Growth and collapse of the 2018–2019 lava dome of Merapi volcano

Co-auteur étrangerInternational audienceLava dome collapses are a major threat to the population living near such volcanoes. However, it is not possible to forecast collapses reliably because the mechanisms are not clearly understood, due partly to the lack of continuous observations of such events. To address this need for field data, we have developed new monitoring stations, which are adapted to the volcanic environment. The stations tracked the complete evolution of the 2018–2019 lava dome of Merapi volcano (Indonesia) and the associated pyroclastic density currents. During the 14 months of activity, the stations acquired thermal, high-resolution visual images and movies in stereoscopic configurations. The dome developed on a plateau flanked by steep sides (~ 40°–50°) inside the crater, which was open to the SE. We observed that the dome behaved in a viscous manner (with a viscosity of 109 Pa s for the interior to 1013 Pa s for external parts of the dome) on gentle slopes, and in a brittle way (friction angle ~ 35°, cohesion < 100 kPa) on slopes steeper than 35°. Thus, the lava dome was unable to grow on the outer slopes of the plateau and a significant volume of lava (350–750 × 103 m3) accumulated and collapsed daily to the SE in relatively small volumes (< 10,000 m3), preventing the lava dome from reaching the critical volume necessary for pyroclastic density currents to form and threaten the surrounding population. The cause of the small and frequent collapses was purely gravitational during the dome activity. This suggests that relatively small differences in the summit morphology can control dome evolution, favouring either a lava dome restricted to a small volume and leading to only a minor crisis, or more voluminous dome growth and a catastrophic collapse