Fig. 4
From: Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour
Explosive behaviour at Merapi volcano thought to be prompted by hydrothermal alteration. a The outgassing stage. Photograph of Merapi on May 3 showing focused outgassing on the dome rim. We interpret this as the result of outgassing through open fractures. b Alteration, sealing, and pressurisation stage. Focussed outgassing stopped on May 5 (the photograph shown here is from May 7). We interpret this as the result of the hydrothermal alteration of the dome (alteration of the rock mass and sealing of fractures). This causes pore pressure to rise beneath the dome. c Explosion. After 5 days of no to little outgassing, an explosion occurred on May 11. The explosion is interpreted as a consequence of the pore pressure augmentation beneath the dome. Some fractures are re-opened and new fractures form allowing for passive outgassing following the explosion (as shown in Supplementary Fig. 4)
