ERUPTIVE ACTIVITY AT MOUNT-ST-HELENS, WASHINGTON, USA, 1984-1988 - A GAS GEOCHEMISTRY PERSPECTIVE

The results from two different types of gas measurement, telemetered i n situ monitoring of reducing gases on the dome and airborne measureme nts of sulfur dioxide emission rates in the plume by correlation spect rometry, suggest that the combination of these two methods is particul arly effective in detecting periods of enhanced degassing that intermi ttently punctuate the normal background leakage of gaseous effluent fr om Mount St Helens to the atmosphere. Gas events were recorded before lava extrusion for each of the four dome-building episodes at Mount St Helens since mid-1984. For two of the episodes, precursory reducing g as peaks were detected, whereas during three of the episodes, COSPEC m easurements recorded pre cursory degassing of sulfur dioxide. During o ne episode (October 1986), both reducing gas monitoring and SO2 emissi on rate measurements simultaneously detected a large gas release sever al hours before lava extrusion. Had both types of gas measurements bee n operational during each of the dome-building episodes, it is thought that both would have recorded precursory signals for all four episode s. Evidence from the data presented herein suggests that increased deg assing at Mount St Helens becomes detectable when fresh upward-moving magma is between 2 km and a few hundred meters below the base of the d ome and between about 60 and 12 hours before the surface extrusion of lava.