Meet Polyvotis!
Nisyros is the youngest of the largest volcanic centers in Greece and is included along with Methana, Milos and Santorini in the active volcanoes of our country, in the so-called Volcanic Arc of the South Aegean. The entire island is entirely covered by volcanic rocks, as a result of volcanic activity over the last 160,000 years. The oldest rocks are found west of Mandraki (in the characteristic form of pillow-shaped handles) while the youngest rocks of magmatic origin are found in the Rhydacitic volcanic dykes that fill the western part of the caldera.
The wonderfully preserved caldera of Nisyros has a diameter of 4 km and its flat part is dominated by active and impressive hydrothermal craters. Most of the island is mountainous, very fertile and consists of various types of volcanic rocks (basalts, andesites, rhyodacites, rhyolites, etc.) which have been used for the construction of various buildings such as the Paleokastro and the bell tower of St. John the Theologian outside Nikia, while they were also used for the construction of millstones in the past.
Geological evolution of the island: The oldest rocks on the island, which constitute the lowest geological formation exposed at the surface, are glass clastites and pillow lavas, together with their muddy products, formed underwater during the early stages of volcanic formation. The volcanic edifice then emerged from the Aegean Sea waters and formed a calcined volcano with successive lava and pyroclastic product ejections. Its evolution continued, forming a high cone (stratovolcano formation) with parasitic cones, during the homonymous eruptive cycle, before ending with the two major eruptive episodes that formed the present-day caldera (124,000 and 70,000 years ago). From then until about 15,000 years ago, when the last magmatic episode on the island took place, lava was poured into the interior of the caldera, forming the rhyodacite domes in its western part, with some of the highest peaks of Nisyros, such as the Prophet Elias.
The hydrothermal system that remains active today on Nisyros is due both to the presence of magma at a depth of 3-4km from the surface, and to the existence of two aquifers fed by both seawater and meteoric water. The system is responsible for the abundance of fumaroles and hot springs found on the island, as well as for the formation of the unique system of hydrothermal craters in Lakki (central part of the caldera), with the deposits of native sulphur crystals and the strong smell of hydrogen sulphide prevailing in the area. The activity of the system was known from a long time ago, as attested by the texts of travelers to Nisyros as early as the 14th-15th century. Stephanos Crater, the largest and most impressive hydrothermal crater with dimensions of 260x350m and a maximum depth of 27m, was probably formed by a huge hydrothermal explosion up to 6000 years ago. In 1871, another such eruption took place, accompanied by ‘earthquakes, eruptions and red and yellow flames’, as described by eyewitnesses. The result was the creation of the craters Great Polyvotis and Flegetros, which remained active until 1873, when new eruptions took place both in the land and in the submarine area between Mandraki and Giali. The most recent hydrothermal eruption of the volcano was in 1887, when the Little Polyvotis crater was formed.
During your stay on the island, and before or after your visit to Polyvotis, Stefanos and the other craters, visit the Volcanological Museum and the Volcanological Observatory housed in the Environmental Research and Education Centre under the auspices of the Laboratory of Climatology and Atmospheric Environment of the University of Athens and the UNESCO Con – E – Ect. In Nikia the first and in Emporios the second, are a first-class opportunity for a little… extra learning, apart from resting, since the active volcano of Nisyros “is a unique geological attraction, an open geological museum, not found elsewhere in Greece”.
