## 4 Discussion  
Between 28 March and 1 April 2020, the summit craters of Stromboli volcano produced two overflows, on 28 and 30-31 March. The two episodes lasted \(\sim\)7 h and \(\sim\)23 h, respectively, and were accompanied by the descent of PDCs down the SdF slope and on the sea surface. Integrating several monitoring data, comprising visual and thermal images from a network of fixed cameras, ground deformation from GBInSAR, seismicity, and satellite images, we could reconstruct the sequence of events that occurred between 28 March and 1 April 2020 and understand their eruptive processes, gaining useful insights for hazard assessment. The explosive activity, in terms of the number of explosions versus time, increased before the 28 March and 30 March lava flows (Figure 3a), which suggested that the magma level was becoming shallower within the feeder conduit [19; 39; 49; 50] prior to the lava flow output. Lava flows were heralded and followed by rock-falls and landslides, which triggered the descent of PDCs down the SdF slope (Figure 3b). The number and duration of these events, as recorded by the seismic network, appear much greater during the 30-31 March lava flow, which lasted longer than the previous event. The first effusive episode was not preceded by significant deformation of the summit zone as detected by the GBInSAR (Figure 3c,d and Figure 4), whereas the second was accompanied by a sudden widening and narrowing of the NE crater zone (Figure 3c,d), and followed by a deflation of the summit crater terrace (max 17 mm recorded away from the sensors, Figure 4b). It should be noted that deflation of the crater terrace (Figure 3c,d) did not occur after the 28 March lava flow, confirming that its erupted volume was probably much smaller compared to the 30-31 March event. The number of explosions at the summit craters decreased after both the lava flow outputs, but this decline was greater and lasted longer on 30-31 March (Figure 3a), yet again suggesting a greater drainage of the shallow conduit, consistent with a larger erupted volume, which generated this lava flow. Hence, during the eruptive phase that took place between 28 March and 1 April at Stromboli, the shift at the summit craters from the persistent Strombolian explosions to lava flow output was heralded by an increase in the rate of explosions, and by a shallower magma level within the feeder conduit [19; 39; 49; 50]. Conversely, the decrease of explosive activity following the lava flow output suggests that the drainage of the uppermost conduit was efficient, requiring a certain amount of time (of the order of h, Figure 3a) in order to allow the magma level to rise again after drainage in order to restore the persistent Strombolian activity at the summit craters. This is consistent with similar events, which were observed during the much longer 2002-2003 and 2007 effusive phases [19; 39; 90; 91]. Satellite data allowed us to obtain the map of the lava flows expanding on the SdF slope between 28 March and 1 April, which was estimated at 94,500 \(\pm\) 3380 m\({}^{2}\) (Figure 6), the radiant heat flux over time (Figure 3e), and, consequently, the TADR (Figure 5), providing an estimation of the cumulative erupted lava flow volume at 37-69 \(\times\) 10\({}^{3}\) m\({}^{3}\) DRE. This compares to the volume of the NEC flank eroded during the overflows that was obtained from the camera images, resulting in 13.4 \(\times\) 10\({}^{3}\) m\({}^{3}\). The behavior of the lava flow spreading from the craters down to the steep SdF slope was consistent with an increase of the lava viscosity and of the yield strength, caused by a decrease in gas-content and bulk temperature of the flow, as well as its crystallization [92]. As a consequence, the lava flow front fragmented and hot blocks detached and descended the slope, the fragmentation generating several PDCs [19; 39; 44]. Again, PDCs formation was more relevant during the 30-31 March lava flow than during the previous 28 March lava flow, confirming a lower volume and/or shorter extent of the former lava flow.