The last volcanic activity in Nisyros occurred thousands of years ago. Nevertheless, Nisyros is characterized as a sleeping volcano, and this characterization is based on three basic facts:
- The relatively recent age of the last volcanic eruptions. The last magmatic activity in Nisyros region occurred about 20,000 years ago, while there is evidence that the volcanic eruption in Giali probably happened less than 10,000 years ago.
- The processes that generate volcanic activity (the sinking of the African lithosphere beneath the Aegean Sea and the large cracks) are still active phenomena in the region.
- The thermal condition of the region which was studied during the soil drilling that took place in Nisyros in order to find a geothermal reservoir capable of producing electricity. At the bottom of the first drilling, 1700 meters below sea level, researchers noticed temperatures close to 500 degrees Celsius which proves the existence of molten rock.
The most spectacular “event” of Nisyros is the steam activity in the region of Ramos and particularly in the crater of Stefanos. The gases emitted in the region have a maximum temperature of 1001 degrees Celsius. These gases consist primarily of water vapor (94%), carbon dioxide (5%), hydrogen sulfide, nitrogen and methane (less than 0.5%). When water vapor liquefies it goes into the surrounding soil and because of the dilute sulfuric acid any fabrics that come in contact with the ground are destroyed very quickly. The characteristic smell of "rotten eggs" in the region is also due to the existence of infinitesimal quantities of hydrogen sulfide.
The emission of hot gases (water vapor) occurs in several places, mainly along major active faults of the island. The most famous ones are located on the south side on the rim of the caldera and the region of Pyria. The temperature of the steam at these points is between 36 and 40 degrees.