Zur Geologie und Petrologie der Inseln Nisyros und Jali (Dodekanes)

Nisyros as well as the islet of Yiali are found in the crossing of two fault lines, that of Episcopi — Patmos and the one of southern Aegean (Soussaki-Gulf of Saronic — Milos — Santorini — Dodecanese) it consists almost exclusively of volcanic rocks, that is lavas and tuff and very limited appearence of Holocene formations. The presence of two volcanic effusions is certified, everyone of which began from basic magma, poor in SiO₂ (olivinic pyroxenic andesites), and gave as final products acidic petrological types (liparites and volcanic glasses). The basic magmas of the first volcanic periods are found only in few places and in very limited areas of appearence. After the ascension of volcanic fragmental products the cycle of the first volcanic period closed with the creation of very thick beds of pumice. The second volcanic period gave more petrological types with transitive forms. After the creation of olivinic — pyroxenic andesites followed the exit of trachyandesites, trachytic and dacitic lavas with final creation of rhyolite and of volcanic glasses. The extension of the mentioned rocks is noted in detail on the geological map. The classification of the exact geological age is difficult because there is no place where they come in contact with sediments. In general the researches think that the volcanity in the region of Dodecanese began during the middle or the end of the Miocene with successive explosions up to the young geological ages. Only on the islet Yiali which consists exclusively of acid lavas (pumice stones, rhyolite, perlite) fossiliferous layers of marly limestone, sandstone and konglomerate are found, as much at the sea level as in the top of the layers of pumice stone, of tyrrhenian age. The discovery of tools of obsidian in the layers of pumice stone is perhaps a sign that these explosions continued up to the Holocene period. Among the post volcanic phenomena are classified emanations, sulfur-genetic activity with the creation of sulfur deposits of the island and the presence of the thermal healing springs in many parts. Petrochemically the lavas of the island Nisyros and Yiali belong to calcium alkaline magmas.