rmed by magmatic eruptions. Nisyros has had many explosive magmatic eruptions, which have ejected material high into the sky and sent pyroclastic flows billowing down its flanks; and also more effusive, quieter eruptions of lava, which have dribbled more slowly down the sides of the island and built up large domes. Over time, these alternating explosive and effusive eruptions built up the complex stratocone that makes up the current island.
The largest and most recent explosive eruptions on Nisyros created the Island's central caldera. These were Plinian events, that generated eruption columns several kilometers high and many large pyroclastic flows. During these events, so much magma was erupted that it emptied the magma chamber, causing the ground above to collapse down into it - creating the subsidence caldera we see today.
The Nisyros magma chamber then re-filled and erupted the six large lava domes that now fill the western side of the Nisyros caldera. These domes were the last magmatic eruptions on Nisyros.
Phreatic eruptions do not involve magma (other than as a heat source) - they are steam explosions that occur in the hot geothermal reservoir beneath the suface. These powerful explosions can destroy the ground above them to create craters (like a bomb buried beneath the ground). This is how the many craters inside the Nisyros caldera were generated, including Stefanos crater. There have been thirteen phreatic eruptions in the Nisyros caldera, most recently during 1871-1873 and in 1888. Phreatic explosions can occur without any warning, and are often triggered by earthquakes.
Nisyros has shown signs of volcanic unrest in recent years, which suggest that there is still magma moving about beneath, and off-shore of, the island. In 1995-98 Nisyros experienced a 'volcano-seismic crisis', with many earthquakes, ground deformation and changes in the chemistry of its fumaroles. Since then, a large fracture and sinkholes have developed in the caldera, and in 2014-15 there was another period of increased local seismicity. The volcano's active hydrothermal system continues to emit fumaroles and hotsprings.
Another volcanic eruption at Nisyros is therefore possible. We would expect to see further warning signs before a magmatic eruption, but it is much harder to predict a phreatic event.
Learn more on a guided tour of Nisyros volcano...
If you are interested in learning more about volcanoes and would like to visit Nisyros on a tour with a fully qualified, professional volcanologist - check out Kinvig Geoscience! They have unrivalled expertise on the volcanic deposits and activity of Nisyros, and offer the opportunity to go into the craters and measure the temperatures of its active features for yourselves.