## 2 Study Area  
Nisyros is an almost circular island located between Kos and Tilos islands (SE Greece) with an average diameter of 8 km. It constitutes the youngest volcano of the South Aegean Active Volcanic Arc [68] (Figure 1a,b). Figure 1: (**a**) Location of Nisyros island in the South Aegean Active Volcanic Arc (SAVA) area (Greece). (**b**) Red square: atmospherically corrected Sentinel-2A MSI image of Nisyros island. The Nisyros volcano, located at the eastern end of South Aegean Active Volcanic Arc (SAAVA), is one of the most known volcanic environments of the Volcanic Arc of Greece. Nisyros is a Quaternary composite stratovolcano, which was formed during the Late Pleistocene-Holocene within an ENE-WSW trending neotectonic graben [69; 70]. It is a volcano of 698-m height with a central caldera depression of 4-km diameter [66; 67; 69; 71; 72]. Its steep walls present a drop of 300-400 m [68]. The Nisyros volcanic edifice was formed during the Late Quaternary and lies above an Alpine basement of Mesozoic limestones [73; 74]. It is characterized by calc-alkaline pyroclastic deposits and lavas of basaltic andesitic, dacitic, rhyodacitic, and rhyodacitic composition formed during several explosive cycles from 160 to 25 ka [69; 75]. The caldera hosts a known hydrothermal field that includes prehistoric and historic hydrothermal craters (e.g., 1871-1873) (Figure 1b) [69]. In this study, we utilized the most recent \"Geological Map of the Island of Nisyros (Dodecanese Archipelago)\" 1:15,000 [68] which we digitized and georeferenced. The geological map synthesizes information from several previous geological maps of the area [71; 72; 76; 77; 78; 79; 80]. Within the caldera, 11 lithoststratigraphic units (LSUs) are present (in terms of age and relative lithology) from the most recent to the older explosive cycles, as shown in Figure 2 and [68]. More specifically, the units are: (LSU1) debris flow and lacustrine deposits related to hydrothermal explosions that took place on the SW sector of the Nisyros caldera; (LSU2) rhyodacitic lava domes and voluminous lavas; (LSU3) rhyoditic lava flows with glass-rich flows, vitrophyric and perlitic facies, internal layering and folding, and amphibole-rich enclaves of basalt-andesitic composition creating a blocky flow surface covered by a paleosol; (LSU4) mainly lag-breccia (near-vent accumulated large lithics); (LSU5) rhyoditic dark grey, glass-rich lava flows and neck with reddish banding, perlitic textures, minor block, and ash flows deposits; (LSU6) basaltic andesite pyroclastic succession, which consists of base surges and pyroclastic flows; (LSU7) andesitic lava flow (max. 40 m) with sole of red scoria derived from northern and central eastern eruptive centers; (LSU8) andesitic pyroclastic sequence of (max. 35 m), from bottom to top, reddish pumice fall with fine layers of laminated surges, pyroclastic flows, surges, and final pumice fall located at the northern and northeastern caldera walls; (LSU9) basaltic andesite scoria-rich lava flows located at the northern and northeastern caldera walls; (LSU10) Quaternary volcano-sedimentary alluvial/beach deposits; and (LSU11) Quaternary volcano-sedimentary old and recent scree deposits. Figure 2: Digitized extract of the geological map of Nisyros (sheet “Map of the Island of Nisyros (Dodecanese Archipelago)”) focusing on the caldera hydrothermal field (scale 1:15,000) [68] with its legend. The lithoststratigraphic units (LSU1 to LSU11) are presented in detail in the main text. There are five main hydrothermal craters, the two areas of Lofos dome (epithermal gold deposits) and Lakki plain. Nis-1 and Nis-2 correspond to the location of two official geothermal wells [73; 74]. The background image is the Sentinel-2 MSI image. Furthermore, Nisyros volcano is characterized by continuous surface expressions of hydrothermal activity. The caldera hosts a hydrothermal field with recorded prehistoric and historic hydrothermal craters (e.g., 1871-1873) placed along calc-alkaline series from basaltic andesites to rhyolites.