Nisyros is a dormant active volcano, and this classification is based on three features:

• The first is the relatively recent date of the latest volcanic eruptions. The last magmatic activity on Nisyros was seen some 20,000 years ago, while the latest eruption of Yali probably dates back only 10,000 years.
• The second feature is that the processes which set off volcanic activity, the subduction of the African lithosphere under the Aegean and the big faults, remain active in the region.
• The third feature is the thermal state of the region, mainly the data from deep bore holes carried out on Nisyros to locate a geothermal reservoir capable of generating electric power. At the bottom of the first deep borehole, 1,700 meters below the sea level, temperatures of over 500 degrees were recorded, suggesting the existence of molten rock at shallow depths of between 3 and 5 kilometres.

The illustration presented here combines the results of geological, volcanological and geothermal research on Nisyros, reconstructing a section of the earth’s crust below the island. We can make out the older non-volcanic rocks, the volcanic edifice with the caldera depression, as well as the apophyses of older and more recent magma chambers. Also reported are the two main reservoirs of geothermal fluids, which were identified by the drilling.

The upper of these two reservoirs is that which feeds all thermal activity on the island’s surface. It fuels the hot springs and the vents through which hot gases escape: – the island’s fumaroles.

The most intense and spectacular thermal activity on the island is to be seen in the fumaroles at the area of Ramos and especially in the crater of Stefanos and the area of Lofos. The gases released from hundreds of points in the vicinity reach a maximum temperature of 100.1 Celsius. The ground around the fumaroles is of the same temperature. The gases are composed mainly of water steam (94% of volume) and carbon dioxide (5%). Hydrogen sulphide, nitrogen and methane are also emitted, but in quantities lower than 0.5%. At the rim of the apertures through which the gases are emitted, beautiful sulphur crystals are deposited, while the steam issuing from the vents waters the ground with a weak solution of sulphuric acid, owing to the dissolution of the hydrogen sulphide in the steam. It is for this reason that materials coming into contact with the ground are quickly ruined. The characteristic smell of rotten eggs in the area is also due to the presence of hydrogen sulphide, detectable by the sense of smell even when present only in very low concentrations.

Outside the area of Ramos, hot gases (steam) are released at various points, mainly along the great active faults and fractures of the island. The best known, on the southern side are on the rim of the caldera and in the area of Pyria. On the northern side they are to be found at the base of the cliff of the caldera, at the peak of the Nifios dome and at the entrance to the village of Emporio. The temperature of the steam at all these points ranges from 36 to 40 degrees.