CONTINUOUS MONITORING OF HIGH-TEMPERATURE FUMAROLES ON AN ACTIVE LAVADOME, VOLCAN COLIMA, MEXICO - EVIDENCE OF MASS-FLOW VARIATION IN RESPONSE TO ATMOSPHERIC FORCING

Rapid, multichannel monitoring of fumaroles on Volcan Colima, Mexico, provides new insight into the time-scales and magnitudes of fumarole t emperature variation. Temperatures in five fumaroles, all located alon g a single fracture cutting the summit lava dome of the volcano, were monitored at 20-min intervals between May 1991 and May 1992. Measureme nts were made using a programmable data logger deployed near the fumar ole field, and data were radiotelemetered to a nearby volcano observat ory at regular intervals. Mean fumarole temperatures varied between 35 0-degrees-C and 550-degrees-C. Statistical analysis of these time seri es shows that significant diurnal variation occurs in each fumarole. M agnitudes of these daily fluctuations are generally between 25-degrees -C and 50-degrees-C, although larger-amplitude variations occur, espec ially in cooler fumaroles. Simultaneous monitoring of atmospheric pres sure at the fumaroles indicates that these variations in temperature a re inversely correlated with barometric pressure. These observations i ndicate that fumarole temperatures respond to atmospheric forcing. A n umerical model developed to explore the dependence of fumarole tempera ture on mass flow demonstrates that many aspects of observed temperatu re variation are accounted for by mass flow variation, resulting from small changes in barometric pressure. The relationship between mass fl ow and fumarole temperature is nonlinear: the response of fumarole tem peratures to a given change in mass flow is greatest in fumaroles with low mass flow (and cool temperatures). The nature of this dependence is little affected by fumarole geometry for the cases considered. Cont inuous measurement of fumarole temperatures may be an effective means of monitoring local mass flow on volcanoes. At Volcan Colima, average temperatures changed by less than 100-degrees-C during the 1-year samp ling period. During and immediately following effusive activity, chang es in degassing were abrupt and inconsistent along the length of the f racture. Following this period, temperatures decreased gradually, and there was a higher degree of correlation between fumaroles. The method described here represents a substantial improvement over traditional fumarole-monitoring techniques because subtle variation can be quickly identified using standard statistical techniques, and the method prov ides regular information about thermal activity on a volcano, minimizi ng the hazards normally associated with the collection of these data o n a regular basis.