Nisyros (Greece)

The island of Nisyros is a Quaternary volcano located at the easternmost end of the Aegean Volcanic Arc, within the Dodecanese archipelago, situated south of Kos (Fig. 1). The island is almost circular, with an average diameter of 8 km, and covers an area of approximately 42 km2. It lies above a basement of Mesozoic limestone and a thinned crust, with the mantle-crust transition (Moho discontinuity) located at a depth of approximately 27 km. The volcanic edifice of Nisyros comprises a succession of calc-alkaline lavas and pyroclastic rocks, with a summit caldera of an average diameter of 4 km.

Fig. 1 The Kos-Yali-Nisyros-Tilos volcanic field with Nisyros volcanic island featured in the centre. Swiss Federal Institute of Technology (ETHZ)
(Click on image to enlarge).

Nisyros island was chosen as a test site due to its unique situation, which includes complex volcanic and related hazards, the increasing impact of tourism on the island and in the Kos region, and easy and low access cost for all monitoring systems.

Fig. 2 Nisyros volcano; inner caldera with Profitis Ilias dome (700 m) and hydrothermal explosion craters visible (Stefanos crater in the foreground)
(Click on image to enlarge).

Nisyros island is a remnant of a prehistoric volcanic field from which 160,000 years ago the largest eruption in the eastern Mediterranean (Kos plateau tuff) devastated the entire Dodecanese islands. Although the last magmatic volcanic activity on Nisyros dates back at least 25,000 years, the present geodynamic activity encompasses high seismic unrest and widespread fumarolic activity.

Violent earthquakes and steam blasts accompanied the most recent hydrothermal eruptions in 1871-1873 and 1887 and left large crater holes behind. Mud and hydrothermal vapours rich in CO2 and H2S were emitted from fracture zones which cut the caldera and extend towards the NNW through the vicinity of the village of Mandraki, into the island of Yali, and even towards Kos. In 1996 and 1997 seismic activity started with earthquakes of magnitudes up to 5.5 and with hypocenters down to 10 km depth, damaging 30 houses in Mandraki.

Today, the Nisyros volcano and the hydrothermal craters are visited daily by hundreds of tourists (Fig. 2) attracted by its fuming (degassing) hydrothermal explosion craters.

Five different kinds of natural hazards could occur:

• Seismic activity due to the regional tectonic movements;
  
• Magmato-tectonic seismic activity related to magmatic unrest in the crust;
  
• Gas and steam hydrothermal eruptions within the Nisyros crater field;
  
• Landslides and tsunami hazards subsequent to earthquakes, magmatic and volcanic activity;
  
• A volcanic eruption.
  

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