Direct gas sampling and laboratory analysis
Sampling volcanic gases is ideally suited for long-term study of volcanic systems because it produces a detailed chemical analysis of specific fumaroles and vents. This method is not well suited for monitoring rapidly changing conditions, however, because of the time necessary for completing a laboratory analysis--often days or weeks. Furthermore, this method is often an unsafe activity for scientists when a volcano becomes increasingly restless because of the necessity of sampling at or near the potential eruptive vent.
Sampling the gases
Volcanic gases are typically collected by inserting a chemically inert and durable tube (made of titanium, quartz, alumina, or mullite) into a hot fumarole. After allowing the tube to heat until condensation in the tube has reached equilibrium with the escaping gases, usually about 5 minutes, either a specially-designed evacuated-sample bottle or a flow-through sample bottle is attached to the collection tubing.In the flow-through bottle method a glass bottle with a stopcock
at each end and a hand-operated pump are attached to the sampling tube.
The hand pump flushes out air and draws the fumarole gases into the bottle.
Because this collection method is faster, it is used when a complete gas
analysis is not necessary, or where field conditions are too hazardous to
safely make an evacuated-bottle collection. Sulfur gases are stable for only
a short time so this type of sample must be analyzed within a few hours
of collection.
Analyzing the gases
Both the Hawaiian Volcano Observatory and the Cascades Volcano Observatory are equipped to analyze gas samples for SO2 and CO2. If a complete and detailed analysis is required, samples are sent to an analytical laboratory at the USGS Western Region Center in Menlo Park, California. The advantage of a detailed analysis is that it provides the necessary information to perform thermodynamic calculations and construct models in order to provide insight into the condition of the magma at depth from which the gases originated.References
Sutton, A.J., McGee, K.A., Casadevall, T.J., and Stokes, B.J., 1992, Fundamental volcanic-gas-study techniques: an integrated approach to monitoring: in Ewert, J.W., and Swanson, D.A. (eds.), 1992, Monitoring volcanoes: techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-90: U.S. Geological Survey Bulletin 1966, p. 181-188.Other methods of monitoring volcanic gases
- Measuring gas-emission rates in volcanic plumes
- Continuous on-site gas monitoring
- Soil-efflux measurements