Chemical precursors to the 1998-1999 eruption of Colima volcano, Mexico
Colima volcano is the most active in Mexico. including a significant eruption in 1991. Geochemical monitoring of the volcano started in January 1996 with: 1) airborne measurements of the S02 flux by COSPEC; 2) collection of fumarolic gases from the summit crater every three months, and 3) collection of water samples from three springs on thc S-SW slopes of the volcano every two months. Chemical compositions of volcanic gases did not show systematic trends. except for the isotopic composition of the volcanic vapor. Starting from the su mmer of 1997, the volcanic water became gradually enriched in deuteriurn, which indicates a contribution from deep, less degassed magma. Three other possible precursor signa is were identified for fhe 1998-99 eruption: 1) a two to three-fold "peak" in the boron concentration approximately 3 months before the eruption, 2) a step-wise increase in HC03 and the calculated partial pressure of dissolved C02 , approximately l.5 years before the eruption , 3) until September 1998, the S02 flux from the volcano was at a low level of less than 100 t/d, often below the detection limit of the instrument. A sharp incrcase in the S02 flux was recorded a month before the eruption started, from 400±50 t/d up to 1,600 t/d two days before the new lava, emerged on 20 November, 1998. The S02 flux was in a good correlation with seismicity.
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