Volcanic Gases


-Tobias Fischer, United States

-Eduardo Gutiérrez, El Salvador

-David M. Harris, United States

-Martin Heiligmann, United States

-Erick Fernández Soto, Costa Rica

-Vince Realmuto, United States


The Santa María Workshop provided the possibility to assess the value of volcanic gas monitoring to supplement other approaches of monitoring. Santiaguito's activity is presently characterized by small and numerous explosions with essentially no emissions of blocks and only a small proportion of ash in the eruption columns. At present these small eruptions are of no hazard. Since the eruptive style of the volcano may change over time due to increased magmatic input or other factors, it is necessary to continuously monitor the volatiles released by the system. This will improve forecasting larger and hazardous eruptions. Volcanic gases, hot springs, and radon can provide information about subsurface conditions related to magmatic and hydrothermal systems.

The chief value of sampling and analyzing fluid phases in volcanic environments is due to the role of volatiles as propellants for eruptions (magmatic, phreatic, and phreatomagmatic) and as potential quantitative indicators of subsurface processes including crystallization, magma ascent, fracturd development, and fluid/rock interactions. We emphasize the importance of an integrated approach which involves sampling, analyzing, and interpreting the fluid phase observations through modeling the governing processes.


1) Obtain baseline measurements of elemental fluxes of S, Cl, C, of presumed magmatic origin for use in monitoring, prediction, and modeling magma dynamics.

2) Monitor temporal changes in S/Cl, S/F, and C/S in volcanic gas as possible indicators of changes (magma intrusion, vent sealing, depth of degassing) in the magma system, which may later be useful in eruption forecasting.

3) Measure and detect temporal changes in the daily and weekly fluxes of total volatiles by steam and ash eruptions. Direct estimates of sulfur fluxes during steam and ash eruptions ved monitoring at Santiaguito. must4, Cl, pH, and other constituents This technology relies on using cloud liquid water to c

4) Rates of radon gas leakage from subsurface magma bodies may vary temporally due to changes in productiollect volcanic gases and aerosols. Optimu H2m sit CO2) and changes ing of tfracture system at magma body margins. Radon gas monitorihe collector requires a long-terpotential in detecting pressure build up and fracture development before eruptions.

5) Volcanic gas emissions should be correlated with other contemporary geophysim field study of the wind field at possible sites., leaks from the magma, and esc

Recommendations and Methodology

Frequent sampling of volcanic gases from fumaroles on Santiaguito dome is extremely dangerous and not recommended. the hazards include eruptions, ash flows, pyroclastic flows, rock avalanches, and fog (getting lost). The potential information obtainable from a few gas samples probably does not justify the risk.

If any monitoring instruments are placed on Santiaguito dome they should be simple, light weight, capable of rapid installation, and should not require maintenance at intervals of less than six months.

Hot Spring Water Chemistry

Water samples of warm and hot springs at the base of Santiaguito dome should be collected on a weekly basis. The samples should be analyzed for pH, temperature, and chemical composition to obtain a database of spring chemistry and to detect changes in SO4, Cl, and F concentrations possibly related to volcanic activity and magmatic volatile input. Analyses should be made by either titrations or selective ion electrodes. Possible involvement of local university students is also recommended.

Sulfur Dioxide Flux Measurements (COSPEC)

The Flux of SO2 at Santiaguito should be measured by COSPEC on at least three consequtive days every three weeks when there is no change in seismic or eruptive activity. The flux of SO2 should be measured more frequently (e.g. every day) if the SO2 flux or eruptive activity increases by a factor of two, until the cause of the change is known.

The remote sensing of volcanic gas emissions is primarily based on measurements of SO2 with the correlation spectrometer (COSPEC). The guatemalan government (INSIVUMEH) owns a COSPEC, but the cost of deploying the instrument limits its usage. We strongly recommend that the COSPEC surveys of Santiaguito be given a higher priority. Weekly surveys can serve to both establish a baseline flux of SO2 and to detect deviations from this baseline.

Fourier Transform Spectroscopy of Plumes

Fourier transform infrared spectrometers (FTIR) may allow remote detection of several constituents in the plume from Santiaguito. SO2 and HCl have been detected by this method in other volcanic plumes. Future capabilities of FTIR may include H2SO4, CO2,and water vapor. The present disadvantage of FTIR is the difficulty in reducing the measurements to estimates of plume concentrations. Where as the COSPEC has a direct read out of the SO2 column concentration in the field of view, the FTIR measurements must be corrected with radiative transfer models. This added complication may be offset by the ability to remotely detect and measure several plume constituents with a single instrument. After further development, this technique may contribute to improved monitoring at Santiaguito.

Volcanic Ash Leachate Measurements

Ash samples should be collected at times of increased explosive activity to obtain leachates. Ash must be collected before rainfall. The leachates can be analyzed by the same methods as recommended for analyses of water samples.

Cloud Liquid Water Sampling and Analyses

An experimental system for collecting and analyzing cloud liquid water for SO4, Cl, pH, and other constituents should be developed installed, and operated continuously at the Santa María volcano observatory. The system should automatically collect cloud liquid water from fog and clouds which have a trajectory over the active gas vents on Santiaguito dome and arrive at a fixed collection system. The sample collector would open and close depending on wind direction and meteorological conditions. Chemical analyses would be performed at one to five day intervals using microchemical methods (e.g. specific ionelectrodes). This technology relies on using cloud liquid water to collect volcanic gases and aerosols. Optimum siting of the collector requires a long-term field study of the wind field at possible sites.

Rainfall Sampling and Analyses

Rainfall data should be collected at the observatory (and other sites), and analyzed for SO2, Cl, and pH by using selective ion electrodes. This method relies on rainfall scavenging of volcanic gas and aerosols in the atmosphere in the vicinity of the observatory.

Radon Monitoring in Soil and Hot Springs

Radon gas exsolves early from the magma, leaks from the magma chamber, and escapes diffusely through the edifice of the volcano. Depending on the equipment and the requirements, radon can be measured continuously or in intervals in volcanic soil or hydrothermal springs. Once a baseline for volcanic degassing has been established, changes in the degassing pattern of the volcano can be monitored and evaluated. Temporal variations in ground radon fluxes can be compared with other moitoring data, including seismicity, tilt, strain, and activity.

Practical Field Recommendations

In order to keep logistics and costs to a minimum, several monitoring systems should be set up at one location, if scientifically possible. For example a seismograph or inclinometer could serve also as a location for collecting ash or rainwater samples.