## 6 Discussion
### Discussion of the Results
The false-color representations clearly show the evolution of the active lava flow on the NNW flank of Karangetang and are consistent with reports from the GVP [31]. Here, the derived lava area includes nearly all of the detected thermal anomalies. In contrast to reports [32], no lava deposits were identified at the N crater during the Karangetang II eruption phase (Section 5.2). Only a small area within the crater, which can be attributed to a hotspot detection in the NHI layer, was classified as lava. The derived lava area was assigned to the main crater by the region-growing algorithm (Figure 7b). The combination of the NHI and _dNDVI_ layers covers mainly the more northerly lava deposits on the W flank (Figure A1). Based on these surfaces, the texture and brightness changes also identified areas located farther south. Brightness was an important factor in detecting these areas of change. Visually, the lava overprinting can be seen in the significantly darker areas in the post-scene, which also make the entire flank area appear much more homogeneous (Figure 7). Furthermore, incandescent debris avalanches are frequently mentioned in the reports of the GVP [32; 33]. These deposits could not be identified both visually and by OBIA change detection. Furthermore, the overlap of individual lava flows throughout the eruptive phase makes detection of individual lava flows nearly impossible [24]. The analysis of the multispectral false-color imagery (Sentinel-2 and Landsat-8) allows the identification of individual lava flows during the eruption (Figure A3). There, the development of the lava flow in WNW direction at the beginning of the eruption period is visible. In the following months, sporadic thermal anomalies occurred in the W and WSW direction, but the main activity remained in the WNW. No lava effusion from N crater can be observed in the false-color imagery, confirming the derived lava area. Interrupted and fragmented thermal anomalies with a tongue-like structure are common (Figure A3c,f,h,j,m-p,u). These anomalies may represent the block debris avalanches described in the GVP reports [32; 33], which contain incandescent pyroclastic material and usually form at the front of lava flows [66]. On the other hand, fragmented thermal anomalies can also represent differ entially cooled regions of a coherent lava flow [11; 65; 66]. Because the derived lava area includes nearly all of the thermal anomalies in the multispectral false-color time series, a high-quality detection of lava areas can be assumed. The analyzed eruption phase Krakatau I (Section 5.2) can be seen as a precursor of the catastrophic event on 22 December 2018 (Krakatau II). [PERSON] et al. [10] were able to detect movement of the SW and S flanks within this phase using interferometric SAR (InSAR) time series. This ranged from 4 to 10 mm per month, with higher rates of movement correlating with increased volcanic activity, especially in September and October 2018 [10]. The location of the lava deposits in the S-SW region of the island (Figure 8b) is consistent with the observations of [PERSON] et al. [10]. The results of the Krakatau I event prove that the OBIA method also works using only the NHI pixels as the input (Figure 10b). In addition, the newly formed coastline in the SW and S of Anak Krakatau due to the lava effusion was delineated and confirms the increment of the island quantified by [PERSON] et al. [19]. In the SE of the island, there is a section of a recent lava deposit, which was formed shortly before the pre-scene (Figure 8). This was partially overprinted by the lava flows of the eruption phase. Here, the part bordering the sea, which was not overprinted during the 2018 activities, could be well demarcated from the lava deposits. Figure 9: Multispectral false-color time series (Sentinel-2) in the SWIR2/SWIR1/NIR band combination during the Karangetang I eruption period. The classified lava area (in green) outlines the thermal anomalies that occurred. (**a–I**) Volcanic activity and evolution of lava flow towards the NNW flank.