Morphology and Instability of the Merapi Lava Dome Monitored by Unoccupied Aircraft Systems

Episodic growth and collapse of the lava dome of Merapi volcano is accompanied by significant hazards associated with material redeposition processes. Some of these hazards are preceded by over-steepening of the flanks of the dome, its destabilisation, fracturing and gravitational collapse, producing lethal pyroclastic density currents. With the emergence of unoccupied aircraft systems (UAS), these changes occurring high up at Merapi can now be monitored at unprecedented levels of detail. Here we summarise the use of UAS at Merapi to better understand the evolution of the lava dome following the 2010 eruption. Systematic UAS overflights and photogrammetric surveys were carried out in 2012, 2015, 2017, 2018 and 2019, allowing identification of the progression of major structures and a three-stage morphological evolution of the dome. We first highlight the significant morphological changes associated with steam-driven explosions that occurred in the period 2012–2014. A large open fissure formed and split the dome into two parts. In the years 2014–2018, hydrothermal activity dominated and progressively altered the dome rock. Lastly, in May–June 2018, a series of steam-driven explosions occurred and was followed by dome extrusion in August 2018, initially refilling the formerly open fissure. This work demonstrates the importance of reactivating pre-existing structures, and summarises the unique contribution realised by high resolution photogrammetric UAS surveys.