## 4 Discussion
### Data and Method Limitations  
Over the past 20 years, SAR data have been increasingly available for studying active volcanoes and their hazards. The SAR interferometry approach (InSAR and time series) was successfully applied to constrain surface deformations at volcanoes [52; 53]. During the 2021 Tajogaite eruption, however, near-field InSAR were not successful due to the loss of coherence during the ash-rich eruption, which is why deformation data remained limited. Therefore we followed earlier studies that exploited the amplitude signal of SAR data, which helped to identify surface changes due to their ability to operate throughout the day and night, independent of visibility and weather conditions [54]. However, as many volcanic processes, such as eruption changes and formation of craters, occur rapidly during hours or days only [22; 36], we could not possibly capture all changes occurring at La Palma. Although we used a large number of satellite images, on average one SAR image every two days, monitoring faster changes occurring on Tajogaite remained challenging. Another important limitation of the SAR data we used is related to its geometric distortion. During the Tajogaite eruption, the topography was rapidly evolving, and the elevation of the cone was growing and partly collapsing again. The eruption cone was built by the accumulation of scoria material, increasing its relative height to 187 m [35]. We can identify a geometric distortion increasing during the course of the eruption and new cone buildup, which is largely pronounced in the ascending data, and to a smaller degree also seen in our descending data. Furthermore, the geometric distortions varied depending on the look angle, which was evident when analyzing our ascending data for two tracks: foreshortening and shadowing were found to be stronger in track 12 than in track 06. We find that this makes the crater flanks in track 12 look approximately half the size of those in track 06. A similar difference was found for the shadowing effect of track 12, where craters look more elongated, and consequently, the crater row appears wider in scale (see supplementary material Figure S2). Therefore, for correct interpretation of the location of craters, comparing SAR images acquired from different tracks (looks) helps to avoid false interpretations. Due to these geometric distortions, we also combined and validated findings with independent optical imagery. By comparing our findings with high-resolution drone orthophotos and a post-eruptive digital elevation model, we found the number of craters to be the same as the ones found in the SAR data. Similarly, the general orientation of the aligned structure is comparable, although the strike locally changes. We also performed a correction on the later SAR data (e.g., the 6 December 2021 image) with the post-eruptive digital elevation model and found some distortion effect compared to the data terrain corrected with a pre-eruptive digital elevation model (Supplementary Material Figure S7). This implies that a regularly updated digital elevation model would be required to achieve improved results. [PERSON] et al. (2006) [23] and [PERSON] et al. (2018) [20] pointed out that for X-band, a height variation of a few millimeters only can cause surfaces to look brighter in the SAR data. The overprinting of materials of different grain sizes might cause variations in theamplitude signal. This could explain the expression of visible craters, especially in Phase 1, where we see five craters on 29 September 2021 and only three on 1 October 2021. This may suggest that finer material covered the new cone and craters, resulting in lower contrasts of the SAR amplitude and, therefore, less defined or partially indistinguishable crater rims. The camera and kymography data were found to be highly instructive, but were limited by the single angle perspective, which prevented us from tracking the activity ofvents perpendicular to the field of view.