## 4 Methods
### Lava Flow Mapping  
The separation of cooled lava flows or other volcanic deposits and the background in unvegetated areas is hardly possible due to spectral similarities in the VIS and NIR [22], which is why a _dNDVI_ analysis (Section 4.2) does not yield any valuable results [18]. Especially at volcanoes with high eruptive activity, more overlapping lava effusions take place. Due to these dynamics, the overlapping lava deposits exhibit mineralogical and morphological similarities that lead to identical spectral signatures [22; 24; 26]. Accordingly, at the end of an eruptive phase, it is difficult to distinguish which individual lava flows drained where and what its age structures are [22; 24]. Vegetation overprinting of cooled lava or lahar deposits can also complicate the detection with optical remote sensing data [22; 24; 61]. To address this issue, a change detection method was developed to map lava deposits in unvegetated areas. The methodology follows an object-based image analysis (OBIA) approach [62]. This allows for the inclusion of spatial, structural, and contextual information into the classification, leading to the exploitation of the full data potential, especially in the case of VHR satellite imagery [62; 63]. Input data of the OBIA change detection are the PlanetScope scenes before and after an eruption event (pre- and post-scenes), the classified _dNDVI_ layer (threshold \(<\)\(-0.3\)) (Section 4.2), the temporally aggregated NHI layer, and the extracted drainage system (Section 4.1). The schematic workflow of the OBIA is shown in Figure 2.  
In the first step (Figure 2), the _dNDVI_ and NHI surfaces are intersected with the drainage system. In this way, the drains can be identified which lie within the _dNDVI_ or NHI areas and therefore could potentially have carried a lava flow because of a volcanic or thermal overprint. Then, starting from the identified drainage sub-areas in the _dNDVI_ or NHI surfaces, the entire coherent drainage flow is detected using a region-growing algorithm. In this process, starting from the runoff flow segments within the _dNDVI_ or NHI areas, the object grows exclusively into areas that are unvegetated in the post-scene (_NDVIposI_ layers <0.4). This step ensures that only runoff networks within unvegetated areas are detected in the post-scene, as these are the only areas where lava deposits may be present. If no vegetation change occurred, only the NHI pixels can be used as the initial area for intersection with the drainage system. Similarly, if no NHI pixels are present during the eruption period, only the _dNDVI_ layer can also be used as the initial change area. The outflows identified in this step are then used as the basis for OBIA lava flow mapping. In the second step (Figure 2), the extracted drains and the VHR PlanetScope data are used to identify areas where lava could potentially have flowed. These are inferred from the distance to the drainage system and texture and brightness changes derived from the pre- and post-event PlanetScope imagery. This combination can be used to ensure that differentiation between lava flows and background can be achieved even in the unvegetated area. For this purpose, the unvegetated area in the post-scene is first identified using a threshold value (_NDVIposI_ < 0.4). Then, a multi-resolution segmentation [63] is performed within this area to create the segments in the highest hierarchical object level 1 (Figure 2). Lava flows are not bound to a specific shape and can take on irregular shapes, especially for smaller objects that do not cover an entire lava flow. At the same time, despite spectral similarities between lava flows and background, differentiations can be made between objects based on spectral properties [24]. For this reason, the used multi-resolution segmentation placed a stronger emphasis on spectral properties rather than geometric properties. After segmentation, the distance of the segment center of each object to the nearest point of the flow line is calculated.