Just Gas, Part I

Tuesday, October 01, 1996
For the past four years, geochemist Tobias Fischer at Arizona State University has been monitoring gas emissions from the 14,000-foot Galeras volcano in southern Colombia. What his measurements now show is that changes in the cocktail of gases emanating from a volcano may signal an imminent eruption.

A volcano is fueled by a pool of molten rock, or magma, at its center, which releases a mix of gases that include hydrochloric acid, sulfur dioxide, and carbon dioxide. The pressure of the escaping gas is so great that it forces out rainwater that has seeped into fissures in the solid rock above. Eventually, some of the fissures become clogged--either by cooling rock at the surface of the magma chamber or by minerals precipitating from volcanic gases dissolved in the water forced from the fissures. The clogged channels slow the flow of gases and pressure builds. That pressure buildup, many geologists suggest, is what ultimately leads to explosive eruptions.

Fischer began taking weekly gas samples at Galeras in June 1992-- just over a month, it turned out, before an eruption. He suspected that if Galeras’s gas channels were becoming blocked, water would again be seeping into the rock around the magma. If that was so, then soluble gases trapped in the channels would dissolve in the invading water and their absence from the emissions could be detected. In fact, Fischer found that levels of hydrogen chloride--which dissolves readily in water--plummeted by a factor of 30 in the week before the eruption. Levels of insoluble carbon dioxide, though, held steady. At the same time, Fischer noted that the gas temperature dropped from 750 degrees to about 415 degrees, indicating that an influx of water might be cooling the system.

Although Japanese researchers suggested decades ago that changes in the mix of volcanic gases might foretell eruptions, no one has ever carried out a frequent, long-term study of emissions before and after an eruption. When an eruption happens, we’re usually either too early or too late to really bracket it nicely, explains Fischer, who just published the results of his research last June. The way we did it at Galeras was very lucky--it’s a unique time series and it shows that the gas chemistry can really change on a very short-term basis. Fischer hopes that in places like Colombia, where many people live on volcanoes’ grassy slopes, emissions will be monitored along with seismicity as a means of more surely targeting eruptions.
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