Geology of Nisyros Volcano

Abstract

The heart of the book is found in this Chapter, the geology of Nisyros Volcano. The compilation of the new “Geological Map of the Island of Nisyros (Dodecanese Archipelago)” with cross sections serves as the basis for the interpretation of the eruptive history of Nisyros volcano. The volcano-tectonic structures of the 698 m high stratovolcano are marked by the central, almost circular caldera with a diameter of 3.6 km and a drop of 300–400m of its steep walls. This dominant structural feature is crosscut by two conjugate major and three minor fault systems. Since accurate geochronological data are still missing due to the rarity of potassium bearing minerals, such as sanidine and biotite as well as to other petrologic and chemical reasons, the eruptive history can only be based on relative ages of a detailed lithostratigraphy of thirty-two mappable eruptive units. A major division into five volcanic cycles has been devised using eruptive successions, contact relationships, epiclastic deposits and intercalated paleosols. Volcanic activity started with shallow submarine basaltic-andesitic lavas during the “Early Shield Volcano Cycles” evolving into a subaerial environment with local lacustrine lakes. Progressively, a composite stratovolcano built up the circular shape of Nisyros Island, with eruptions on the northern and southern part of the island, the latter with effusion of rhyolitic lava. Along the east coast, mainly explosive eruptions produced two larger tuff cones of andesitic and dacitic compositions. During an evolved stratovolcanic stage, basaltic-andesites and andesites were emitted as lavas and pyroclastics from several smaller cinder, spatter and scoria cones covering the southern, western and northern slopes of the island. The “Composite Stratovolcanic Cycles” ended with the effusion of a large dacitic dome in the northern part of the island, which generated a major dome collapse of the northern slopes into the sea. With an unknown time gap, voluminous eruptions of rhyolitic pyroclastics started from the northern and southern eruptive centers, “Caldera Eruptive Cycles”, followed by a first caldera collapse and successive voluminous rhyolitic lava flows, which extruded towards southeast. The youngest major explosive rhyolitic eruption occurred after an unknown time interval from a northern centre, covering the northern and eastern slopes with major pumice deposits and inducing a second-stage of the caldera collapse. Afterwards, during the “Post Caldera Eruptive Cycle”, voluminous effusions of rhyodacitic magma filled the western part of the caldera with large domes and flows that extend towards the sea, in the southwest. Since then, Nisyros Volcano remains in a dormant stage. Magmatic counterparts, such as gabbroic and dioritic intrusives, as well as mafic cumulates have generated a large hydrothermal system at depth, which is still highly active today.