Galeras volcano reactivated in early 1988, as evidenced by increased seismicity, gas emissions, and fumarole temperatures, and had a series of explosive eruptions in May 1989. During 1989-1990 the volcano was characterized by high SO2 flux, reaching 5000 t (metric tons)/day. This period of strong degassing was followed by upward movement of magma (July-September 1991) and the emplacement of a lava dome (October-November 1991). Since November 1991, long- period seismicity and SO2 flux have decreased, indicating that the conduit has partially sealed itself, probably by crystallization and solidification of the magma. On July 16, 1992, the dome was destroyed by an explosion, consistent with pressurization due to sealing and degassing in a nonporous medium. Continued pressurization beneath the volcano is indicated by eruptions on January 14, March 23, April 4, April 13, and June 7, 1993. Glass inclusion and matrix glass analyses support a hypothesis of significant magma degassing at depth in the magma chamber and in the conduit. This degassing has reduced the H2O, S, and Cl contents of the magma, but the F content appears to have increased due to crystallization of anhydrous mineral phases and to the low fluid-melt partition coefficient for F. Despite its degassed state, Galeras is currently a dangerous volcano because the sealing process forces gas-saturated magma to release volatile components into confined spaces, thereby causing pressurization and explosive eruptions.