Abstract: The detailed analysis of the craters of hydrothermal eruptions and related products present on Nisyros Island demonstrates the ephemerality of these morphological forms. In other words, the mere recognizable existence of the craters and associated deposits implies recency of hydrothermal activity. The minimum temperature required to cause the explosive phenomenon and, possibly, the depth of the reservoir (which can be evaluated on the basis of the correlation between the diameter of the crater and the depth of explosion as proposed by Fytikas and Marinelli, 1976) are therefore closely representative of the current hydrothermal circulation.Both field evidence and historical records indicate that all the deposits of hydrothermal eruption recognized on Nisyros Island were emplaced as debris flows. Almost all the ballistic ejecta were entrained in these debris flows and either redeposited far from their landing sites or involved in later crater collapse and erosion. This emplacing mechanism implies that the original products were characterized by a water content higher than about 5% by weight.Steam-driven hydrothermal eruptions, one of which took place in 1871, originated deposits of limited dispersion, as no sign of these erodible products can be found in the field today.Surface geology and fluid geochemistry, together with subsurface information (e.g., primary and hydrothermal lithologies, distribution of temperature with depth, physical-chemical characteristics of deep water-bearing zones) indicate that two distinct hydrothermal aquifers are present underneath the southeastern part of the caldera floor. Both aquifers were probably involved in the most important historically documented hydrothermal eruptions, which occurred in 1873.At that time, violent earthquakes fractured the brittle aquiclude separating the two aquifers and caused a sudden transfer of fluids from the deep to the shallow aquifer, thus triggering the hydrothermal eruptions.Hydrothermal eruptions will probably occur in future, and this hazard must be taken into serious consideration. The southern half of Lakki plain, where all past eruptions took place and active fumaroles are concentrated is the zone at highest risk.At present, gas geochemistry represents an effective tool to detect changes in the P,T conditions of the shallow aquifer, and particularly the phenomena of pressure build-up that may lead to a hydrothermal eruption.

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   |wiki=    Wicri/Terre
   |area=    NissirosV1
   |flux=    Istex
   |étape=   Corpus
   |type=    RBID
   |clé=     ISTEX:885CAA988AFD75EDF26F62CA78978C115306CA63
   |texte=   Hydrothermal eruptions of Nisyros (Dodecanese, Greece). Past events and present hazard
}}