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

The recent eruptive cycle at Poas Volcano was notable for the dramatic disa ppearance and subsequent reappearance of the summit crater lake. This cycle consisted of discrete phases of activity associated with a range of geophy sical and geochemical signatures that illustrate the relative value of the various techniques for identifying precursory phenomena. intrusive episodes in 1980 and 1986-1989 at Poas were preceded by A-type seismicity. Magma ro se close to the surface on both occasions but the focus shifted from the do me (1980), when the lake remained stable, to the crater lake (1986-1989). T he 1986-1989 event, which culminated in the complete loss of the crater lak e and explosive eruptions, was characterised by concurrent increases in mic ro-gravity (on the southern crater floor), B-type seismicity and lake tempe rature and by changes in lake geochemistry. The calculated mass of magma in truded 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 se ries of magma-filled dendritic conduits intruded beneath the lake facilitat ed enhanced heat and gas flux from a deeper magma feeder body. A model is e nvisaged where brittle fracture of the magma carapace at about 500 m depth allows magma to rise up through the conduit system beneath the crater and t o fall again or solidify in situ when pressure drops. Whilst active, this p rocess transfers heat and gas upwards driven by the convection of buoyant, volatile-rich magma displacing colder, relatively Volatile-poor magma. As m agma pressure from below decreases, the link between the deeper magma feede r and the upper conduit system is broken and the hydrothermal system resume s 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 it s disappearance in 1989 was accompanied by enhanced eruptive activity and g as emissions with considerable local environmental impact. The lake therefo re acts as both a moderator and index of volcanic processes at Poas. (C) 20 00 Elsevier Science B.V. All rights reserved.