THE 1984 TO 1996 CYCLIC ACTIVITY OF LASCAR VOLCANO, NORTHERN CHILE - CYCLES OF DOME GROWTH, DOME SUBSIDENCE, DEGASSING AND EXPLOSIVE ERUPTIONS

Lascar Volcano (5592 m; 23 degrees 22'8, 67 degrees 44'W) entered a ne w period of vigorous activity in 1984, culminating in a major explosiv e eruption in April 1993. Activity since 1984 has been characterised b y cyclic behaviour with recognition of four cycles up to the end of 19 93. In each cycle a lava dome is extruded in the active crater, accomp anied by vigorous degassing through high-temperature, high-velocity fu maroles distributed on and around the dome. The fumaroles are the sour ce of a sustained steam plume above the volcano. The dome then subside s back into the conduit. During the subsidence phase the velocity and gas output of the fumaroles decrease, and the cycle is completed by vi olent explosive activity. Subsidence of both the dome and the crater f loor is accommodated by movement on concentric, cylindrical or inward- dipping conical fractures. The observations are consistent with a mode l in which gas loss from the dome is progressively inhibited during a cycle and gas pressure increases within and below the lava dome, trigg ering a large explosive eruption. Factors that can lead to a decrease in gas loss include a decrease in magma permeability by foam collapse, reduction in permeability due to precipitation of hydrothermal minera ls in the pores and fractures within the dome and in country rock surr ounding the conduit, and closure of open fractures during subsidence o f the dome and crater floor. Dome subsidence may be a consequence of r eduction in magma porosity (foam collapse) as degassing occurs and pre ssurisation develops as the permeability of the dome and conduit syste m decreases. Superimposed upon this activity are small explosive event s of shallow origin. These we interpret as subsidence events on the co ncentric fractures leading to short-term pressure increases just below the crater floor.