Istituto di Geoscienze e Georisorse     
Marini L., Principe C., Chiodini G., Cioni R., Fytikas M., Marinelli G. Hydrothermal eruptions of Nisyros (Dodecanese, Greece). Past events and present hazards. In: Journal of Volcanology and Geothermal Research, vol. 56 pp. 71 - 94. Elsevier, 1993.
The detailed analysis of the craters of hydrothermal eruptions and related products 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 resevoir (wich can be evaluated on the basis of correlation between the diameter of the crater and the depth of explosion as proposed Fytikas and Marinelli, 1976) are therefore closely representative of the current hydrothermal circulation. Both field evidence and historical records indicate that all deposits of hydrothermal eruption recognized on Nisyros Island were emplaced as debris flow. Almost all the ballistic ejecta were entrained in these debris-flow and either redeposit far from their landing sites or involved in later crater collapse and erosion. This emplacing mechanism implies that the original products were chracterized by water content higher than about 5% by weight. Steam-driven hydrothermal eruptions, one of wich 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 chemistry, together with the 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 caldera floor. Both aquifers were probably involved in most important historically documented hydrothermal eruptions, wich occurred in 1873. At that time, violent earthquakes fracture the brittle aquiclude separating the two aquifers and caused a sudden transfer of fluids from the deep to shallow aquifer, thus triggering the hydrothermal eruption. 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 concentranted is the zone at highest risk. At present, gas geochemistry reprents 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
URL: http://https://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/
Subject Greece
Nisyros island
hydrothermal eruptions
eruption mechanism
eruption products
geothermal implications

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