Geophysical studies of the recent 15-year eruptive cycle at Poás Volcano, Costa Rica

Abstract

The recent eruptive cycle at Poás Volcano was notable for the dramatic disappearance and subsequent reappearance of the summit crater lake. This cycle consisted of discrete phases of activity associated with a range of geophysical and geochemical signatures that illustrate the relative value of the various techniques for identifying precursory phenomena. Intrusive episodes in 1980 and 1986–1989 at Poás were preceded by A-type seismicity. Magma rose close to the surface on both occasions but the focus shifted from the dome (1980), when the lake remained stable, to the crater lake (1986–1989). The 1986–1989 event, which culminated in the complete loss of the crater lake and explosive eruptions, was characterised by concurrent increases in micro-gravity (on the southern crater floor), B-type seismicity and lake temperature and by changes in lake geochemistry. The calculated mass of magma intruded in this period is far too small to account for the observed increase in surface heat flux and subsequent loss of the lake; we suggest that a series of magma-filled dendritic conduits intruded beneath the lake facilitated enhanced heat and gas flux from a deeper magma feeder body. A model is envisaged where brittle fracture of the magma carapace at about 500m depth allows magma to rise up through the conduit system beneath the crater and to fall again or solidify in situ when pressure drops. Whilst active, this process transfers heat and gas upwards driven by the convection of buoyant, volatile-rich magma displacing colder, relatively volatile-poor magma. As magma pressure from below decreases, the link between the deeper magma feeder and the upper conduit system is broken and the hydrothermal system resumes its role of cooling the magma feeder. The role of the lake as a physical and chemical buffer to the volcanic system was clearly demonstrated when its disappearance in 1989 was accompanied by enhanced eruptive activity and gas emissions with considerable local environmental impact. The lake therefore acts as both a moderator and index of volcanic processes at Poás.