The 1984 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions

Abstract

Lascar Volcano (5592 m; 23°22'S, 67°44'W) entered a new period of vigorous activity in 1984, culminating in a major explosive eruption in April 1993. Activity since 1984 has been characterised by cyclic behaviour with recognition of four cycles up to the end of 1993. In each cycle a lava dome is extruded in the active crater, accompanied by vigorous degassing through high-temperature, high-velocity fumaroles distributed on and around the dome. The fumaroles are the source of a sustained steam plume above the volcano. The dome then subsides back into the conduit. During the subsidence phase the velocity and gas output of the fumaroles decrease, and the cycle is completed by violent explosive activity. Subsidence of both the dome and the crater floor is accommodated by movement on concentric, cylindrical or inward-dipping conical fractures. The observations are consistent with a model in which gas loss from the dome is progressively inhibited during a cycle and gas pressure increases within and below the lava dome, triggering 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 minerals in the pores and fractures within the dome and in country rock surrounding the conduit, and closure of open fractures during subsidence of the dome and crater floor. Dome subsidence may be a consequence of reduction in magma porosity (foam collapse) as degassing occurs and pressurisation develops as the permeability of the dome and conduit system decreases. Superimposed upon this activity are small explosive events of shallow origin. These we interpret as subsidence events on the concentric fractures leading to short-term pressure increases just below the crater floor.