ACID GAS EMISSIONS FROM GALERAS-VOLCANO, COLOMBIA, 1989-1994

Characterization of acid gases discharged by Galeras Volcano have allo wed us to define a clear change in the pattern of degassing during 198 9 through 1994, probably associated with pressure buildup that ended w ith the destruction of the dome by an explosive eruption on July 16, 1 992. Using Japanese boxes for sampling, it was possible to establish c haracteristic discharge (absorption) levels for the period of study an d to conclude that the absolute discharge of acid gases was declining at least until the middle of 1991. A staggered temporal response in ga s compositions, particularly CO2 and St, for the different fumaroles s uggests at least three possibilities: (1) magma resurgence, which woul d increase relative CO2 discharge as a result of new gas contribution and its relatively low solubility in magma; (2) solidification and/or crystallization of magma, first in a zone of higher thermal gradient w here there is a greater interaction with the hydrothermal system (Calv ache fumarole), and second toward the volcanic-magmatic fumaroles (Def ormes and Besolima); and (3) contribution of the hydrothermal system, produced by water saturation and/or temperature increase that would re duce CO2 solubility in water. The similar behavior of the Calvache fum arole and Japanese Box 3, as well as the existence of La Trucha lake i n the northeastern moat between the active cone and the amphitheater m argin, help us to define the location of the principal hydrothermal sy stem. The abrupt change toward a hydrothermal component in Deformes fu maroles in August 1990, and the variations in chloride concentrations and other cation concentrations in condensate samples, allow us to obs erve very large changes over short periods in the composition of the f luid phase of the volcanic system. This suggests that the density of s ampling should be increased, together with:the development of alternat ive techniques that could reduce risks and augment the resolution in t he observation of the chemical changes. On the other hand, the depende nce of gas absorption (e.g., CO2) on the sampling period, seen at the Japanese boxes, suggests that fixed sampling intervals would improve t his method by reduction of sampling variables.