Distal degassing of radon and carbon dioxide on Galeras volcano, Colombia.
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| Authors: | Heiligmann, M; Stix, J; Williams-Jones, G; Sherwood Lollar, B; Garzón, GV |
| Year: | 1997 |
| Journal: | Journal of Volcanology and Geothermal Research 77: 267-283 PDF Article Link (DOI) |
| Title: | Distal degassing of radon and carbon dioxide on Galeras volcano, Colombia. |
| Abstract: | Diffuse degassing at Galeras volcano, Colombia, was studied during three consecutive field seasons from 1993 to 1995. Measurements of222Rn and CO2 were made at 30 stations which were distributed on the volcano and on regional faults intersecting the edifice. Time series data show a decline of radon soil gas of up to 50% prior to a M 2.8 earthquake on 12 August 1993 at stations located near the epicenter and on the volcano near the location of earthquake swarms which occurred in April 1993, November–December 1993 and March 1995. The onset of volcanic seismic activity (‘tornillos’) on 9 August 1994 was preceded by anomalous soil gas increases at six stations located on the flanks of the volcano. On the southwestern flank, radon increased from 51 to 130 pCi/1 between 7 and 14 August, while on the northern flank, radon concentrations began to increase 19 days before the appearance of tornillos. In general, stations close to the crater showed the largest radon increases. Soil gas distributions and carbon isotope data suggest that diffuse degassing on the volcano is structurally controlled and that the abundance of CO2 in soil gas on the edifice cannot be taken as an indicator for the presence of magmatic gases. Radon soil gas concentrations and the222Rn emanating226Ra concentration increase near faults, whereas CO2 concentrations are more variable but commonly are higher on the volcano than near faults. δ13C values in soil CO2 vary between −8.5 and −23.2‰, with δ13C values more enriched than −15‰ found only in the vicinity of faults or sites prone to earthquake swarms. This suggests a magmatic origin of CO2 soil gas only near faults and an almost impermeable edifice in unfractured areas. The observed correlations between seismic activity and soil degassing provide further evidence that soil gas studies, especially when correlated to other methods of volcano surveillance such as seismicity and deformation, may be useful in forecasting volcanic and seismic events. |
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