The Hydrothermal System and Geothermal Activity

Nisyros volcano’s activity is characterized by high seismic unrest, hydrothermal explosions, degassing through fumarolic activity and diffuse degassing structures. Hydrothermal explosions in the caldera floor of unknown age until recently created a complex set of intersecting crater structures with evident structural controls. Low-temperature thermal springs and fumaroles occur along known and inferred fracture systems and their crossing points, springs preferentially along the coast and fumaroles within the caldera and its hydrothermal explosion craters. Their chemical and isotopic signatures reflect phase separation and mixing to varying degrees with magmatic, hydrothermal, meteoric, and seawater components. A deep hot hydrothermal brine with seawater and magmatic influence feeds thermal fluid and gases by vapor separation into a set of intermediate thermal aquifers, which have been explored by drilling. A shallow low-temperature aquifer results from steam condensation and the resulting surface manifestations emit hot steam-gas mixtures at up to boiling temperature. Widespread diffuse degassing of CO2 demonstrates the permeable nature of the Nisyros hydrothermal system, focusing along fault systems, releasing 68 t d−1 of CO2 and 58 MW of thermal energy. The last magmatic activity of Nisyros, even if of unknown age, is considered relatively old, based on morphological considerations and on the absence of historical magmatic eruptions. Nevertheless, there are many geochemical indications supporting the presence of a magmatic source at depth, which feeds the hydrothermal system. This evidence is mainly constituted by the isotopic composition of H2O, S, and He discharged by the Nisyros fumaroles.