## 6 Discussion
### The Hydrothermal Alteration Field  
It is well known that the presence of clays and sulphates, which are arggillic alterations, can be indicators of hydrothermally altered rocks in regions of known geothermal activity such the Nisyros caldera. Based on this fact, in this study, clays were represented by the hydroxyl-bearing alteration endmember, which corresponded to the pixel with the highest NHASI value, a well-known SI that takes into account the Al-OH feature at 2.2 \(\upmu\)m.  
From the results obtained from the application of the proposed methodology on the three multispectral datasets, it was observed that, in general, they all satisfactorily mapped the exposed alteration area of interest, especially ASTER and Sentinel-2. The result demonstrates that mapping alteration using a related hydroxyl-bearing alteration endmember can provide a reliable result (especially for the aforementioned two sensors). The results sufficiently delineated the spatial distribution of the arggillic hydrothermal alteration (which encompasses the arggillic alteration minerals kaolinite, montmorillonite, illite). Moreover, we were able to calculate the percentage of the total surface per lithology affected by alteration, due to the availability of a relative geological map [68]. In the sequel, we highlight the main locations where each lithology is met and we comment on the associated hydrothermal alteration results provided by the proposed methodology. 1. **Acidic to intermediary lavas and breccias** (LSU2, 3, 4, and 5) are present in Lofos dome, locally at the five central sector hydrothermal craters and the west-northwest part of the study area (Figure 2 in red color). According to the results shown in Table 4, hydrothermal alteration seems to affect 10.3-12.5% of their total surface (Table 4). However, we observed that the only LSU that is partially affected by high to very high alteration (23-27% of the total surface) is LSU2, namely, the rhyodacitic lava domes and lavas of the last volcanic cycle of Post-Caldera Eruptive Cycle located at Lofos Dome and the central sector craters (Table 5). Accordingly, the supplementary sulfur deposits cover only 4.5-17% of the acidic to intermediary lavas' surface (Table 4) and, as it is the case of hydroxyl-bearing alteration, high to very high sulfur abundances cover a very small surface of only LSU2 outcrops (0.4% to 4.7%) (Table 5). This result is in accordance with the documented severely altered terrains concentrated in the LSU2 zone due to their contact with hydrothermal fluids [68,76]. It is also worth noting here that SU also succeeded in mapping the very characteristic halo that is created by mud and altered rocks in the area of Megalos and Mikros Polybotes from the explosions of 1887 [68]. 2. **Basic to intermediary composition lava outcrops** (LSU 6-9): The outcrops of these formations are very limited, located in the southeastern part of the study area (Figure 2 in green color) as well as in the southeastern wall of Stefanos crater. In contrast with acidic to intermediary lavas, basic to intermediary lava compositions show low to intermediate hydrothermal alteration abundances, which cover less than 1% of their total surface (Table 5). A characteristic example is the presence hydroxyl-bearing alteration abundances mapped in the Stefanos crater wall (especially from ASTER data) which are in accordance with [86] where it is referred that the bottom of Stefanos crater consists of fragments of andesitic lavas exposed along the steep inner crater walls. Kaminakia crater is also partly filled by the talus of the caldera wall and by the deposits of Stefanos crater. 3. Quaternary deposits: They include volcano-sedimentary alluvial/beach deposits and volcano-sedimentary old and recent scree deposits. In particular: _Volcano-sedimentary Alluvial/Beach Deposits_ (_LSUI0_) They are loose materials of Quaternary alluvial/beach deposits constituting a mixture of epiclastic and sandy materials [68].