During the Caldera Forming eruptive cycle, two significant and violent plinian-type eruptive events took place at Nisyros.
Between these events, there was a first caldera that formed after the first eruption, followed by the emission of at least three major rhyolitic lava flows with a thickness up to 150 m and a total volume bigger than 1 cubic km. This type of lava is characteristic in that it has a high silica content and high viscosity, meaning that it flows like a thick, viscous fluid down the surface, at a relatively slow speed rate. Magmas generating this lava also feature high amounts of gas, thus creating violent eruptions accompanied by pyroclastic flows and tuff deposits, like the caldera-forming eruptive events.
During the effusion of these lavas, a neck at present day Parletia was piled up along with domes that formed the eastern rim of the first caldera. The neck forms the Parletia mound that was used as one of the Medieval castles of Nisyros, where some remnants and water tanks can be found. As a result, this geotope not only consists of an important volcanological formation but also demonstrates the connection between geological and cultural heritage, which make Nisyros a textbook example of a UGGp.
