined SO2 fluxes and RSAM (Real Time Seismic Amplitude Measurements: average amplitude over 10-minutes intervals; Ewert et al., 1993) have proven to be a good tool for detecting changes in volcanic activity in 2005 at Santa Ana Volcano, El Salvador (Olmos et al., 2007; Figure 1). SO2 fluxes and RSAM increased about 2 months before the eruption pointing to an accumulation of gas-rich magma at shallow level. Shortly before the eruption, RSAM decreased sharply but SO2 fluxes remained high. During the eruption variations in these two parameters were closely related to explosive activity. Cross-correlation studies indicate that the time lag between changes in the two parameters was on the order of minutes or hours. A systematic increase in SO2 emission and RSAM was observed prior to explosive events indicating that accumulation of magmatic gases and consequent increase in pressure within the magma chamber was leading to an explosive event, which was followed by peak values in the two parameters. These results illustrate that SO2 fluxes and RSAM together can provide useful indications of upcoming volcanic activity (e.g. for the volcanoes Santa Ana (El Salvador), Pinatubo (Philippines) and Soufriere Hills (Montserrat)). Decreasing SO2 fluxes accompanying diminishing post-eruptive activity have also been observed at many volcanoes, including Mt. St. Helens from 1980 to 1988 (e.g. Gerlach and McGee,1994), where a decline in CO2 flux and increase in H2O flux were also observed following the 1980 eruption. The decrease in the CO2 and SO2 emissions suggested that the magma reservoir was not being replenished, consistent with the decreased eruption rates. Lack of large SO2 emissions after large dome collapses at Soufriere Hills in 2000 and 2001 suggested that risk of explosive activity after a large collapse was smaller than during the first phase of dome building (Edmonds et al., 2003).
1.2 Literature
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Gerlach TM, McGee KA (1994) Total sulfur dioxide emissions and pre-eruption vapor-saturated magma at Mount St. Helens, 1980-88. Geophys Res Lett 21, 2833-2836
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Intergovernmental Panel on Climate Change (2007), Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge Univ. Press, New York.
McGonigle AJS, Oppenheimer C, Galle B, Mather T, Pyle D (2002) Walking traverse and scanning DOAS measurements of volcanic gas emission rates. Geophys Res Lett 29, Art. No. 1985, doi:10.1029/2002GL015827
McGonigle AJS, Oppenheimer C, Hayes AR, Galle B, Edmonds M, Caltabiano T, Salerno G, Burton M, Mather TA (2003) Sulphur dioxide fluxes from Mount Etna, Vulcano, and Stromboli measured with an automated scanning ultraviolet spectrometer. J Geophys Res 108, Art. No. 2455, doi:10.1029/2002JB002261
Olmos R, Barrancos J, Rivera C, Barahona F, Lopez DL, Henriquez B, Hernandez A, Benitez E, Hernandez PA, Perez N M.,Galle B (2007) Anomalous emissions of SO2 during the recent eruption of Santa Ana volcano, El Salvador, Central America. Pure Appl Geophys 164, 2489-2506
Robock, A. (2002), Pinatubo eruption: The climatic aftermath, Science, 295(5558), 1242 – 1244.
Sparks, R. S. J. (2003), Dynamics of magma degassing, in Volcanic Degassing, edited by C. Oppenheimer, D. Pyle, and J. Barclay, Geol. Soc. Spec. Publ., 213, 5 – 22
Symonds, R. B., W. I. Rose, G. J. S. Bluth, and T. M. Gerlach (1994), Volcanic-gas studies: Methods, results, and applications, Rev. Mineral. Geochem., 30(1), 1 – 66.