## 2 Study Area
The hydrothermal activity is expressed on the surface through an extensive fumaradic field at the southern part of Lakki plain (Figure 2) with characteristic hydrothermal craters [81; 82; 83; 84; 85; 86]. The more severely altered terrains are located (1) at the central sector, where the debris and mudflow deposits of the historical hydrothermal eruptions occurred, and (2) at the Stefanos crater [68]. According to the records of two geothermal exploration wells (named Nis-1 and Nis-2 in Figure 2), the main zone that is expressed on the surface is the argillic zone [73; 74]. The hydrothermal processes have led to the alteration of primary minerals of the initial volcanic lithologies to clay minerals followed by supplementary sulfur deposits. The complexity of the surface within the hydrothermal field of Nisyros is extremely high due to several mineralogical transformations linked to the advanced argillic alteration phenomena. Hydrothermal alteration has also occurred outside the diffuse degassing structures along NE-SW striking faults. The NE-SW striking faults strongly control the location of the recent domes emplaced in the plain as well as the craters of the historical hydrothermal eruptions, located in the intersections between two conjugate major and three minor faults systems [69; 70; 81; 86; 87; 88; 89; 90]. The hydrothermal activity has created a complex of intersecting craters, which are controlled by diffuse degassing structures [68]. In particular, the hydrothermal system is represented by five known hydrothermal craters, namely, Stefanos, Kaminakia, Phlegethon, Megalos Polybotes, and Mikros Polybotes in the southern Lakki plain [68; 81]. In particular, (1) Stefanos crater has a floor of 240 m along the major NE-trending axis and 180 m along the minor NW-trending axis and a depth of 27 m [68]. It consists of fragments of andesitic lavas. From northwest toward the center of the caldera, the fragments decrease, their roundness increases, and they appear in alluvial beds. The grey color of this material is due to the advanced argillic alteration that has preceded the hydrothermal eruptions [86]. There is a red, fine-grained layer, which upon it hosts another hydrothermally altered deposit with 5-6-m thickness. The final product of alteration in volcanic rocks is usually white material rich in kaolinite clay and silica. This material is abundant in Nisyros caldera and especially within the crater. (2) The Kaminakia craters are located in the eastern part of the hydrothermal field with an average diameter of about 150 m and are partly filled by the talus of the caldera wall and by the deposits of Stefanos [86]. Kaminakia are the oldest hydrothermal craters whose exact age is unknown. (3) The central sector craters are located in the western part of the hydrothermal field. They are made up of fragments of the lava domes supported by fine-grained materials, rich in small gypsum crystals [86]. The Nisyros caldera is also known for the presence of epithermal gold in the Lofos dome area (82 ppb) and in the adjacent Profitis Ilias area (2500 ppb), which indicates the occurrence of important hydrothermal circulation phenomena [86]. During 1995-1998, Nisyros depicted signals of renewing activity, which included intense seismicity, ground deformations, significant variations in the geochemical parameters of fumaroles, and a progressive uplift and E-W extension of the central parts of the island and a possible magma input at greater crustal depth [76; 91; 92]. This progressive uplift led to a large N-S trending fracture, known as \"Lakki rupture\", in the Lakki plain of the caldera in early December 2001 [91; 93], where a surface stress release took place in the consolidated and cemented epiclastic and hydrothermal sediments of the caldera floor [68].