Transition from passive to pre-extrusion degassing of ascending andesitic magma before the lava dome-building eruption as derived from the seismic signals and tilt changes: Volcán de Colima, México, August–September 2004

Volcanoes emit volcanic gas during various types of activity. In the case of andesitic and dacitic volcanoes the high viscosity of silicic magma inhibits the release of volatiles. This increases the possibility that gas escape will occur explosively. The explosive emission of volatile constituents, presented in magmas, generates the seismic signals which may give information about the process of degassing at different stages of eruption. Transition between the eruptive episodes of lava dome-building at andesitic volcanoes represents the transition from passive degassing of the final stage of one episode to pre-extrusion degassing of the next episode. In this article we use the seismic parameters of volcanic explosions, volcanic tremor and micro-explosions, recorded at Volcán de Colima during August–September 2004, to describe the process of transition from passive to pre-extrusion degassing from episode 2 (May 2001–September 2004) to episode 3 (30 September 2004–January 2007). Variations in the tilt changes allowed us to identify three periods in development of this transition: period of passive degassing at the final stage of episode 2 (until 6 September) and two periods of pre-extrusive degassing at the initial stage of episode 3 defined by processes of inflation and deflation of volcanic structure from 6 to 30 September. Intensity of degassing of magma, expressed in the energy of volcanic explosions, decreased during transition from passive to pre-extrusive stages. During pre-extrusion periods of inflation and deflation of volcanic structure the depths of explosive events became shallower. Period of deflation was characterized also by a dominance of degassing processes occurring within the permeable cap of ascending magma and generating volcanic tremors and swarms of micro-earthquakes. Comparison of degassing of magma during pre-extrusive period and co-extrusive and post-extrusive periods showed the similarity in intensity of degassing of magma during the pre-extrusive and co-extrusive periods but a sharp increase in degassing intensity during the post-extrusive period of lava dome destruction with large explosions. It was shown also minor intensity of degassing processes just preceding the beginning of the lava dome building comparing with intensity of degassing processes just preceding the beginning of the lava dome destruction with great explosions.