The crater lake at Santa Ana Volcano (El Salvador) was monitored during 1992–1993 and 2002–2007. Crater lake chemistry was generally similar until the 2005 eruption. Acidification of the hydrothermal system by condensing magmatic gases yielded fluids that sustained a cool acid sulfate-chloride lake roughly 200 m in diameter (temperature = 16–28 °C, pH = 0.7–2.0, SO42− = 4500–14,000 mg/L, Cl− = 1100–9200 mg/L, total dissolved solids [TDS] = 7000–25,000 mg/L).
The phreatomagmatic eruption Volcanic Explosivity Index (VEI) 3 of October 2005 modified the summit crater morphology, leading to physical, thermal, and chemical changes in the lake over the next few years. The lake became hotter and more acidic, with variable chemistry and color (temperature = 24–66 °C, pH = 0.4–1.3, SO42− = 2500–9800 mg/L, Cl− = 3200–22,000 mg/L, TDS = 10,000–36,000 mg/L, turquoise-gray-yellow color). The SO42−/Cl− ratio dropped below 1, indicating an increase in the rate of volcanic gas input and coincident S depletion by abundant precipitation of native sulfur and secondary S-bearing minerals (alunite, gypsum, iron sulfides, and anhydrite). An increase in rare earth element (REE) concentrations in lake waters indicated leaching of the newly intruded magma. The eruption likely enhanced permeability in the edifice, further increasing the amount of available fresh wall rock to react with acidic fluids, and the concentration of rock-forming elements in the lake increased fivefold to a maximum of 90 g rock dissolved per kg water.
The magma continued to degas through the lake bottom at the drowned eruptive vent, providing a large, direct gas input into the lake. Direct gas discharge into the lake led to sulfur saturation and formation of hollow sulfur spherules by percolation of gas bubbles through the molten sulfur bottom layer. Increased heat input into the lake (8–830 MW, equivalent SO2 flux of 16–1600 t/d) led to enhanced evaporation and highly variable lake mass. Consequently, on three occasions during 2006 and 2007, the lake area diminished to 70% of its former size.
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