Chemical evolution and volcanic activity of the active crater lake of Poás volcano, Costa Rica, 1993–1997 |
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| Authors: | M Martínez E Fernndez J Valds V Barboza R Van der Laat E Duarte E Malavassi L Sandoval J Barquero T Marino |
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| Institution: | M. Martínez, E. Fernández, J. Valdés, V. Barboza, R. Van der Laat, E. Duarte, E. Malavassi, L. Sandoval, J. Barquero,T. Marino |
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| Abstract: | Concentrations of chloride and sulfate and pH in the hot crater lake (Laguna Caliente) at Poás 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 70°C to a minimum value of 25°C, 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 l−1, respectively. Minimum concentrations of chloride and sulfate after the lake refilled to its maximum volume were 2630 and 4060 mg l−1, 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 pyroclastic cone 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. |
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| Keywords: | crater lakes fumaroles seismicity chloride sulfate acid rain environmental impact Poá s volcano |
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