DEFORMATION ASSOCIATED WITH THE EXTRUSION OF A DOME AT GALERAS-VOLCANO, COLOMBIA, 1990-1991

During 1989-1992, two electronic tilt meters named Crater and Peladito s, and a short level Line named Las Piedras, were used for monitoring Galeras volcano. The two tilt meters detected considerable changes of uplift mainly from June to November 1991. The Crater station showed cu mulative values of more than 600 microradians, while the Peladitos sta tion exhibited values of more than 100 microradians. The short level l ine showed inflationary tilt of more than 20 microradians in 1989. Thi s behavior was related to increases in the daily occurrence of long-pe riod seismic events during July-November 1991 caused by the rise and e xtrusion of a lava dome, which was subsequently destroyed in July 1992 . At the end of 1990, the deformation source was located at a depth of 2-3 ion. By December 1991, the source had migated to less than 500 m in depth. The magma ascent rate was 1-4 m/day during this period. The dome emplaced in the crater had a volume between 300,000 to 400,000 m( 3). Using a hydrodynamic model and the tilt changes registered by the Peladitos station, the volume of the magma body is estimated at approx imately 2.8 x 10(6) m(3).