Tracking the evolution of the summit lava dome of Merapi volcano between 2018 and 2019 using DEMs derived from TanDEM-X and Pleiades data

At andesitic volcanoes, effusive lava flows and dome emplacement alternate with explosive, sometimes very destructive events. It is thus crucial to obtain quantitative information on the dome volume emplaced as well as on the extrusion rate. However, steep slopes and continuous activity make it difficult to install field instruments near many volcano summits. In this study, we take advantage of two high resolution remote-sensing datasets, Pleiades (optical acquisitions in tri-stereo mode) and TanDEM-X (radar acquisitions in bistatic mode), to produce twenty Digital Elevation Models (DEMs) over the summit area of Merapi volcano, Indonesia, between July 2018 and December 2019. We calculate the difference in elevation between each DEM and a reference DEM derived from Pleiades images acquired in 2013, in order to track the evolution of the dome in the crater. Uncertainties are quantified for each dataset by a statistical analysis of areas with no change in elevation. We show that the DEMs derived from Pleiades and TanDEM-X data are consistent with each other and provide good spatiotemporal constraints on the evolution of the dome. Furthermore, the remote-sensing estimate of the lava volume is consistent with local drone measurements carried out by BPPTKG at the time of dome growth. From our DEMs, we show that the dome growth was sustained by a relatively small effusion rate of about 0.0336 +/- 0.0067 m(3).s(-1)(2900 +/- 580 m(3)/day) from August 2018 to February 2019, when it reached a height of 40 m (+/- 5 m) and a volume of 0.64 Mm(3) (+/- 0.03 Mm(3)). The lava dome initially grew radially, and then extended asymmetrically to the northwest and southeast starting in October 2018. From February 2019 onwards, the dome elevation remained constant, but lava was continuously emitted. Lava supply was balanced by destabilization southwards downhill producing an accumulation zone of 400 m long and maximum 15 m (+/- 5 m) high with a volume of 0.37 Mm(3) (+/- 0.29 Mm(3)). The measured accumulation rate between February and September 2019 is 0.0094 +/- 0.001 m(3).s(-1)(810 +/- 90 m(3)/day). In late 2019, several minor explosions partially destroyed the center of the dome. This study highlights the strong potential of the joint use of TanDEM-X and Pleiades DEMs to quantitatively monitor domes at andesitic stratovolcanoes.