The island has a 3 kilometres (1.9 mi) to 4 kilometres (2.5 mi) wide caldera, and was formed within the past 150,000 years, with 3 separate eruptive stages, ranging from explosive and effusive andesitic eruptions to effusive and extrusive dacitic and rhyolitic activity.
The volcano is currently active (but not erupting), and fumaroles are found at the craters. It has had four historical eruptions, all of which had a VEI of 2. Almost all of its eruptions involved phreatic activity. The latest eruptive activity was a steam explosion in 1888, after small ash eruptions in 1871 and 1873 and earthquakes are not infrequent. A period of seismic unrest in 1996–1997 led an international team of scientist to initiate monitoring of the volcanic unrest in the European Union sponsored Geowarn project. The entire volcanic complex includes the seafloor between Nisyros and Kos, the island of Gyali and a part of Kos island.
Source: wikipedia.org