Investigaciones Vulcanología

Abstract: Bouguer gravity surveys have long been used to investigate sub-surface density contrasts. The main sources of error in previous surveys have been the determination of relative elevations of stations and the effect of topography (removed via the terrain correction). The availability of high precision Kinematic GPS data now facilitates generation of high-resolution Digital Elevation Models that can help to improve the accuracy of relative elevation determination and the terrain correction. Here we describe a highresolution gravity survey at Poa´s volcano, Costa Rica. Our gravity modelling (i) identifies small pockets of magma at shallow depths which relate to successive magma intrusion through time and (ii) shows that the persistent degassing in the eastern part of the crater is related to local deformation at the top of the volcano and changes in the fracture network, rather than to the presence of a shallow magma intrusion.

Concentrations of chloride and sulfate and pH in the hot crater lake (Laguna Caliente) at Poa´s volcano and in acid rain varied over the period 1993–1997. These parameters are related to changes in lake volume and temperature, and changes in summit seismicity and fumarole activity beneath the active crater. During this period, lake level increased from near zero to its highest
level since 1953, lake temperature declined from a maximum value of 708C to a minimum value of 258C, and pH of the lake water increased from near zero to 1.8. In May 1993 when the lake was nearly dry, chloride and sulfate concentrations in the lake water reached 85,400 and 91,000 mg l21, respectively. Minimum concentrations of chloride and sulfate after the lake refilled to its maximum volume were 2630 and 4060 mg l21, respectively. Between January 1993 and May 1995, most fumarolic activity was focused through the bottom of the lake. After May 1995, fumarolic discharge through the bottom of the lake declined and reappeared outside the lake within the main crater area. The appearance of new fumaroles on the composite pyroclastic cone coincided with a dramatic decrease in type B seismicity after January 1996. Between May 1995 and December 1997, enhanced
periods of type A seismicity and episodes of harmonic tremor were associated with an increase in the number of fumaroles and the intensity of degassing on the composite pyroclastic cone adjacent to the crater lake. Increases in summit seismic activity (type A, B and harmonic tremor) and in the height of eruption plumes through the lake bottom are associated with a period of enhanced volcanic activity during April–September 1994. At this time, visual observations and remote fumarole temperature measurements suggest an increase in the flux of heat and gases discharged through the bottom of the crater lake, possibly related to renewed magma ascent beneath the active crater. A similar period of enhanced seismic activity that occurred between August 1995 and January 1996, apparently caused fracturing of sealed fumarole conduits beneath the composite pyroclasticcone allowing the focus of fumarolic degassing to migrate from beneath the lake back to the 1953–1955 cone. Changes in the chemistry of summit acid rain are correlated changes in volcanic activity regardless of whether fumaroles are discharging into the lake or are discharging directly into the atmosphere. q2000 Elsevier Science B.V. All rights reserved.

The recent eruptive cycle at Poa´s 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 Poa´s 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 500 m 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 Poa´s. q2000 Elsevier Science B.V. All rights reserved.

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